计算机科学与技术专业无线局域网毕业论文外文文献翻译及原文
网络工程局域网中英文对照外文翻译文献
中英文对照外文翻译(文档含英文原文和中文翻译)PUTER NETWORKSDATE COMMUNICATIONSThe end equipment which either generates the digital information for transmission or uses the received digital data can be computer ,printers ,keyboards, CRTs, and so on. This equipment generally manipulates digital information internally in word units—all the bits that make up a word in a particular piece of equipment are transferred in parallel. Digital data, when transmitted, are in serial form. Parallel transmission of an 8-bit word require eight pairs of transmission lines—not at all cost-effective. Data terminal (DTE) is a general phrase encompassing all of the circuitry necessary to perform parallel-to-serial and serial-to-parallel conversions for transmission and reception respectively and for data link management. The UART (Universal Asynchronous Receiver/Transmitter) and USART (Universal Asynchronous/Asynchronous Receiver/Transmitter) are the devices that perform the parallel-to-serial and serial-to-parallel conversions. The primary DTE includes a line control unit (LCU or LinCo) which controls the flow of information in a multipoint data link system. A station controller (STACO) is the corresponding that belonged to the subscriber in a data link system. Between the DTEs, starting with the modems, was communications equipment owned and maintained by Telco property.Data communications equipment (DCE) accepts the serial data stream from the DTE and converts it to some form of analog signal suitable for transmission on voice-grade lined. At the receive end, the DCE performs the reverse function of converting the received analog signal to a serial digital data stream. The simplest form of DCE is a modem (modulator/demodulator) or data set. At the transmit end, the modem can be considered a form of digital-to-analog converter, while at the receive end, it can considered a form of analog-to-digital converter. The most common of modulation by modems are frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM). This is a typically data transmission mode using the analog telephone lines. If you transmit data by digital channel (sometimes it is called “Digital T-carrier”), a pulse Code Modulation (PCM) equipment must be used. A microwave transmission system can also be used for the data communication. Finally, you can use the satellite communication system for data transmission.If the cables and signal levels used to interconnect the DTE and DCE were left unregulated, the variations generated would probably be proportional to the number of manufacturers. Electronics industries Association (EIA),an organization of manufactures with establishing the DTE and modem. This is a 25-pincable whose pins have designated functions and specified signal levels. The RS-232C is anticipated to be replaced by an update standard.2.ARCHITECTURE OF COMPUTER NETWORKSComputer network is a complex consisting of two or more conned computing units, it is used for the purpose of data communication and resource resource sharing. Design of a network and its logical structure should comply with a set of design principles, including the organization of functions and the description of data formats and procedure. This is the network architecture and so called as a set of layers and protocols, because the architecture is a layer-based.In the next two sections we will discuss two important network architectures, the OSI reference model and the TCP/IP reference model.1.The OSI Reference ModelThe OSI model is shown in Fig.14-2(minus the physical medium). This model is based on a proposal developed by the International Standards Organizations (OSI) as the first step toward international standardization of the protocols used in the various layers. The model is called the ISO OSI (Open System Interconnection) Reference Model because it deals with connecting open systems--that is, systems that are open for communication with other systems, We will usually just call it the OSI model for short.The OSI model has seven has seven layers. Note that the OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do. However , However, ISO has also produced standards for all the layers, although these are not part of the reference model itself. Each one has been published as a separate international standard.2.The TCP/IP Reference ModelThe TCP/IP reference model is an early transport protocol which was designed by the US Department of Defence (DOD) around in 1978. It is often claimed that it gave rise the OSI “connectionless”mode of operation. TCP/IP is still usedextensively and is called as a industrial standard of internet work in fact, TCP/IP has two parts: TCP and IP. TCP means it is on the transport layer and IP means it is on the network layer separately.1.There are two end-to-end protocols in the transport layer, one of which is TCP (Transmission Control Protocol) , another is UDP (User Datagram Protocol). TCP is a connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine in the internet. UDP is an unreliable, connectionless protocol for application that do not want TCP’s sequencing of flows control flow control and wish to provide their own.2.The network layer defines an official packet format and protocol called IP (Internet protocol). The job of the network layer is to deliver IP packets where they are supposed to go.The TCP/IP Reference Model is shown in Fig.14.3. On top of the transport layer is the application layer, It contains all the higher-level protocols. The early ones included virtual terminal (TELNET), file transfer (FTP), electronic mail (SMTP) and domain name service(DNS).3.WIDE AREA NETWORKA wide area network, or WAN, spans a large geographical area, often a country or continent . It contains a collection of machines intended for running user (i. e. , application) programs. We will follow traditional usage and call these machines hosts. By a communication subnet, or just subnet for short. The job of the subnet is to carry messages from host to host, just as the telephone system carries words from speaker to listener. By separating the pure communication aspects of the network (the subnet) from the application aspects (the hosts), the complete network design is greatly simplified. Relation between hosts and the subnet is shown in Fig.14-4.One of many methods that can be used to categorize wide area networks is with respect to the flow of information on a transmission facility. If we use this method to categorize wide area networks, we can group them into three basic types: circuit switched, leased line and packet switched.1.CIRCUIT SWITCHED NETWORKSThe most popular type of network and the one almost all readers use on a daily basis is a circuit switched network. The public switched telephone network, however,is not limited to the telephone company, By purchasing appropriate switching equipment, any organization can construct their own internal circuit switched network and, if desired, provide one or more interfaces to the public switched network to allow voice and data transmission to flow between the public network and their private internal network2．LEASED LINE NETWORKSThis is a dedicated network connected by leased lines. Leased line is a communications line reserved for the exclusive use of a leasing customer without inter-exchange switching arrangements. Leased or private lines are dedicated to the user. This advantage is that the terminal or computer is a always physically connected to the line. Very short response times are met with service.3.PACKET SWITCHING NETWORKSA packet network is constructed through the use of equipment that assembles and disassembles packets, equipment that routes packet, and transmission facilities used to route packets from the originator to the destination device. Some types of data terminal equipment (DTE) can create their own packets, while other types of DTE require the conversion of their protocol into packets through the use of a packet assembler / disassemble (PAD). Packets are routed through the network by packet switches. Packet switches examine the destination of packets as they flow through the network and transfer the packets onto trunks interconnecting switches based upon the packet destination destination and network activity.Many older pubic networks follow a standard called X.25. It was developed during 1970s by CCITT to provide an interface between public packet-switched network and their customers.CCITT Recommendation X.25 controls the access from a packet mode DTE, such as a terminal device or computer system capable of forming packets, to the DCE at a packet mode. CCITT Recommendation X.28 controls the interface between non-packet mode devices that cannot interface between the PAD and the host computer. CCITT Recommendation X.3 specifies the parameter settings on the PAD and X.75 specifies the interface between packet network.4.LOCAL AREA NETWORKLocal area data network , normally referred to simply as local area network or LANs, are used to interconnect distributed communities of computer-based DTEs located within a building or localized group of building. For example, a LAN may be used to interconnect workstations distributed around offices within a single building or a group of buildings such as a university campus. Alternatively, it may be complex. Since all the equipment is located within a single establishment, however, LANs are normally installed and maintained by the organization. Hence they are also referred to as private data networks.The main difference between a communication path established using a LAN and a connection made through a public data network is that a LAN normally offers much higher date transmission rates because of the relatively short physical separations involved. In the context of the ISO Reference Model for OSI, however, this difference manifests itself only at the lower network dependent layers. In many instances the higher protocol layers in the reference model are the same for both types of network.Before describing the structure and operation of the different types of LAN, it is perhaps helpful to first identify some of the selection issues that must be considered. It should be stressed that this is only a summary; there are also many possible links between the tips of the branches associated with the figure.1.TopologyMost wide area networks, such as the PSTN, use a mesh (sometimes referred to as a network) topology. With LANs, however, the limited physical separation of the subscriber DTEs allows simpler topologies to be used. The four topologies in common use are star, bus ,ring and hub.The most widespread topology for LANs designed to function as data communication subnetworks for the interconnection of local computer-based equipment is the hub topology, which is a variation a variation of the bus and ring. Sometimes it is called hub/tree topology.2.Transmission mediaTwisted pair, coaxial cable and optical fiber are the three main type of transmission medium used for LANs.3. Medium access control methodsTwo techniques have been adopted for use of the medium access control in the LANs. They are carrier-sense-multiple-access with collision detection (CSMA/CD), for bus network topologies, and control token, for use with either bus or ring networks.CSMA/CD is used to control multiple-access networks. Each on the network “listens” before attempting to send a message, waiting for the “traffic” to clear. If two stations try to sent their messages at exactly the same time, a “collision” is detected, and both stations are required to “step back” and try later.Control token is another way of controlling access to a shared transmission medium that is by the use of a control (permission) token. This token is passed form one DTE to another according to a defined set of rules understood and adhered to by all DTEs connected to the medium. ADTE may only transmit a frame when it is in possession of the token and, after it has transmitted the frame, it passed the token on to allow another DTE to access the transmission medium.1．计算机网络数据通信端设备可以是计算机、打印机、键盘、CRT等，它们可以产生要发送的数字信息，也可使用所接收的数字数据。
无线局域网毕业论文中英文对照资料外文翻译文献
无线局域网毕业论文中英文对照资料外文翻译文献中英文对照资料外文翻译文献WLANWhy use WLANFor one of the main local area network management, for the laying of cables, or check the cable is disconnected this time-consuming work, it is easy to upset, not easy to break in a short time to find out where. Furthermore, for the business and application environment constantly updating and development of enterprise network must be matched with the original re-layout, need to re-install the network lines, although the cable itself is not expensive, but requested the technical staff to the high cost of wiring, especially the old building, wiring project costs even higher. Therefore, the construction of wireless local area network has become the best solution.What conditions need to use WLANWLAN is not limited to alternative local area network, but to make up for lack of wired local area networks, in order to achieve the purpose of extending the network, the following circumstances may have wireless local area network.●no fixed workplace users●wired local area network set up by the environmental constraints●As a wired local area network backup systemWLAN access technologyCurrently manufacturers in the design of wireless local area network products, there are quite a variety of access design methods can be divided into three categories: narrowband microwave, spread spectrum (Spread Spectrum) technology, andinfrared have their advantages and disadvantages, limitations, and more, followed by detailed discussion of these techniques. (Infrared) technology, each technique has their advantages and disadvantages, limitations, and more, followed by detailed discussion of these techniques.Technical requirementsAs wireless local area network needs to support high-speed, burst data services, need to be addressed in the indoor use of multipath fading, as well as issues such as crosstalk subnets. Specifically, wireless local area network must achieve the following technical requirements:1)Reliability: Wireless LAN system packet loss rate should be lower than 10-5,the error rate should be lower than 10-8.2)Compatibility: For indoor use of wireless local area network, so as far aspossible with the existing wired LAN network operating system and networksoftware compatible.3)Data rate: In order to meet the needs of local area network traffic, wirelessLAN data transfer rate should be more than 1Mbps.4)The confidentiality of communications: As the data transmitted in the air viawireless media, wireless local area networks at different levels must takeeffective measures to improve communication security and data security.5)Mobility: support for all mobile networks or semi-mobile network.6)Energy Management: When receiving or sending data to the site when themachine is in sleep mode, when activated again when the data transceiver toachieve the savings in power consumption.7)small size and low price: This is the key to the popularity of wireless local areanetwork can be.8)Electromagnetic environment: wireless LAN should consider thehumanbodyand the surrounding electromagnetic environment effects.AndroidGoogle Android is a Linux-based platform for developing open-source phone operating system (registered trademark in China called "Achi;). It includes operating systems, user interface and applications - mobile phone work required by the software, but there is no past, the exclusive right to impede innovation and barriers to mobile industry, called mobile terminal is the first to create a truly open and complete mobile software. Google and Open Handset Alliance to develop the Android, the alliance by including China Mobile, Motorola, Qualcomm and T-Mobile, including more than 30 technology and the composition of a leader in wireless applications. Google with operators, equipment manufacturers, developers and other interested parties to form deep-level partnerships, hoping to establish a standardized, open software platform for mobile phones in the mobile industry to form an open ecosystem .It uses software stack layers (software stack, also known as the software stack) architecture, is divided into three parts: thecore of the underlying Linux-based language developed by the c, only basic functions. Middle layer consists of library. Library and Virtual Machine Virtual Machine, developed by the C +. At the top are a variety of applications, including the call procedures, SMS procedures, application software is developed by the companies themselves to write java.To promote this technology, Google, and dozens of other phone company has established the Open Handset Alliance (Open Handset Alliance).Characteristic●application framework to support component reuse and replacement●Dalvik virtual machine specifically for mobile devices i s optimized●Internal integrated browser, the browser-based open-source WebKit engine●optimization of 2D and 3D graphics library includes graphics library, 3Dgraphics library based on OpenGL ES 1.0 (hardware-accelerated optional)●# SQLite for structured data storage●Multimedia support includes the common audio, video and static image fileformats (such as MPEG4, H.264, MP3, AAC, AMR, JPG, PNG, GIF)●GSM phone (depending on hardware)●Bluetooth Bluetooth, EDGE, 3G, and WiFi (hardware dependent)●Camera, GPS, compass, and accelerometer (hardware dependent)●Rich development environment including a device emulator, debugger,memory and performance analysis charts, and the Eclipse integrateddevelopment environment plug-insApplicationsA core Android application package together with the release of the application package, including email client, SMS short messaging program, calendar, maps, browser, contact management procedures. A ll applications are written using JA V A.Android Application Framework Developers have full access to core applications used by the API framework. The application framework designed to simplify the reuse of software components; any application can publish its functional blocks and any other applications can use the function block its release (but must follow the framework of security restrictions). Reuse mechanism allows the application form can be user replaced.All of the following applications by the composition of a range of services and systems, including:●an expanded view (V iews) can be used to build applications, including a list of(lists), grid (grids), text boxes (text boxes), buttons (buttons), and even an embeddable web browser.●Content Manager (Content Providers) allows applications to access data fromanother application program (such as the contact database), or to share their own data.● A resource manager (Resource Manager) to provide access to non-coderesources, such as local strings, graphics, and hierarchical file (layout files).● A notification manager (Notif ication Manager) allows applications to customersin the status bar display notification information.●An activity class Manager (Activity Manager) to manage the application lifecycle and provides common navigation rollback feature.Ordering the systemOrdering the system information using automated software tools to achieve la carte, side dishes, stir fry vegetables to the transfer of all management processes; completion point, the computer management menu, point the menu and the kitchen, front-end checkout synchronization print; achieved without the menu paper-based operation; backstage manager of inquiry; warehouse inventory management and so on.In addition, ordering the system can also effectively manage customer data, archiving and future reference, put an end to the restaurant "leakage List", "run list" phenomenon; help restaurants using computer data processing capability and powerful ability to process optimization to achieve automated management, streamline workflow restaurant, reduce waste and man-made phenomenon of management oversight, re-optimal allocation of corporate resources, the operating costs to a minimum.Powerful addition to ordering the system to support the general application of stand-alone and LAN in addition to support head office / branch of multi-level framework used for remote network using the POS system to achieve front store sales cashier, sales of small-ticket instantly print sales day-end, reporting sales data and receive information of new featuresdishes.There are three currently ordering the system to achieve mode:First, the touch screen a la carte model: It uses the currently most popular touch-computer ordering process to achieve that members can to order the software screen prompts, simply click on the screen with your fingers can complete the entire ordering process and convenient This model applies to the practice of rich dishes and large restaurants, restaurants, and restaurant, etc..Second,the wireless PDA ordering mode: it uses a wireless WiFi technology, a la carte interface by PDA display, use touch pen to complete the ordering process, virtuallyanywhere, anytime to order real-time response, this model is more suitable for dishes and practices simple restaurant, features a restaurant and special mood of senior restaurants.Third, the wireless ordering Po mode: it uses the ISM band, can be a floor or other obstruction in the case of seamless coverage up to 10 meters away, while the signal remained stable, which is the ratio of the wireless PDA ordering model's greatest strength, this model applies to simple dishes and practices and other requirements with fewer fast food restaurants, pot shops.。
无线局域网技术外文翻译文献
无线局域网技术外文翻译文献(文档含中英文对照即英文原文和中文翻译)翻译：无线局域网技术最近几年，无线局域网开始在市场中独霸一方。
越来越多的机构发现无线局域网是传统有线局域网不可缺少的好帮手，它可以满足人们对移动、布局变动和自组网络的需求，并能覆盖难以铺设有线网络的地域。
无线局域网是利用无线传输媒体的局域网。
就在前几年，人们还很少使用无线局域网。
原因包括成本高、数据率低、职业安全方面的顾虑以及需要许可证。
随着这些问题的逐步解决，无线局域网很快就开始流行起来了。
无线局域网的应用局域网的扩展在20世纪80年代后期出现的无线局域网早期产品都是作为传统有线局域网替代品而问世的。
无线局域网可以节省局域网缆线的安装费用，简化重新布局和其他对网络结构改动的任务。
但是，无线局域网的这个动机被以下一系列的事件打消。
首先，随着人们越来越清楚地认识到局域网的重要性，建筑师在设计新建筑时就包括了大量用于数据应用的预先埋设好的线路。
其次，随着数据传输技术的发展，人们越来越依赖于双绞线连接的局域网。
特别是3类和5类非屏蔽双绞线。
大多数老建筑中已经铺设了足够的3类电缆，而许多新建筑里则预埋了5类电缆。
因此，用无线局域网取代有线局域网的事情从来没有发生过。
但是，在有些环境中无线局域网确实起着有线局域网替代品的作用。
例如，象生产车间、股票交易所的交易大厅以及仓库这样有大型开阔场地的建筑；没有足够双绞线对，但又禁止打洞铺设新线路的有历史价值的建筑；从经济角度考虑，安装和维护有线局域网划不来的小型办公室。
在以上这些情况下，无线局域网向人们提供了一个有效且更具吸引力的选择。
其中大多数情况下，拥有无线局域网的机构同时也拥有支持服务器和某些固定工作站的有线局域网。
因此，无线局域网通常会链接到同样建筑群内的有线局域网上。
所以我们将此类应用领域成为局域网的扩展。
建筑物的互连无线局域网技术的另一种用途是邻楼局域网之间的连接，这些局域网可以是无线的也可以是有线的。
计算机科学与技术专业毕业设计外文翻译IP协议及IPSec协议安全分析
郑州轻工业学院本科毕业设计——文献翻译题目IP协议及IPSec协议安全分析学生姓名专业班级计算机科学与技术2003-1班学号56院(系) 计算机与通信工程学院指导教师)完成时间2007 年6 月6 日英文原文The Analysis Of IP and IPSec Protocol’s Security Problem1.OSI model in time for TCP/IP protocol Overview1.1OSI model in time for TCP/IP protocol introduceIt was repose International Or ganization for Standardization’ suggest to whereas extend arisen', it broke into seven layer into to that OSI model (open system interconnection reference model). The overdone bulkiness, intricacy incur know clearly heap criticism of the is ISO constitutive OSI reference model.As shown in the following chartThe practical application sense nope very large, thereof forsooth toward fathom network protocol interior wield did very avail out of the refer to network cannot but talk OSI reference model, notwithstanding OSI reference model. In reins network world liner, TCP/IP protocol suite obtain know clearly still for extensive application. These OSI seven layer model suffer, per layer big city provide thereon thickness with, combine one visit mouthpiece or interface.The homology hierarchy entitled peer layer of the differ mainframe of compartment. Stand for and mainframe B menses presentation layer each other forpeer layer, mainframe A menses session layer and mainframe B menses session layer each other for peer layer grade among as if mainframe A. Session layer and stand for (these double-deck function by merge to application layer realize) among at TCP/IP reference model suffer, take out know clearly OSI reference model.1.2TCP/IP presence frangibilityIt was run low of virtual security authentication and crypto system, there into up most factor namely IP address problem that IP layered major defect. R command , NFS, X window grade big city is repose IP address versus user proceed authentication and authorization to among that of TCP/IP protocol with IP address came by way of network node alone one identification, heap TCP/IP serve, include Berkeley. Source IP address estimation compact technique authenticity and security among both that of currently TCP/IP cellular security mechanism primarily repose IP address WRAPT filtration(packet filtering) and authentication (authentication) technology, its validity incarnate at could warranty IP WRAPT. The shield, sans versus IP fold misogyny IP address authenticity 'authentication mechanism and security measure in of whereas IP address lie heap problem, consultative maxima shortcoming namely sans versus IP address. The shield, sans versus IP fold misogyny IP address authenticity 'authentication mechanism and security measure in of whereas IP address lie heap problem, consultative maxima shortcoming namely sans versus IP address.It was repose IP protocol of last, TCP subsection and UDP protocol data packet is encapsulation be on the security menace of IP WRAPT suffer at network upper Tran missive, wherefore sameness be confronted with IP layer station encounter that owing to UDP. Now that people all the while in thought method set, yet still avoid less namely as per TCP tie hour in at rest "thrice handshake" Machine-made attack. Either these attack summarize arisen include：One: source address cheat(source address spoofing) or IP cheat(IP spoofing)；Two: source routing select cheat(source routing spoofing)；Three: rip attack(rip attacks)；Four: discriminate attack(authentication attacks)；Five: TCP serial number cheat(TCP sequence number spoofing)；Six: TCP/IP protocol data stream adopt plaintext transmission；Seven: TCP serial number BOMB attack(TCP SYN flooding attack), for short SYN attack；Eight: easy fraudulence (ease of spoofing).1.3Network SecurityBoth moiety end user versus thereof visit, furthermore enterprise network proper no more will exterior closed among be mainframe computer system suffer among it was one relatively walkway that it was resource that both that of one be indispensable to whereas important factor among Network security right through data network. Preparatory network suffer, none but calculator proper and application. Shield this resource. Confidential data by memory at one fit on glass hall. Past crypto guard make inspect weight, nothing but allowed at enterprise network. Versus heap government sector and academic institution came said, internet except one design to transponder electronic mail and proceed document transmission’ instrument.There be the resource completeness exposure at hacker, juggler and those hellion’ attack of down, there into likely to return include some immoral numerator in order to or certain still ugly motive station ongoing destroy of it was completeness visible that the revolution transfer know clearly possession all these of the both internet and computer technology. Now, calculator full high speed, cheapness combine possess resolvability, general design to depot private and confidential information. Internet in the range of get folk in an OSI to at global proceed without a hitch corresponding also, thereof unreliability no more will conceivable. These purport end user transmitting data station utilized network element likely to met absence their dam within sight back. If data proper yes confidential, it with met expose likely to burglar, by have no right limit user check or bowdlerize through to.In the meanwhile, special enterprise network too need for use internet combine therewith mutual to. Internet at advertisement and e-business aspect repose whereabouts huge business opportunity, versus user came said internet must. Figure full high speed, cheapness combine possess resolvability, general design to depotprivate and confidential information up lead folk in a OSI to at global within range proceed without a hitch corresponding also, thereof unreliability no more will conceivable to it means end user transmitting data station utilized network element likely to met absence their dam within sight back. But do so should gotten enterprise network T-number now that, figure full high speed, cheapness combine possesses resolvability, general design to depot private and confidential information up. In despite of cause how about, now versus network security requirements ratio anciently tighten up, too still necessity to know clearly.It was be on the foundation upward strain for security insure of IP-layer or still definitely said yes at each IP grouping that both data stream among as a matter of fact us has manifold means useful for protective network. Could through the medium of be on the fringe erect one fire wall, filter come off those undefeated data stream out for of dedicated network. Application and transport protocols command thereof own security mechanism. Other kind of technology, considering hereinafter several cause lead such approach possess definite meaning to：1. The Intranet big city yes repose IP 'of the both internet and enterprise.Proprietary data stream quantity big city must through IP-layer. Proprietarydata too big city yes by IP grouping came load-supporting;2. Both it could shield and isolation higher level application exempt meets withsafeness attack;3. It took the part of in being higher level security mechanism;4. It could took the part of above internet erect one extendible, secure VPN.Both it is time for in order to met the needs of above IP-layer realize safeness, IETF came into existence know clearly IP security (IPSec) workgroup. Transit effort, to workgroup already fetch round robin at IPV four and IPV six upward strains for network layer safeness agreement, mechanism kimono devote one's efforts to.IPSec frame station took the part of serve include hereinafter content：1.access control;2.data origin authentication(demonstration every last IP grouping)；3.replay protection (Prevent attacker eavesdrop to certain grouping combineafter some hour playback)；4.data integrity (Test withal make IP grouping at transport process suffer haveno by distort certain out);5.data confidentiality & encryption (Part of past encrypt should grouping stash)6.limited traffic flow management (The IP address of the conceal originalitydispatcher)7.key managementIPSec frame initially definitive agreement include inspect weight head (AH), encapsulation security net lotus (ESP) and key management.2IPSec:IP layer protocol security2.1IPSec protocol bring necessity forthIPSec at IP layer endue safety service, it lead system be able to according to require select secure protocol, take serve station utilized algorithm in time for clap demand serve required key to relevant OSI in for to with. The path of the IPSec be used to shield a stick of or multiyear mainframe and mainframe compartment, safety net shut and safety net shut compartment, safety net shut and mainframe compartment. Both IPSec be able to submitted safety service multitude include access control, connectionless integrality, data source authentication, reject retransmitted packet(partial sequence integrality form), privacy and finitude transmission current privacy. For these serve equal at IP layer endue, so any higher level protocol use they, for instance TCP, UDP, ICMP, BGP and so on. It was through the medium of twain large transmission secure protocol, header authentication(AH)and encapsulation safe load(ESP), and key manager harmonize discuss 'use came finished as well that these object. It was by user, application, and / or site, organize versus security and systemic demand came decision that that of required IPSec protocol multitude content very utilized mode.Both the instant correct realize, use these mechanism, they ought not versus use these security mechanism shield tarn missive user, mainframe and rest hero specialnet part bring negative impact forth. This mechanism too by is designed for algorithm independent. Such modularity permit select different algorithm multitude instead of impact rest segmental realize for to. For example：In the event of, different user communication is available to different algorithm multitude.It was a sort of away hair homology serial number 'WRAPT lead system crash' method of attack grade means came attack to that of whereas IPv6 WRAPT proper without supply any security protection, hacker could past information packet detect, IP spoofing, joint captive, replay attack. The data packet be in existence hereinafter hazard of the wherefore, us receive：No came from legal dispatcher; data at transport process suffer by human amend; data content afterwards by human pick (for instance military secret equiponderance ask informational dialogue) for brains. Both that of that of IPSec purpose namely by way of realize data transfer integrality (source address demonstration and guarantee data have no modify) and confidentiality (without by human run over) and endue to a certainty degree 'versus replay offensive shield with as well to. IPSec usable it supplies security protection with IP very upper layer protocol (TCP and UDP grade). It was throng accident prevention 'syntheses, whereas IPv6IPSec mechanism yes there into important constituent, endue know clearly protocol layer plane last consistency set, these no more will IPv6 compare IPv6'graveness superiority at rest with that the log, routing protocol event and error logging grade, for administration of networks personnel make fault analysis, orient and statistics of the both the security of the both the attack；past MAC address and IP address binding, confine per port 'MAC address use quantity, establish per port broadcast packet flow threshold, use repose port and VLAN’ACL, foundation security user tunnel grade came kept away aim at two tiered attack；past route filtration, versus route informational encrypt and authentication, orient multicasting control, bump route rapidity of convergence withal relieve route oscillation impact grade measure, came muscle three-ply Network security update of the cow originate in equipments drawing unreliability, such as sheet caloric spoil, physical interface 'operating characteristic of electrical apparatus and EMC environmental deteriorate grade into of the usually, physical layer the shield instrument include of the potential safety hazard, such as both encapsulation attack, broadcast packet attack, MAC water-flooding, spanning tree attack grade second floor attack, as well as mendacious ICMP message, ICMP water-flooding, source address beguile , route oscillation gradeaim at three-ply consultative attack into of the that of usually, physical layer menace be from equipments drawing unreliability, such as sheet caloric spoil, physical interface' operating characteristic of electrical apparatus and EMC environmental deteriorate grade. Versus such potential safety hazard, could past allocate redundancy unit, redundancy circuit, security power supply, insure EMC environment and muscle supervise came shield up as well. At physical layer upwards stratification plane, subsistent potential safety hazard mostly has be from aim at different kinds of consultative security menace, as well as with a view of illegality occupancy network resource or exhaust network resource. At application layer mostly has direct http, FTP/TFTP, telnet and through the medium of electronic mail blaze viral attack abroad as well to. toward these attack, be available to through the medium of allocate redundancy unit, redundancy circuit, security power supply, insure EMC environment and muscle supervise came shield up as well have got direct http, FTP/TFTP, telnet and through the medium of electronic mail blaze viral attack abroad as well to：past AAA, Tacacs+, radius grade security access control protocol, Control user versus Network access authority, phony catch aim at application layer be on the shield instrument include of application layer, mostly have got direct http, FTP/TFTP, telnet and through the medium of electronic mail blaze viral attack abroad as well to. toward these attack, be available to in such a way that route filtration, versus route informational encrypt and authentication, orient multicasting control, advance route rapidity of convergence withal alleviate route oscillation impact grade measure, filter , versus route informational encrypt and authentication, orient multicasting control, bump route rapidity of convergence withal relieve route oscillation impact grade measure, came muscle three-ply Network security up at up out concurrent with EMC environmental deteriorate grade. Versus such potential safety hazard, could through the medium of allocate redundancy unit, redundancy circuit, security power supply, insure EMC environment and muscle supervise came shield up as well. At physical layer upwards stratification plane, subsistent potential safety hazard mostly has be from aim at different kinds of consultative security menace, as well as with a view of illegality occupancy network resource or exhaust network resource 'potential safety hazard, such as both encapsulation attack, broadcast packet attack, MAC water-flooding, spanning tree attack grade second floor attack, as well as mendacious ICMP messenger, ICMP water-flooding, source address beguile , route oscillation grade aim at three-ply consultative attack into. At application layer, mostly have gotdirect http, FTP/TFTP, telnet and through the medium of electronic mail blaze viral attack abroad as well to. toward these attack, be available to' shield instrument include：past AAA, Tacacs+, radius grade security access control protocol, Control user versus Network access authority, phony catch aim at application layer 'attack；past MAC address and IP address binding, confine per port' MAC address use quantity, establish per port broadcast packet flow threshold, use repose port and VLAN 'ACL, foundation security user tunnel grade came kept away aim at two tiered attack；Past route filtration, versus route informational encrypt and authentication, orient multicasting control, bump route rapidity of convergence withal relieve route oscillation impact grade measure, came muscle three-ply Network security up at up. In order to compose construct safety net T-number, return ought adopt rest safety precautions.(one)incorporation AAA authentication, nat-pt, two/three-ply mpls VPN, repose ACL standard visit list and static state spread visit list, phony sliver fold attack grade came realize safety precautions in.(two)past route filtration, static route, policy route and route load-sharing came realize security route.(three)through the medium of sshv2(secure shell beta two edition), snmpv3(Simple Network Management Protocol beta three edition), ex., endue tenor visit security, line visit security with.(four)through the medium of Hierarchical Direct, custom-tailor privilege class supervise grade instrument came realize Network security management.(five)past sophisticated alarm, log and audit function realize network clock.(six)supply visit list and in such a way that sshv2(secure shell beta two edition), snmpv3(Simple Network Management Protocol beta three edition), endue tenor visit security, line visit security with.(four)past Hierarchical Direct, custom-tailor privilege class supervise grade instrument critical event deplete network resource of potential safety hazard, such as both encapsulation attack, broadcast packet attack, MAC water-flooding, spanning tree attack grade second floor attack, as well as mendacious ICMP messenger, ICMP water-flooding, source address beguile , route oscillation grade aim at three-ply consultative attack into erect security user tunnel grade came kept away aim at two tiered attack；in such a way that route filtration, versus route informational encrypt and authentication, orient multicasting control, advance route rapidity of convergence withal alleviate route oscillation impact grade measure, filter , versus route informational encrypt and authentication, orient multicasting control, bump route rapidity of convergence withal relieve route oscillation impact grade measure, camemuscle three-ply Network security up at up out for above physical layer upwards stratification plane, subsistent potential safety hazard mostly has be from aim at different kinds of consultative security menace, as well as with a view of illegality occupancy network resource or exhaust network resource' potential safety hazard, such as both encapsulation attack, broadcast packet attack, MAC water-flooding, spanning tree attack grade second floor attack, as well as mendacious ICMP messenger, ICMP water-flooding, source address beguile , route oscillation grade aim at three-ply consultative attack into. On the shield instrument include of application layer, mostly have got direct http, FTP/TFTP, telnet and through the medium of electronic mail blaze viral attack abroad as well to. toward these attack, be available to：past AAA, Tacacs+, radius grade security access control protocol, Control user versus Network access authority, phony catch aim at application layer 'attack；past MAC address and IP address binding, confine per port' MAC address use quantity, establish per port broadcast packet flow threshold, use repose port and VLAN 'ACL, foundation security user tunnel grade came kept away aim at two tiered attack；in such a way that route filtration, versus route informational encrypt and authentication, orient multicasting control, advance route rapidity of convergence withal alleviate route oscillation impact grade measure, filter , versus route informational encrypt and authentication, orient multicasting control, bump route rapidity of convergence withal relieve route oscillation impact grade measure, came muscle three-ply Network security up at up out. At the same time, in order to compose construct safety net T-number, return ought adopt rest safety precautions.(one)incorporation AAA authentication, nat-pt, two / three-ply mpls VPN, repose ACL standard visit list and static state spread visit list, phony sliver fold attack grade came realize safety precautions in.(two)through the medium of route filter , static route, policy route and route load-sharing came realize security route out.(three)through the medium of sshv2(secure shell beta two edition), snmpv3(Simple Network Management Protocol beta three edition), ex., endue tenor visit security, line visit security with.(four)through the medium of Hierarchical Direct, custom-tailor privilege class supervise grade instrument came realize Network security management.(five)through the medium of sophisticated alarm, log and audit function realize network clock' security.(six)endue visit list and critical event 'log, routing protocol event and error logging grade, for administration of networks personnel make fault analysis, orient and statistics with. put together station state, secure network.2.2IPSec basic structure analysesIt was avail authentication header (AH) and encapsulation melt security net lotus (esp.) came realize compact technique authentication and encrypt of that that of IPSec basic structure. And that be used to realize integrity of data, this be used to realize compact technique confidentiality. The transmission provision know clearly amphipods of the at the same time logarithm according to：Transmission mode and channel mode. Either at transmission mode suffer, IP head inscribe layer protocol head of compartment embed one new IPSec head (AH or esp.); at passage mode suffer, ask protective wholly IP fold big city encapsulation to the other one IP data packet liner, at the same time outwards and interior IP head of compartment embed one new IPSec head in for. IPSec head big city could at the same time withal transmission mode sum channel mode wrought.Both IPSec structure include a multitude of protocol and algorithm. The correlation as follows station notify of the protocols of compartment.IPSec structure includes a large number of agreements and algorithms. These agreements are between the mutual relations.Graph 1: Structural Drawing of IPSec protocols2.2.1Encapsulating Security Payload(ESP)Esp. supply integrity checking, authentication and encrypt, could set down as with IP data packet "super AH" it was addressable, therefore in the event of invocation encrypt, then too with concurrent selection know clearly integrity checkingand authentication that of, for it endue confidentiality combine preventable distort with. There into, ESP encrypt serve. For in the event of hardly use encryption, intruder with likely to fake fold withal launch cryptanalytic attack in up.There into, esp. message preamble field include：1.Security Parameters Index2.Sequence Number3.Padding Length4.Next Header2.2.2Authentication Header(AH)AH agreement supply data source authentication, data integrity and bob weight sow warrant, it be able to guard correspondence from doctor, therefore be incapable of prevent intercept, fit design to transmission not confidential data up to with IP correspondence.There into, AH message preamble field include：1.Next Header2.Length3.Security Parameters Index4.Sequence Number5.Authentication Data2.3IPSec protocol future vistaToday, information super highway ratio ever before big city bulk, but it too be confronted with EVER-LARGER security menace, hence versus muscle T-number suffer every last node hardware security sexual demand in course of on the increase up. It was one postmortem take problem into consideration that but, versus heap network equipments drawing Design whereas character, security. Us need for versus whence, when and on what occasion enrolls safety properties precede reconsider in as well to. For the moment have got three strain primary means could above network hardware equipments enroll security function in. First strain no more will principals means namely treat coherency with one coprocessor importance loaf networkprocessing unit or currency. It was eke one security get base up at the back of network processing unit done out that second strain means. It was should encryption circuit integration at maintain know clearly transfer speed and farthest dam know clearly chip area back with network processing unit analogous chip suffer, thereby at network processing unit suffer join know clearly security function at the same time that third strain means. Wherefore, high integration density should inspire IPSec ‘develop with rush.3Follow on IP agreement-IPv63.1Change of IPv6Change incarnate infra five importance aspect among IPv6：(1)spread address(2)predigestion head format(3)Tone toward spread and option 'support up(4)Stream tag(5)Both identity authentication and secrecy1. Spread addressThe clean culture address and design to appoint by one or more of mainframe intercept 'multicasting address base these immovability of the address structure bed cleaning in feeding stage of an instars know clearly do with three two bit address space reach up to know clearly 1 two 8-bit besides, return versus IP mainframe likely to obtain different type address did know clearly some adjust of the IPv6. IPv6 suffer Cancel know clearly broadcast address whereas instead arbitrary point sow address. IPv6 useful appoint one network interface.2. Predilection head formatBoth IPv6 suffer include total length for four 0 syllabic eight words segment (there into two is source address and subsection can but by source node proceed among IPv6 suffer include total length for four 0 syllabic eight words segment (there into two is source address differ with destination address). it and IPv6 toe 'consist,IPv6 suffer contain at least 1 two loaf differ field, even length at without option bear date two 0 byte, therefore at contain option hour approve reach six 0 byte. IPv6 by the exercise of F-format' toe combine be reduce by needs check sum get field the quantity of, these should so as to channel selection 'efficiency still high done. Toe' predilection so as to IP' certain operate mode occur know clearly change. On the one hand, possession toe length unify, hence had no further use for toe length field into. Furthermore, through the medium of make amendment of fold sectional rule could at toe suffer chip some field away in. IPv6 have no option bear date two 0 byte, therefore at contain option hour approve reach six 0 byte reduce need for test get field the quantity of, these should so as to channel selection' efficiency still high done out to by：Said fold station middle Router be incapable of reenter row any subsection in. in conclusion, take out IP head check sum shan't impact reliability, and these above all for head check sum should by still higher level agreement (UDP and TCP) preside.3. Tone toward spread and option 'support upThe WRAPT demand came in for made an exception of, for not that possession chain circuit big city suffice get those long transmission cell, even Router expect as best as one can keep out of do with they forward to mishandled 'network upper of done of the along with it dispose hop-by-hop option of the in IPv6 suffer could at IP cephalic tail join option, therewith differ, IPv6 suffer do with option plus at separate spread head suffer. Through the medium of such means, option heads none but in case of need but need for test dispose out to. IPv6 manse subsection only take place on source node upper, hence need for take subsection spread cephalic node none but source node and destination node into consideration to. Source node preside subsection combine begin spread head, said spread head should lay in IPv6 head and next higher level agreement head of compartment. destination node take over said fold combine use spread head proceed reshipment in. possession intermediate node big city could in security overlook said subsection spread head, in this way with bump know clearly fold channel selection' efficiency in up. Other kind of selection scheme suffer, hop-by-hop option spread head oblige clad path last every last node big city get said head field done to. instance, each Router must at dispose IPv6 toe enroll option,。
计算机专业毕业外文翻译原文+封面+中文翻译
本科毕业论文外文翻译外文译文题目（中文）：具体数学：汉诺塔问题学院: 计算机科学与技术专业: 计算机科学与技术学号:学生姓名:指导教师:日期: 二○一二年六月1 Recurrent ProblemsTHIS CHAPTER EXPLORES three sample problems that give a feel for what’s to c ome. They have two traits in common: They’ve all been investigated repeatedly by mathe maticians; and their solutions all use the idea of recurrence, in which the solution to eac h problem depends on the solutions to smaller instances of the same problem.1.1 THE TOWER OF HANOILet’s look first at a neat little puzzle called the Tower of Hanoi,invented by the Fr ench mathematician Edouard Lucas in 1883. We are given a tower of eight disks, initiall y stacked in decreasing size on one of three pegs:The objective is to transfer the entire tower to one of the other pegs, movingonly one disk at a time and never moving a larger one onto a smaller.Lucas furnished his toy with a romantic legend about a much larger Tower of Brah ma, which supposedly has 64 disks of pure gold resting on three diamond needles. At th e beginning of time, he said, God placed these golden disks on the first needle and orda ined that a group of priests should transfer them to the third, according to the rules abov e. The priests reportedly work day and night at their task. When they finish, the Tower will crumble and the world will end.It's not immediately obvious that the puzzle has a solution, but a little thought (or h aving seen the problem before) convinces us that it does. Now the question arises：What' s the best we can do？That is，how many moves are necessary and suff i cient to perfor m the task?The best way to tackle a question like this is to generalize it a bit. The Tower of Brahma has 64 disks and the Tower of Hanoi has 8；let's consider what happens if ther e are TL disks.One advantage of this generalization is that we can scale the problem down even m ore. In fact, we'll see repeatedly in this book that it's advantageous to LOOK AT SMAL L CASES first. It's easy to see how to transfer a tower that contains only one or two di sks. And a small amount of experimentation shows how to transfer a tower of three.The next step in solving the problem is to introduce appropriate notation：NAME ANO CONQUER. Let's say that T n is the minimum number of moves that will t ransfer n disks from one peg to another under Lucas's rules. Then T1is obviously 1 , an d T2= 3.We can also get another piece of data for free, by considering the smallest case of all：Clearly T0= 0，because no moves at all are needed to transfer a tower of n = 0 disks! Smart mathematicians are not ashamed to think small,because general patterns are easier to perceive when the extreme cases are well understood(even when they are trivial).But now let's change our perspective and try to think big；how can we transfer a la rge tower? Experiments with three disks show that the winning idea is to transfer the top two disks to the middle peg, then move the third, then bring the other two onto it. Thi s gives us a clue for transferring n disks in general：We first transfer the n−1 smallest t o a different peg (requiring T n-1moves), then move the largest (requiring one move), and finally transfer the n−1 smallest back onto the largest (req uiring another T n-1moves). Th us we can transfer n disks (for n > 0)in at most 2T n-1+1 moves：T n≤2T n—1+1，for n > 0.This formula uses '≤' instead of '=' because our construction proves only that 2T n—1+1 mo ves suffice; we haven't shown that 2T n—1+1 moves are necessary. A clever person might be able to think of a shortcut.But is there a better way? Actually no. At some point we must move the largest d isk. When we do, the n−1 smallest must be on a single peg, and it has taken at least T moves to put them there. We might move the largest disk more than once, if we're n n−1ot too alert. But after moving the largest disk for the last time, we must trans fr the n−1 smallest disks (which must again be on a single peg)back onto the largest；this too re quires T n−1moves. HenceT n≥ 2T n—1+1，for n > 0.These two inequalities, together with the trivial solution for n = 0, yieldT0=0；T n=2T n—1+1 , for n > 0. (1.1)(Notice that these formulas are consistent with the known values T1= 1 and T2= 3. Our experience with small cases has not only helped us to discover a general formula, it has also provided a convenient way to check that we haven't made a foolish error. Such che cks will be especially valuable when we get into more complicated maneuvers in later ch apters.)A set of equalities like (1.1) is called a recurrence (a. k. a. recurrence relation or r ecursion relation). It gives a boundary value and an equation for the general value in ter ms of earlier ones. Sometimes we refer to the general equation alone as a recurrence, alt hough technically it needs a boundary value to be complete.The recurrence allows us to compute T n for any n we like. But nobody really like to co m pute fro m a recurrence，when n is large；it takes too long. The recurrence only gives indirect, "local" information. A solution to the recurrence would make us much h appier. That is, we'd like a nice, neat, "closed form" for Tn that lets us compute it quic kly，even for large n. With a closed form, we can understand what T n really is.So how do we solve a recurrence? One way is to guess the correct solution,then to prove that our guess is correct. And our best hope for guessing the solution is t o look (again) at small cases. So we compute, successively,T3= 2×3+1= 7; T4= 2×7+1= 15; T5= 2×15+1= 31; T6= 2×31+1= 63.Aha! It certainly looks as ifTn = 2n−1，for n≥0. (1.2)At least this works for n≤6.Mathematical induction is a general way to prove that some statement aboutthe integer n is true for all n≥n0. First we prove the statement when n has its smallest v alue,no; this is called the basis. Then we prove the statement for n > n0,assuming that it has already been proved for all values between n0and n−1, inclusive; this is called th e induction. Such a proof gives infinitely many results with only a finite amount of wo rk.Recurrences are ideally set up for mathematical induction. In our case, for exampl e，(1.2) follows easily from (1.1)：The basis is trivial，since T0 = 20−1= 0.And the indu ction follows for n > 0 if we assume that (1.2) holds when n is replaced by n−1：T n= 2T n+1= 2(2n−1−1)+1=2n−1.Hence (1.2) holds for n as well. Good! Our quest for T n has ended successfully.Of course the priests' task hasn't ended；they're still dutifully moving disks,and wil l be for a while, because for n = 64 there are 264−1 moves (about 18 quintillion). Even at the impossible rate of one move per microsecond, they will need more than 5000 cent uries to transfer the Tower of Brahma. Lucas's original puzzle is a bit more practical, It requires 28−1 = 255 moves, which takes about four minutes for the quick of hand.The Tower of Hanoi recurrence is typical of many that arise in applications of all kinds. In finding a closed-form expression for some quantity of interest like T n we go t hrough three stages：1 Look at small cases. This gives us insight into the problem and helps us in stages2 and 3.2 Find and prove a mathematical expression for the quantity of interest.For the Tower of Hanoi, this is the recurrence (1.1) that allows us, given the inc lination，to compute T n for any n.3 Find and prove a closed form for our mathematical expression.For the Tower of Hanoi, this is the recurrence solution (1.2).The third stage is the one we will concentrate on throughout this book. In fact, we'll fre quently skip stages I and 2 entirely, because a mathematical expression will be given tous as a starting point. But even then, we'll be getting into subproblems whose solutions will take us through all three stages.Our analysis of the Tower of Hanoi led to the correct answer, but it r equired an“i nductive leap”；we relied on a lucky guess about the answer. One of the main objectives of this book is to explain how a person can solve recurrences without being clairvoyant. For example, we'll see that recurrence (1.1) can be simplified by adding 1 to both sides of the equations：T0+ 1= 1;T n + 1= 2T n-1+ 2, for n >0.Now if we let U n= T n+1，we haveU0 =1；U n= 2U n-1，for n > 0. (1.3)It doesn't take genius to discover that the solution to this recurrence is just U n= 2n；he nce T n= 2n −1. Even a computer could discover this.Concrete MathematicsR. L. Graham, D. E. Knuth, O. Patashnik《Concrete Mathematics》，1.1 ，The Tower Of HanoiR. L. Graham, D. E. Knuth, O. PatashnikSixth printing, Printed in the United States of America1989 by Addison-Wesley Publishing Company，Reference 1-4 pages具体数学R.L.格雷厄姆，D.E.克努特，O.帕塔希尼克《具体数学》，1.1，汉诺塔R.L.格雷厄姆，D.E.克努特，O.帕塔希尼克第一版第六次印刷于美国，韦斯利出版公司，1989年，引用1-4页1 递归问题本章将通过对三个样本问题的分析来探讨递归的思想。
计算机专业毕业设计外文翻译
外文翻译Birth of the NetThe Internet has had a relatively brief, but explosive history so far. It grew out of an experiment begun in the 1960's by the U.S. Department of Defense. The DoD wanted to create a computer network that would continue to function in the event of a disaster, such as a nuclear war. If part of the network were damaged or destroyed, the rest of the system still had to work. That network was ARPANET, which linked U.S. scientific and academic researchers. It was the forerunner of today's Internet.In 1985, the National Science Foundation (NSF) created NSFNET, a series of networks for research and education communication. Based on ARPANET protocols, the NSFNET created a national backbone service, provided free to any U.S. research and educational institution. At the same time, regional networks were created to link individual institutions with the national backbone service.NSFNET grew rapidly as people discovered its potential, and as new software applications were created to make access easier. Corporations such as Sprint and MCI began to build their own networks, which they linked to NSFNET. As commercial firms and other regional network providers have taken over the operation of the major Internet arteries, NSF has withdrawn from the backbone business.NSF also coordinated a service called InterNIC, which registered all addresses on the Internet so that data could be routed to the right system. This service has now been taken over by Network Solutions, Inc., in cooperation with NSF.How the Web WorksThe World Wide Web, the graphical portion of the Internet, is the most popular part of the Internet by far. Once you spend time on the Web,you will begin to feel like there is no limit to what you can discover. The Web allows rich and diverse communication by displaying text, graphics, animation, photos, sound and video.So just what is this miraculous creation? The Web physically consists of your personal computer, web browser software, a connection to an Internet service provider, computers called servers that host digital data and routers and switches to direct the flow of information.The Web is known as a client-server system. Your computer is the client; the remote computers that store electronic files are the servers. Here's how it works:Let's say you want to pay a visit to the the Louvre museum website. First you enter the address or URL of the website in your web browser (more about this shortly). Then your browser requests the web page from the web server that hosts the Louvre's site. The Louvre's server sends the data over the Internet to your computer. Your web browser interprets the data, displaying it on your computer screen.The Louvre's website also has links to the sites of other museums, such as the Vatican Museum. When you click your mouse on a link, you access the web server for the Vatican Museum.The "glue" that holds the Web together is called hypertext and hyperlinks. This feature allow electronic files on the Web to be linked so you can easily jump between them. On the Web, you navigate through pages of information based on what interests you at that particular moment, commonly known as browsing or surfing the Net.To access the Web you need web browser software, such as Netscape Navigator or Microsoft Internet Explorer. How does your web browser distinguish between web pages and other files on the Internet? Web pages are written in a computer language called Hypertext Markup Language or HTML.Some Web HistoryThe World Wide Web (WWW) was originally developed in 1990 at CERN, the European Laboratory for Particle Physics. It is now managed by The World Wide Web Consortium, also known as the World Wide Web Initiative.The WWW Consortium is funded by a large number of corporate members, including AT&T, Adobe Systems, Inc., Microsoft Corporation and Sun Microsystems, Inc. Its purpose is to promote the growth of the Web by developing technical specifications and reference software that will be freely available to everyone. The Consortium is run by MIT with INRIA (The French National Institute for Research in Computer Science) acting as European host, in collaboration with CERN.The National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign, was instrumental in the development of early graphical software utilizing the World Wide Web features created by CERN. NCSA focuses on improving the productivity of researchers by providing software for scientific modeling, analysis, and visualization. The World Wide Web was an obvious way to fulfill that mission. NCSA Mosaic, one of the earliest web browsers, was distributed free to the public. It led directly to the phenomenal growth of the World Wide Web.Understanding Web AddressesYou can think of the World Wide Web as a network of electronic files stored on computers all around the world. Hypertext links these resources together. Uniform Resource Locators or URLs are the addresses used to locate thesefiles. The information contained in a URL gives you the ability to jump from one web page to another with just a click of your mouse. When you type a URL into your browser or click on a hypertext link, your browser is sending a request to a remote computer to download a file.What does a typical URL look like? Here are some examples:/The home page for study english.ftp:///pub/A directory of files at MIT* available for downloading.news:rec.gardens.rosesA newsgroup on rose gardening.The first part of a URL (before the two slashes* tells you the type of resource or method of access at that address. For example:∙http - a hypertext document or directory∙gopher - a gopher document or menu∙ftp - a file available for downloading or a directory of such files∙news - a newsgroup∙telnet - a computer system that you can log into over the Internet∙WAIS* - a database or document in a Wide Area Information Search database∙file - a file located on a local drive (your hard drive)The second part is typically the address of the computer where the data or service is located. Additional parts may specify the names of files, the port to connect to, or the text to search for in a database.You can enter the URL of a site by typing it into the Location bar of your web browser, just under the toolbar.Most browsers record URLs that you want to use again, by adding them to a special menu. In Netscape Navigator, it's called Bookmarks. In Microsoft Explorer, it's called Favorites. Once you add a URL to your list, you can return to that web page simply by clicking on the name in your list, instead of retyping the entire URL.Most of the URLs you will be using start with http which stands for Hypertext Transfer Protocol*. http is the method by which HTML files are transferred over the Web. Here are some other important things to know about URLs:∙ A URL usually has no spaces.∙ A URL always uses forward slashes (//).If you enter a URL incorrectly, your browser will not be able to locate the site or resource you want. Should you get an error message or the wrong site, make sure you typed the address correctly.You can find the URL behind any link by passing your mouse cursor over the link. The pointer will turn into a hand and the URL will appear in the browser's status ba r, usually located at the bottom of your screen.Domain NamesWhen you think of the Internet, you probably think of ".com." Just what do those three letters at the end of a World Wide Web address mean?Every computer that hosts data on the Internet has a unique numerical address. For example, the numerical address for the White House is198.137.240.100. But since few people want to remember long strings of numbers, the Domain Name System (DNS)* was developed. DNS, a critical part of the Internet's technical infrastructure*, correlates* a numerical address to a word. To access the White House website, you could type its number into the address box of your web browser. But most people prefer to use "." In this case, the domain name is . In general, the three-letter domain name suffix* is known as a generictop-level domai n and describes the type of organization. In the last few years, the lines have somewhat blurred* between these categories..com - business (commercial).edu - educational.org - non-profit.mil - military.net - network provider.gov - governmentA domain name always has two or more parts separated by dots and typically consists of some form of an organization's name and the three-letter suffix. For example, the domain name for IBM is ""; the United Nations is "."If a domain name is available, and provided it does not infringe* on an existing trademark, anyone can register the name for $35 a year through Network Solutions, Inc., which is authorized to register .com, .net and .org domains. You can use the box below to see if a name is a available. Don't be surprised ifthe .com name you want is already taken, however. Of the over 8 million domain names, 85% are .com domains.ICANN, the Internet Corporation for Assigned Names and Numbers, manages the Domain Name System. As of this writing, there are plans to add additional top-level domains, such as .web and .store. When that will actually happen is anybody's guess.To check for, or register a domain name, type it into the search box.It should take this form: In addition to the generic top-level domains, 244 national top-level domains were established for countries and territories*, for example:.au - Australia.ca - Canada.fr - France.de - Germany.uk - United KingdomFor US $275 per name, you can also register an international domain name with Net Names. Be aware that some countries have restrictions for registering names.If you plan to register your own domain name, whether it's a .com or not, keep these tips in mind:The shorter the name, the better. (But it should reflect your family name, interest or business.)The name should be easy to remember.It should be easy to type without making mistakes.Remember, the Internet is global. Ideally, a domain name will "read" in a language other than English.Telephone lines were designed to carry the human voice, not electronic data from a computer. Modems were invented to convert digital computer signals into a form that allows them to travel over the phone lines. Those are the scratchy sounds you hear from a modem's speaker. A modem on theother end of the line can understand it and convert the sounds back into digital information that the computer can understand. By the way, the word modem stands for MOdulator/DEModulator.Buying and using a modem used to be relatively easy. Not too long ago, almost all modems transferred data at a rate of 2400 Bps (bits per second). Today, modems not only run faster, they are also loaded with features like error control and data compression. So, in addition to converting and interpreting signals, modems also act like traffic cops, monitoring and regulating the flow of information. That way, one computer doesn't send information until the receiving computer is ready for it. Each of these features, modulation, error control, and data compression, requires a separate kind of protocol and that's what some of those terms you see like V.32, V.32bis, V.42bis and MNP5 refer to.If your computer didn't come with an internal modem, consider buying an external one, because it is much easier to install and operate. For example, when your modem gets stuck (not an unusual occurrence), you need to turn it off and on to get it working properly. With an internal modem, that means restarting your computer--a waste of time. With an external modem it's as easy as flipping a switch.Here's a tip for you: in most areas, if you have Call Waiting, you can disable it by inserting *70 in front of the number you dial to connect to the Internet (or any online service). This will prevent an incoming call from accidentally kicking you off the line.This table illustrates the relative difference in data transmission speeds for different types of files. A modem's speed is measured in bits per second (bps). A 14.4 modem sends data at 14,400 bits per second. A 28.8 modem is twice as fast, sending and receiving data at a rate of 28,800 bits per second.Until nearly the end of 1995, the conventional wisdom was that 28.8 Kbps was about the fastest speed you could squeeze out of a regular copper telephoneline. Today, you can buy 33.6 Kbps modems, and modems that are capable of 56 Kbps. The key question for you, is knowing what speed modems your Internet service provider (ISP) has. If your ISP has only 28.8 Kbps modems on its end of the line, you could have the fastest modem in the world, and only be able to connect at 28.8 Kbps. Before you invest in a 33.6 Kbps or a 56 Kbps modem, make sure your ISP supports them.Speed It UpThere are faster ways to transmit data by using an ISDN or leased line. In many parts of the U.S., phone companies are offering home ISDN at less than $30 a month. ISDN requires a so-called ISDN adapter instead of a modem, and a phone line with a special connection that allows it to send and receive digital signals. You have to arrange with your phone company to have this equipment installed. For more about ISDN, visit Dan Kegel's ISDN Page.An ISDN line has a data transfer rate of between 57,600 bits per second and 128,000 bits per second, which is at least double the rate of a 28.8 Kbps modem. Leased lines come in two configurations: T1 and T3. A T1 line offers a data transfer rate of 1.54 million bits per second. Unlike ISDN, a T-1 line is a dedicated connection, meaning that it is permanently connected to the Internet. This is useful for web servers or other computers that need to be connected to the Internet all the time. It is possible to lease only a portion of a T-1 line using one of two systems: fractional T-1 or Frame Relay. You can lease them in blocks ranging from 128 Kbps to 1.5 Mbps. The differences are not worth going into in detail, but fractional T-1 will be more expensive at the slower available speeds and Frame Relay will be slightly more expensive as you approach the full T-1 speed of 1.5 Mbps. A T-3 line is significantly faster, at 45 million bits per second. The backbone of the Internet consists of T-3 lines. Leased lines are very expensive and are generally only used by companies whose business is built around the Internet or need to transfer massiveamounts of data. ISDN, on the other hand, is available in some cities for a very reasonable price. Not all phone companies offer residential ISDN service. Check with your local phone company for availability in your area.Cable ModemsA relatively new development is a device that provides high-speed Internet access via a cable TV network. With speeds of up to 36 Mbps, cable modems can download data in seconds that might take fifty times longer with a dial-up connection. Because it works with your TV cable, it doesn't tie up a telephone line. Best of all, it's always on, so there is no need to connect--no more busy signals! This service is now available in some cities in the United States and Europe.The download times in the table above are relative and are meant to give you a general idea of how long it would take to download different sized files at different connection speeds, under the best of circumstances. Many things can interfere with the speed of your file transfer. These can range from excessive line noise on your telephone line and the speed of the web server from which you are downloading files, to the number of other people who are simultaneously trying to access the same file or other files in the same directory.DSLDSL (Digital Subscriber Line) is another high-speed technology that is becoming increasingly popular. DSL lines are always connected to the Internet, so you don't need to dial-up. Typically, data can be transferred at rates up to 1.544 Mbps downstream and about 128 Kbps upstream over ordinary telephone lines. Since a DSL line carries both voice and data, you don't have to install another phone line. You can use your existing line to establish DSLservice, provided service is available in your area and you are within the specified distance from the telephone company's central switching office.DSL service requires a special modem. Prices for equipment, DSL installation and monthly service can vary considerably, so check with your local phone company and Internet service provider. The good news is that prices are coming down as competition heats up.Anatomy of a Web PageA web page is an electronic document written in a computer language called HTML, short for Hypertext Markup Language. Each web page has a unique address, called a URL* or Uniform Resource Locator, which identifies its location on the network.A website has one or more related web pages, depending on how it's designed. Web pages on a site are linked together through a system of hyperlinks* , enabling you to jump between them by clicking on a link. On the Web, you navigate through pages of information according to your interests.Home Sweet Home PageWhen you browse the World Wide Web you'll see the term home page often. Think of a home page as the starting point of a website. Like the table of contents of a book or magazine, the home page usually provides an overview of what you'll find at the website. A site can have one page, many pages or a few long ones, depending on how it's designed. If there isn't a lot of information, the home page may be the only page. But usually you will find at least a few other pages.Web pages vary wildly in design and content, but most use a traditional magazine format. At the top of the page is a masthead* or banner graphic*, then a list of items, such as articles, often with a brief description. The items in the list usually link to other pages on the website, or to other sites. Sometimes these links are highlighted* words in the body of the text, or are arranged in a list, like an index. They can also be a combination* of both. A web page can also have images that link to other content.How can you tell which text are links? Text links appear in a different color from the rest of the text--typically in blue and underlined. When you move yourcursor over a text link or over a graphic link, it will change from an arrow to a hand. The hypertext words often hint* at what you will link to.When you return to a page with a link you've already visited, the hypertext words will often be in a different color, so you know you've already been there. But you can certainly go there again. Don't be surprised though, if the next time you visit a site, the page looks different and the information has changed. The Web is a dynamic* medium. To encourage visitors to return to a site, some web publishers change pages often. That's what makes browsing the Web so excitingA Home (Page) of Your OwnIn the 60s, people asked about your astrological* sign. In the 90s, they want to know your URL. These days, having a web address is almost as important as a street address. Your website is an electronic meeting place for your family, friends and potentially*, millions of people around the world. Building your digital domain can be easier than you may think. Best of all, you may not have to spend a cent. The Web brims with all kinds of free services, from tools to help you build your site, to free graphics, animation and site hosting. All it takes is some time and creativity.Think of your home page as the starting point of your website. Like the table of contents of a book or magazine, the home page is the front door. Your site can have one or more pages, depending on how you design it. If there isn't a lot of information just yet, your site will most likely have only a home page. But the site is sure to grow over time.While web pages vary dramatically* in their design and content, most use a traditional magazine layout. At the top of the page is a banner graphic. Next comes a greeting and a short description of the site. Pictures, text, and links to other websites follow.If the site has more than one page, there's typically a list of items--similar to an index--often with a brief description. The items in the list link to other pages on the website. Sometimes these links are highlighted words in the body of the text. It can also be a combination of both. Additionally, a web page may have images that link to other content.Before you start building your site, do some planning. Think about whom the site is for and what you want to say. Next, gather up the material that you wantto put on the site: write the copy, scan the photos, design or find the graphics. Draw a rough layout on a sheet of paper.While there are no rules you have to follow, there are a few things to keep in mind:∙Start simply. If you are too ambitious at the beginning, you may never get the site off the ground. You can always add to your site.∙Less is better. Most people don't like to read a lot of text online. Break it into small chunks.∙Use restraint. Although you can use wild colors and images for the background of your pages, make sure your visitors will be able to readthe text easily.∙Smaller is better. Most people connect to the Internet with a modem.Since it can take a long time to download large image files, keep the file sizes small.∙Have the rights. Don't put any material on your site unless you are sure you can do it legally. Read Learn the Net's copyright article for moreabout this.Stake Your ClaimNow it's time to roll up your sleeves and start building. Learn the Net Communities provides tools to help you build your site, free web hosting, and a community of other homesteaders.Your Internet service provider may include free web hosting services with an account, one alternative to consider.Decoding Error MessagesAs you surf the Net, you will undoubtedly find that at times you can't access certain websites. Why, you make wonder? Error messages attempt to explain the reason. Unfortunately, these cryptic* messages baffle* most people.We've deciphered* the most common ones you may encounter.400 - Bad RequestProblem: There's something wrong with the address you entered. You may not be authorized* to access the web page, or maybe it no longer exists.Solution: Check the address carefully, especially if the address is long. Make sure that the slashes are correct (they should be forward slashes) and that all the names are properly spelled. Web addresses are case sensitive, socheck that the names are capitalized in your entry as they are in the original reference to the website.401 - UnauthorizedProblem: You can't access a website, because you're not on the guest list, your password is invalid or you have entered your password incorrectly.Solution: If you think you have authorization, try typing your password again. Remember that passwords are case sensitive.403 - ForbiddenProblem: Essentially the same as a 401.Solution: Try entering your password again or move on to another site.404 - Not FoundProblem: Either the web page no longer exists on the server or it is nowhere to be found.Solution: Check the address carefully and try entering it again. You might also see if the site has a search engine and if so, use it to hunt for the document. (It's not uncommon for pages to change their addresses when a website is redesigned.) To get to the home page of the site, delete everything after the domain name and hit the Enter or Return key.503 - Service unavailableProblem: Your Internet service provider (ISP) or your company's Internet connection may be down.Solution: Take a stretch, wait a few minutes and try again. If you still have no luck, phone your ISP or system administrator.Bad file requestProblem: Your web browser may not be able to decipher the online form you want to access. There may also be a technical error in the form.Solution: Consider sending a message to the site's webmaster, providing any technical information you can, such as the browser and version you use.Connection refused by hostProblem: You don't have permission to access the page or your password is incorrect.Solution: Try typing your password again if you think you should have access.Failed DNS lookupProblem: DNS stands for the Domain Name System, which is the system that looks up the name of a website, finds a corresponding number (similar to a phone number), then directs your request to the appropriate web server on theInternet. When the lookup fails, the host server can't be located.Solution: Try clicking on the Reload or Refresh button on your browser toolbar. If this doesn't work, check the address and enter it again. If all else fails, try again later.File contains no dataProblem: The site has no web pages on it.Solution: Check the address and enter it again. If you get the same error message, try again later.Host unavailableProblem: The web server is down.Solution: Try clicking on the Reload or Refresh button. If this doesn't work, try again later.Host unknownProblem: The web server is down, the site may have moved, or you've been disconnected from the Net.Solution: Try clicking on the Reload or Refresh button and check to see that you are still online. If this fails, try using a search engine to find the site. It may have a new address.Network connection refused by the serverProblem: The web server is busy.Solution: Try again in a while.Unable to locate hostProblem: The web server is down or you've been disconnected from the Net.Solution: Try clicking on the Reload or Refresh button and check to see that you are still online.Unable to locate serverProblem: The web server is out-of-business or you may have entered the address incorrectly.Solution: Check the address and try typing it again.Web BrowsersA web browser is the software program you use to access the World Wide Web, the graphical portion of the Internet. The first browser, called NCSA Mosaic, was developed at the National Center for Supercomputing Applications in the early '90s. The easy-to-use point-and-click interface*helped popularize the Web, although few then could imagine the explosive growth that would soon occur.Although many different browsers are available, Microsoft Internet Explorer* and Netscape Navigator* are the two most popular ones. Netscape and Microsoft have put so much money into their browsers that the competition can't keep up. The pitched battle* between the two companies to dominate* the market has lead to continual improvements to the software. Version 4.0 and later releases of either browser are excellent choices. (By the way, both are based on NCSA Mosaic.) You can download Explorer and Navigator for free from each company's website. If you have one browser already, you can test out the other. Also note that there are slight differences between the Windows and MacIntosh* versions.You can surf to your heart's content, but it's easy to get lost in this electronic web. That's where your browser can really help. Browsers come loaded with all sorts of handy features. Fortunately, you can learn the basics in just a few minutes, then take the time to explore the advanced functions.Both Explorer and Navigator have more similarities than differences, so we'll primarily cover those. For the most up-to-date information about the browsers, and a complete tutorial, check the online handbook under the Help menu or go to the websites of the respective* software companies.Browser AnatomyWhen you first launch your web browser, usually by double-clicking on the icon on your desktop, a predefined web page, your home page, will appear. With Netscape Navigator for instance, you will be taken to Netscape's NetCenter.The Toolbar （工具栏）The row of buttons at the top of your web browser, known as the toolbar, helps you travel through the web of possibilities, even keeping track ofwhere you've been. Since the toolbars for Navigator and Explorer differ slightly, we'll first describe what the buttons in common do:o The Back button returns you the previous page you've visited.o Use the Forward button to return to the page you just came from.o Home takes you to whichever home page you've chosen. (If you haven't selected one, it will return you to the default home page,usually the Microsoft or Netscape website.)。
计算机科学与技术 外文翻译 英文文献 中英对照
附件1：外文资料翻译译文大容量存储器由于计算机主存储器的易失性和容量的限制, 大多数的计算机都有附加的称为大容量存储系统的存储设备, 包括有磁盘、CD 和磁带。
相对于主存储器，大的容量储存系统的优点是易失性小，容量大，低成本, 并且在许多情况下, 为了归档的需要可以把储存介质从计算机上移开。
术语联机和脱机通常分别用于描述连接于和没有连接于计算机的设备。
联机意味着，设备或信息已经与计算机连接，计算机不需要人的干预，脱机意味着设备或信息与机器相连前需要人的干预，或许需要将这个设备接通电源，或许包含有该信息的介质需要插到某机械装置里。
大量储存器系统的主要缺点是他们典型地需要机械的运动因此需要较多的时间，因为主存储器的所有工作都由电子器件实现。
1. 磁盘今天，我们使用得最多的一种大量存储器是磁盘，在那里有薄的可以旋转的盘片，盘片上有磁介质以储存数据。
盘片的上面和（或）下面安装有读/写磁头，当盘片旋转时，每个磁头都遍历一圈，它被叫作磁道，围绕着磁盘的上下两个表面。
通过重新定位的读/写磁头，不同的同心圆磁道可以被访问。
通常，一个磁盘存储系统由若干个安装在同一根轴上的盘片组成，盘片之间有足够的距离，使得磁头可以在盘片之间滑动。
在一个磁盘中，所有的磁头是一起移动的。
因此，当磁头移动到新的位置时，新的一组磁道可以存取了。
每一组磁道称为一个柱面。
因为一个磁道能包含的信息可能比我们一次操作所需要得多，所以每个磁道划分成若干个弧区，称为扇区，记录在每个扇区上的信息是连续的二进制位串。
传统的磁盘上每个磁道分为同样数目的扇区，而每个扇区也包含同样数目的二进制位。
（所以，盘片中心的储存的二进制位的密度要比靠近盘片边缘的大）。
因此，一个磁盘存储器系统有许多个别的磁区, 每个扇区都可以作为独立的二进制位串存取，盘片表面上的磁道数目和每个磁道上的扇区数目对于不同的磁盘系统可能都不相同。
磁区大小一般是不超过几个KB; 512 个字节或1024 个字节。
计算机科学与技术毕业设计(论文)外文翻译
本科毕业设计(论文) 外文翻译（附外文原文）系 ( 院 )：信息科学与工程学院课题名称：学生信息管理系统专业(方向)：计算机科学与技术（应用）7.1 Enter ActionMappingsThe Model 2 architecture (see chapter 1) encourages us to use servlets and Java- Server Pages in the same application. Under Model 2, we start by calling a servlet.The servlet handles the business logic and directs control to the appropriate pageto complete the response.The web application deployment descriptor (web.xml) lets us map a URL patternto a servlet. This can be a general pattern, like *.do, or a specific path, like saveRecord.do.Some applications implement Model 2 by mapping a servlet to each business operation. This approach works, but many applications involve dozens or hundredsof business operations. Since servlets are multithreaded, instantiating so manyservlets is not the best use of server resources. Servlets are designed to handle anynumber of parallel requests. There is no performance benefit in simply creatingmore and more servlets.The servlet’s primary job is to interact with the container and HTTP. Handlinga business operation is something that a servlet could delegate to another component. Struts does this by having the ActionServlet delegate the business operationto an object. Using a servlet to receive a request and route it to a handler is knownas the Front Controller pattern [Go3].Of course, simply delegating the business operation to another componentdoes not solve the problem of mapping URIs [W3C, URI] to business operations.Our only way of communicating with a web browser is through HTTP requests and URIs. Arranging for a URI to trigger a business operation is an essential part of developing a web application.Meanwhile, in practice many business operations are handled in similar ways.Since Java is multithreaded, we could get better use of our server resources if wecould use the same Action object to handle similar operations. But for this towork, we might need to pass the object a set of configuration parameters to usewith each operation.So what’s the bottom line? To implement Model 2 in an efficient and flexibleway, we need to:Enter ActionMappings 195♉ Route requests for our business operations to a single servlet♉ Determine which business operation is related to the request♉ Load a multithreaded helper object to handle the business operation♉ Pass the helper object the specifics of each request along with any configuration detail used by this operationThis is where ActionMappings come in.7.1.1 The ActionMapping beanAn ActionMapping (org.apache.struts.action.ActionMapping) describes howthe framework handles each discrete business operation (or action). In Struts,each ActionMapping is associated with a specific URI through its path property. When a request comes in, the ActionServlet uses the path property to select the corresponding ActionMapping. The set of ActionMapping objects is kept in an ActionMappings collection (org.apache.struts.action.ActionMappings). Originally, the ActionMapping object was used to extend the Action objectrather than the Action class. When used with an Action, a mapping gives a specific Action object additional responsibilities and new functionality. So, it was essentiallyan Action decorator [Go4]. Along the way, the ActionMapping evolved into anobject in its own right and can be used with or without an Action.DEFINITION The intent of the decorator pattern is to attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassingfor extending functionality [Go4].The ActionMappings are usually created through the Struts configuration file.For more about this file, see chapter 4.7.1.2 The ActionMappings catalogThe ActionMappings catalog the business logic available to a Struts application.When a request comes in, the servlet finds its entry in the ActionMappings catalogand pulls the corresponding bean.The ActionServlet uses the ActionMapping bean to decide what to do next. Itmay need to forward control off to another resource. Or it may need to populateand validate an ActionForm bean. At some point, it may have to pass control to an Action object, and when the Action returns, it may have to look up an Action-Forward associated with this mapping.196 CHAPTER 7Designing with ActionMappingsThe ActionMapping works like a routing slip for the servlet. Depending onhow the mapping is filled out, the request could go just about anywhere.The ActionMappings represent the core design of a Struts application. If youwant to figure out how a Struts application works, start with the ActionMappings. Ifyou want to figure out how to write a new Struts application, start with the Action- Mappings. The mappings are at the absolute center of every Struts application.In this chapter, we take a close look at the ActionMapping properties andexplore how they help you design the flow of a Struts application.1.0 vs 1.1 In Struts 1.1, ActionMapping subclasses ActionConfig (org.apache. struts.config.ActionConfig) and adds API methods required forbackward compatibility. ActionMapping is not deprecated, and how thehierarchy will be handled in future releases has not been determined.For now, we refer to the ActionMapping class, but you should note thatin Struts 1.1 all of the action properties are actually defined by the ActionConfigsuper class. The ActionMapping class otherwise works thesame way in both versions.7.2 ActionMapping propertiesTable 7.1 describes the base ActionMapping properties. As with other configuration components, developers may extend ActionMapping to provide additionalproperties.Table 7.1 The base ActionMapping propertiesProperty Descriptionpath The URI path from the request used to select this mapping. (API command) forward The context-relative path of the resource that should serve this request via a forward.Exactly one of the forward, include, or type properties must be specified.orinclude The context-relative path of the resource that should serve this request via aninclude. Exactly one of the forward, include, or type properties must be specified.ortype Optionally specifies a subclass oforg.apache.struts.action.ActionMappingthat should be used when instantiating this mapping.className The fully qualified name of the Action class used by this mapping. SinceStruts 1.1ActionMapping properties 197In the sections that follow, we take a look at each of these properties.7.2.1 The path propertyThe ActionMapping URI, or path, will look to the user like just another file onthe web server. But it does not represent a file. It is a virtual reference to our ActionMapping.Because it is exposed to other systems, the path is not really a logical name, likethose we use with ActionForward. The path can include slashes and an extension—as if it referred to a file system—but they are all just part of a single name.The ActionMappings themselves are a “flat” namespace with no type of internalhierarchy whatsoever. They just happen to use the same characters that we areused to seeing in hierarchical file systems.name The name of the form bean, if any, associated with this action. This is not the classname. It is the logical name used in the form bean configuration.roles The list of security roles that may access this mapping.scope The identifier of the scope (request or session) within which the form bean, if any,associated with this mapping will be created.validate Set to true if the validate method of the form bean (if any) associated with thismapping should be called.input Context-relative path of the input form to which control should be returned ifa validationerror is encountered. This can be any URI: HTML, JSP, VM, or another Action- Mapping.parameter General-purpose configuration parameter that can be used to pass extra informationto the Action selected by this ActionMapping.attribute Name of the request-scope or session-scope attribute under which our form bean isaccessed, if it is other than the bean's specified name.prefix Prefix used to match request parameter names to form bean property names, if any.suffix Suffix used to match request parameter names when populating the properties ofour ActionForm bean, if any.unknown Can be set to true if this mapping should be configured as the default for this application(to handle all requests not handled by another mapping). Only one mappingcan be defined as the default unknown mapping within an application.forwards(s) Block of ActionForwards for this mapping to use, if any.exception(s) Block of ExceptionHandlers for this mapping to use, if any.Table 7.1 The base ActionMapping properties (continued)Property DescriptionSinceStruts 1.1SinceStruts 1.1198 CHAPTER 7Designing with ActionMappingsOf course, it can still be useful to treat your ActionMappings as if they werepart of a hierarchy and group related commands under the same "folder." Theonly restriction is that the names must match whatever pattern is used in the application’s deployment description (web.xml) for the ActionServlet. This is usuallyeither /do/* or *.do, but any similar pattern can be used.If you are working in a team environment, different team members can begiven different ActionMapping namespaces to use. Some people may be workingwith the /customer ActionMappings, others may be working with the /vendor ActionMappings. This may also relate to the Java package hierarchy the team isusing. Since the ActionMapping URIs are logical constructs, they can be organizedin any way that suits your project.With Struts 1.1, these types of namespaces can be promoted to applicationmodules. Each team can work independently on its own module, with its own setof configuration files and presentation pages. Configuring your application to use multiple modules is covered in chapter 4.DEFINITION The web runs on URIs, and most URIs map to physical files. If you want to change the resource, you change the corresponding file. Some URIs, likeStruts actions, are virtual references. They do not have a correspondingfile but are handled by a programming component. To change the resource,we change how the component is programmed. But since thepath is a URI and interacts with other systems outside our control, thepath is not a true logical reference—the name of an ActionForward, forinstance. We can change the name of an ActionForward without consultingother systems. It’s an internal, logical reference. If we change thepath to an ActionMapping, we might need to update other systems thatrefer to the ActionMapping through its public URI.7.2.2 The forward propertyWhen the forward property is specified, the servlet will not pass the request to an Action class but will make a call to RequestDispatcher.forward. Since the operationdoes not use an Action class, it can be used to integrate Struts with otherresources and to prototype systems. The forward, include, and type propertiesare mutually exclusive. (See chapter 6 for more information.)7.2.3 The include propertyWhen the include property is specified, the servlet will not pass the request to an Action class but will make a call to RequestDispatcher.include. The operationActionMapping properties 199does not use an Action class and can be used to integrate Struts with other components. The forward, include, and type properties are mutually exclusive. (Seechapter 6 for more information.)7.2.4 The type propertyMost mappings will specify an Action class type rather than a forward or include.An Action class may be used by more than one mapping. The mappings may specifyform beans, parameters, forwards, or exceptions. The forward, include, andtype properties are mutually exclusive.7.2.5 The className propertyWhen specified, className is the fully qualified Java classname of the ActionMapping subclass that should be used for this object. This allows you to use your own ActionMapping subclass with specialized methods and properties. See alsosection 7.4.7.2.6 The name propertyThis property specifies the logical name for the form bean, as given in the formbean segment of the Struts configuration file. By default, this is also the name tobe used when placing the form bean in the request or session context. Use theattribute property of this class to specify a different attribute key.7.2.7 The roles propertyThis property is a comma-delimited list of the security role names that are allowed access to this ActionMapping object. By default, the same system that is used with standard container-based security is applied to the list of roles given here. Thismeans you can use action-based security in lieu of specifying URL patterns in the deployment descriptor, or you can use both together.The security check is handled by the processRoles method of the Request- Processor (org.apache.struts.action.RequestProcessor). By subclassing RequestProcessor, you can also use the roles property with application-based security. See chapter 9 for more about subclassing RequestProcessor.7.2.8 The scope propertyThe ActionForm bean can be stored in the current request or in the session scope (where it will be available to additional requests). While most developers userequest scope for the ActionForm, the framework default is session scope. Tomake request the default, see section 7.4.SinceStruts 1.1SinceStruts 1.1200 CHAPTER 7Designing with ActionMappings7.2.9 The validate propertyAn important step in the lifecycle of an ActionForm is to validate its data before offering it to the business layer. When the validate property for a mapping is true, the ActionServlet will call the ActionForm’s validate method. If validate returns false, the request is forwarded to the resource given by the input property.Often, developers will create a pair of mappings for each data entry form. Onemapping will have validate set to false, so you can create an empty form. Theother has validate set to true and is used to submit the completed form.NOTE Whether or not the ActionForm validate method is called does not relateto the ActionServlet’s validating property. That switch controlshow the Struts configuration file is processed.7.2.10 The input propertyWhen validate is set to true, it is important that a valid path for input be provided. This is where control will pass should the ActionForm validate methodreturn false. Often, this is the address for a presentation page. Sometimes it willbe another Action path (with validate set to false) that is required to generatedata objects needed by the page.NOTE The input path often leads back to the page that submitted the request.While it seems natural for the framework to return the request to whereit originated, this is not a simple task in a web application. A request is oftenpassed from component to component before a response is sent backto the browser. The browser only knows the path it used to retrieve theinput page, which may or may not also be the correct path to use for theinput property. While it may be possible to try and generate a default inputpage based on the HTTP referrer attribute, the Struts designersdeemed that approach unreliable.inputForwardIn Struts 1.0, the ActionMapping input property is always a literal URI. InStruts 1.1, it may optionally be the name of an ActionForward instead. The ActionForward is retrieved and its path property is used as the input property.This can be a global or local ActionForward.To use ActionForwards here instead of literal paths, set the inputForwardattribute on the <controller> element for this module to true:SinceStruts 1.1ActionMapping properties 201<controller inputForward="true">For more about configuring Struts, see chapter 4. For more about ActionForwards,see chapter 6.7.2.11 The parameter propertyThe generic parameter property allows Actions to be configured at runtime. Severalof the standard Struts Actions make use of this property, and the standardScaffold Actions often use it, too. The parameter property may contain a URI, the name of a method, the name of a class, or any other bit of information an Actionmay need at runtime. This flexibility allows some Actions to do double and tripleduty, slashing the number of distinct Action classes an application needs on hand.Within an Action class, the parameter property is retrieved from the mappingpassed to perform:parameter = mapping.getParameter();Multiple parametersWhile multiple parameters are not supported by the standard ActionMappingsclass, there are some easy ways to implement this, including using HttpUtils, a StringTokenizer, or a Properties file (java.util.Properties).HttpUtils. Although deprecated as of the Servlet API 2.3 specification, theHttpUtils package (javax.servlet.http.HttpUtils) provides a static method that parses any string as if it were a query string and returns a Hashtable(java.util.Hashtable):Hashtable parameters = parseQueryString(parameter);The parameter property for your mapping then becomes just another query string, because you might use it elsewhere in the Struts configuration. stringTokenizer. Another simple approach is to delimit the parameters using the token of your choice—such as a comma, colon, or semicolon—and use the StringTokenizer to read them back:StringTokenizer incoming =new StringTokenizer(mapping.getParameter(),";");int i = 0;String[] parameters = new String[incoming.countTokens()]; while (incoming.hasMoreTokens()) {parameters[i++] = incoming.nextToken().trim();}202 CHAPTER 7Designing with ActionMappingsProperties file. While slightly more complicated than the others, another popular approach to providing multiple parameters to an ActionMapping is with a standard Properties files (java.util.Properties). Depending on your needs, the Properties file could be stored in an absolute location in your file system or anywhere on your application’s CLASSPATH.The Commons Scaffold package [ASF, Commons] provides a ResourceUtils package (mons.scaffold.util.ResourceUtils) with methods forloading a Properties file from an absolute location or from your application’s CLASSPATH.7.2.12 The attribute propertyFrom time to time, you may need to store two copies of the same ActionForm inthe same context at the same time. This most often happens when ActionFormsare being stored in the session context as part of a workflow. To keep their names from conflicting, you can use the attribute property to give one ActionForm bean a different name.An alternative approach is to define another ActionForm bean in the configuration, using the same type but under a different name.7.2.13 The prefix and suffix propertiesLike attribute, the prefix and suffix properties can be used to help avoid naming conflicts in your application. When specified, these switches enable aprefix or suffix for the property name, forming an alias when it is populatedfrom the request.If the prefix this was specified, thenthisName=McClanahanbecomes equivalent toname=McClanahanfor the purpose of populating the ActionForm. Either or both parameters would call getName("McClanahan");This does not affect how the properties are written by the tag extensions. It affects how the autopopulation mechanism perceives them in the request.Nested components 2037.2.14 The unknown ActionMappingWhile surfing the Web, most of us have encountered the dreaded 404— page not found message. Most web servers provide some special features for processing requests for unknown pages, so webmasters can steer users in the right direction. Struts offers a similar service for ActionMapping 404s—the unknown ActionMapping. In the Struts configuration file, you can specify one ActionMapping toreceive any requests for an ActionMapping that would not otherwise be matched:<actionname="/debug"forward="/pages/debug.jsp"/>When this option is not set, a request for an ActionMapping that cannot bematched throws400 Invalid path /notHere was requestedNote that by a request for an ActionMapping, we mean a URI that matches the prefix or suffix specified for the servlet (usually /do/* or *.do). Requests for other URI patterns, good or bad, will be handled by other servlets or by the container:/do/notHere (goes to the unknown ActionMapping)/notHere.txt (goes to the container)7.3 Nested componentsThe ActionMapping properties are helpful when it comes to getting an Action torun a business operation. But they tell only part of the story. There is still much todo when the Action returns.An Action may have more than one outcome. We may need to register several ActionForwards so that the Action can take its pick.7.3.1 Local forwardsIn the normal course, an ActionMapping is used to select an Action object to handle the request. The Action returns an ActionForward that indicates which pageshould complete the response.The reason we use ActionForwards is that, in practice, presentation pages areeither often reused or often changed, or both. In either case, it is good practice to encapsulate the page’s location behind a logical name, like “success” or “failure.”The ActionForward object lets us assign a logical name to any given URI.204 CHAPTER 7Designing with ActionMappingsOf course, logical concepts like success or failure are often relative. What represents success to one Action may represent failure to another. Each Action-Mapping can have its own set of local ActionForwards. When the Action asks for a forward (by name), the local set is checked before trying the global forwards. See chapter 6 for more about ActionForwards.Local forwards are usually specified in the Struts configuration file. See chapter4 for details.7.3.2 Local exceptionsMost often, an application’s exception handlers (org.apache.struts.action. ExceptionHandler) can be declared globally. However, if a given ActionMapping needs to handle an exception differently, it can have its own set of local exception handlers that are checked before the global set.Local exceptions are usually specified in the Struts configuration file. Seechapter 4 for details.7.4 Rolling your own ActionMappingWhile ActionMapping provides an impressive array of properties, developers may also provide their own subclass with additional properties or methods. InStruts 1.0, this is configured in the deployment descriptor (web.xml) for the ActionServlet:<init-param><param-name>mapping</param-name><param-value>app.MyActionMapping</param-value></init-param>In Struts 1.1, this is configured in the Struts configuration file as an attribute to the <action-mappings> element:<action-mappings type="app.MyActionMapping">Individual mappings may also be set to use another type through the className attribute:<action className="app.MyActionMapping">For more about configuring Struts, see chapter 4.SinceStruts 1.1Summary 205The framework provides two base ActionMapping classes, shown in table 7.2. They can be selected as the default or used as a base for your own subclasses.The framework default is SessionActionMapping, so scope defaults to session. Subclasses that provide new properties may set them in the Struts configuration using a standard mechanism:<set-property property="myProperty" value="myValue" /> Using this standard mechanism helps developers avoid subclassing the Action- Servlet just to recognize the new properties when it digests the configuration file. This is actually a feature of the Digester that Struts simply inherits.7.5 SummarySun’s Model 2 architecture teaches that servlets and JavaServer Pages should be used together in the same application. The servlets can handle flow control and data acquisition, and the JavaServer Pages can handle the HTML.Struts takes this one step further and delegates much of the flow control anddata acquisition to Action objects. The application then needs only a single servletto act as a traffic cop. All the real work is parceled out to the Actions and theStruts configuration objects.Like servlets, Actions are efficient, multithreaded singletons. A single Actionobject can be handling any number of requests at the same time, optimizing your server’s resources.To get the most use out of your Actions, the ActionMapping object is used as a decorator for the Action object. It gives the Action a URI, or several URIs, and away to pass different configuration settings to an Action depending on which URIis called.In this chapter, we took a close look at the ActionMapping properties andexplained each property’s role in the scheme of things. We also looked at extendingthe standard ActionMapping object with custom properties—just in case yourscheme needs even more things.Table 7.2 The default ActionMapping classesActionMapping Descriptionorg.apache.struts.action.SessionActionMapping Defaults the scope property to sessionorg.apache.struts.action.RequestActionMapping Defaults the scope property to request206 CHAPTER 7Designing with ActionMappingsIn chapter 8, the real fun begins. The configuration objects covered so far aremainly a support system. They help the controller match an incoming requestwith a server-side operation. Now that we have the supporting players, let’s meet the Struts diva: the Action object.7.1 进入ActionMappingModel 2 架构(第1章)鼓励在同一个应用中使用servlet和JSP页面。
计算机科学与技术外文翻译
Working with Strings
Strings are characters or words. String values must always be enclosed in either single quotesor double quotes. Here are a few examples of strings:
In ActionScript, you’ll work with many different datatypes. However, for the sake of understandinghow these datatypes work, you can consider them in two basic categories: primitivetypes and reference types. The primitive types are called primitive because they are the basicfoundational datatypes, not because they lack importance. The reference datatypes are calledreference types because they reference the primitive types.
计算机外文翻译（完整）
计算机外⽂翻译（完整）毕业设计(论⽂)外⽂资料翻译专业：计算机科学与技术姓名：王成明学号：06120186外⽂出处：The History of the Internet附件： 1.外⽂原⽂ 2.外⽂资料翻译译⽂；附件1：外⽂原⽂The History of the InternetThe Beginning - ARPAnetThe Internet started as a project by the US government. The object of the project was to create a means of communications between long distance points, in the event of a nation wide emergency or, more specifically, nuclear war. The project was called ARPAnet, and it is what the Internet started as. Funded specifically for military communication, the engineers responsible for ARPANet had no idea of the possibilities of an "Internet."By definition, an 'Internet' is four or more computers connected by a network.ARPAnet achieved its network by using a protocol called TCP/IP. The basics around this protocol was that if information sent over a network failed to get through on one route, it would find another route to work with, as well as establishing a means for one computer to "talk" to another computer, regardless of whether it was a PC or a Macintosh.By the 80's ARPAnet, just years away from becoming the more well known Internet, had 200 computers. The Defense Department, satisfied with ARPAnets results, decided to fully adopt it into service, and connected many military computers and resources into the network. ARPAnet then had 562 computers on its network. By the year 1984, it had over 1000 computers on its network.In 1986 ARPAnet (supposedly) shut down, but only the organization shut down, and the existing networks still existed between the more than 1000 computers. It shut down due to a failied link up with NSF, who wanted to connect its 5 countywide super computers into ARPAnet.With the funding of NSF, new high speed lines were successfully installed at line speeds of 56k (a normal modem nowadays) through telephone lines in 1988. By that time, there were 28,174 computers on the (by then decided) Internet. In 1989 there were 80,000 computers on it. By 1989, there were290,000.Another network was built to support the incredible number of people joining. It was constructed in 1992.Today - The InternetToday, the Internet has become one of the most important technological advancements in the history of humanity. Everyone wants to get 'on line' to experience the wealth of information of the Internet. Millions of people now use the Internet, and it's predicted that by the year 2003 every single person on the planet will have Internet access. The Internet has truly become a way of life in our time and era, and is evolving so quickly its hard to determine where it will go next, as computer and network technology improve every day.HOW IT WORKS:It's a standard thing. People using the Internet. Shopping, playing games,conversing in virtual Internet environments.The Internet is not a 'thing' itself. The Internet cannot just "crash." It functions the same way as the telephone system, only there is no Internet company that runs the Internet.The Internet is a collection of millioins of computers that are all connected to each other, or have the means to connect to each other. The Internet is just like an office network, only it has millions of computers connected to it.The main thing about how the Internet works is communication. How does a computer in Houston know how to access data on a computer in Tokyo to view a webpage?Internet communication, communication among computers connected to the Internet, is based on a language. This language is called TCP/IP. TCP/IP establishes a language for a computer to access and transmit data over the Internet system.But TCP/IP assumes that there is a physical connecetion between onecomputer and another. This is not usually the case. There would have to be a network wire that went to every computer connected to the Internet, but that would make the Internet impossible to access.The physical connection that is requireed is established by way of modems,phonelines, and other modem cable connections (like cable modems or DSL). Modems on computers read and transmit data over established lines,which could be phonelines or data lines. The actual hard core connections are established among computers called routers.A router is a computer that serves as a traffic controller for information.To explain this better, let's look at how a standard computer might viewa webpage.1. The user's computer dials into an Internet Service Provider (ISP). The ISP might in turn be connected to another ISP, or a straight connection into the Internet backbone.2. The user launches a web browser like Netscape or Internet Explorer and types in an internet location to go to.3. Here's where the tricky part comes in. First, the computer sends data about it's data request to a router. A router is a very high speed powerful computer running special software. The collection of routers in the world make what is called a "backbone," on which all the data on the Internet is transferred. The backbone presently operates at a speed of several gigabytes per-second. Such a speed compared to a normal modem is like comparing the heat of the sun to the heat of an ice-cube.Routers handle data that is going back and forth. A router puts small chunks of data into packages called packets, which function similarly to envelopes. So, when the request for the webpage goes through, it uses TCP/IP protocols to tell the router what to do with the data, where it's going, and overall where the user wants to go.4. The router sends these packets to other routers, eventually leadingto the target computer. It's like whisper down the lane (only the information remains intact).5. When the information reaches the target web server, the webserver then begins to send the web page back. A webserver is the computer where the webpage is stored that is running a program that handles requests for the webpage and sends the webpage to whoever wants to see it.6. The webpage is put in packets, sent through routers, and arrive at the users computer where the user can view the webpage once it is assembled.The packets which contain the data also contain special information that lets routers and other computers know how to reassemble the data in the right order.With millions of web pages, and millions of users, using the Internet is not always easy for a beginning user, especially for someone who is not entirely comfortale with using computers. Below you can find tips tricks and help on how to use main services of the Internet.Before you access webpages, you must have a web browser to actually be able to view the webpages. Most Internet Access Providers provide you with a web browser in the software they usually give to customers; you. The fact that you are viewing this page means that you have a web browser. The top two use browsers are Netscape Communicator and Microsoft Internet Explorer. Netscape can be found at /doc/bedc387343323968011c9268.html and MSIE can be found at /doc/bedc387343323968011c9268.html /ie.The fact that you're reading this right now means that you have a web browser.Next you must be familiar with actually using webpages. A webpage is a collection of hyperlinks, images, text, forms, menus, and multimedia. To "navigate" a webpage, simply click the links it provides or follow it's own instructions (like if it has a form you need to use, it will probably instruct you how to use it). Basically, everything about a webpage is made to be self-explanetory. That is the nature of a webpage, to be easily navigatable."Oh no! a 404 error! 'Cannot find web page?'" is a common remark made by new web-users.Sometimes websites have errors. But an error on a website is not the user's fault, of course.A 404 error means that the page you tried to go to does not exist. This could be because the site is still being constructed and the page hasn't been created yet, or because the site author made a typo in the page. There's nothing much to do about a 404 error except for e-mailing the site administrator (of the page you wanted to go to) an telling him/her about the error.A Javascript error is the result of a programming error in the Javascript code of a website. Not all websites utilize Javascript, but many do. Javascript is different from Java, and most browsers now support Javascript. If you are using an old version of a web browser (Netscape 3.0 for example), you might get Javascript errors because sites utilize Javascript versions that your browser does not support. So, you can try getting a newer version of your web browser.E-mail stands for Electronic Mail, and that's what it is. E-mail enables people to send letters, and even files and pictures to each other.To use e-mail, you must have an e-mail client, which is just like a personal post office, since it retrieves and stores e-mail. Secondly, you must have an e-mail account. Most Internet Service Providers provide free e-mail account(s) for free. Some services offer free e-mail, like Hotmail, and Geocities.After configuring your e-mail client with your POP3 and SMTP server address (your e-mail provider will give you that information), you are ready to receive mail.An attachment is a file sent in a letter. If someone sends you an attachment and you don't know who it is, don't run the file, ever. It could be a virus or some other kind of nasty programs. You can't get a virus justby reading e-mail, you'll have to physically execute some form of program for a virus to strike.A signature is a feature of many e-mail programs. A signature is added to the end of every e-mail you send out. You can put a text graphic, your business information, anything you want.Imagine that a computer on the Internet is an island in the sea. The sea is filled with millions of islands. This is the Internet. Imagine an island communicates with other island by sending ships to other islands and receiving ships. The island has ports to accept and send out ships.A computer on the Internet has access nodes called ports. A port is just a symbolic object that allows the computer to operate on a network (or the Internet). This method is similar to the island/ocean symbolism above.Telnet refers to accessing ports on a server directly with a text connection. Almost every kind of Internet function, like accessing web pages,"chatting," and e-mailing is done over a Telnet connection.Telnetting requires a Telnet client. A telnet program comes with the Windows system, so Windows users can access telnet by typing in "telnet" (without the "'s) in the run dialog. Linux has it built into the command line; telnet. A popular telnet program for Macintosh is NCSA telnet.Any server software (web page daemon, chat daemon) can be accessed via telnet, although they are not usually meant to be accessed in such a manner. For instance, it is possible to connect directly to a mail server and check your mail by interfacing with the e-mail server software, but it's easier to use an e-mail client (of course).There are millions of WebPages that come from all over the world, yet how will you know what the address of a page you want is?Search engines save the day. A search engine is a very large website that allows you to search it's own database of websites. For instance, if you wanted to find a website on dogs, you'd search for "dog" or "dogs" or "dog information." Here are a few search-engines.1. Altavista (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed2. Yahoo (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed Collection3. Excite (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed4. Lycos (/doc/bedc387343323968011c9268.html ) - Web spider & Indexed5. Metasearch (/doc/bedc387343323968011c9268.html ) - Multiple searchA web spider is a program used by search engines that goes from page to page, following any link it can possibly find. This means that a search engine can literally map out as much of the Internet as it's own time and speed allows for.An indexed collection uses hand-added links. For instance, on Yahoo's site. You can click on Computers & the Internet. Then you can click on Hardware. Then you can click on Modems, etc., and along the way through sections, there are sites available which relate to what section you're in.Metasearch searches many search engines at the same time, finding the top choices from about 10 search engines, making searching a lot more effective.Once you are able to use search engines, you can effectively find the pages you want.With the arrival of networking and multi user systems, security has always been on the mind of system developers and system operators. Since the dawn of AT&T and its phone network, hackers have been known by many, hackers who find ways all the time of breaking into systems. It used to not be that big of a problem, since networking was limited to big corporate companies or government computers who could afford the necessary computer security.The biggest problem now-a-days is personal information. Why should you be careful while making purchases via a website? Let's look at how the internet works, quickly.The user is transferring credit card information to a webpage. Looks safe, right? Not necessarily. As the user submits the information, it is being streamed through a series of computers that make up the Internet backbone.The information is in little chunks, in packages called packets. Here's the problem: While the information is being transferred through this big backbone, what is preventing a "hacker" from intercepting this data stream at one of the backbone points?Big-brother is not watching you if you access a web site, but users should be aware of potential threats while transmitting private information. There are methods of enforcing security, like password protection, an most importantly, encryption.Encryption means scrambling data into a code that can only be unscrambled on the "other end." Browser's like Netscape Communicator and Internet Explorer feature encryption support for making on-line transfers. Some encryptions work better than others. The most advanced encryption system is called DES (Data Encryption Standard), and it was adopted by the US Defense Department because it was deemed so difficult to 'crack' that they considered it a security risk if it would fall into another countries hands.A DES uses a single key of information to unlock an entire document. The problem is, there are 75 trillion possible keys to use, so it is a highly difficult system to break. One document was cracked and decoded, but it was a combined effort of14,000 computers networked over the Internet that took a while to do it, so most hackers don't have that many resources available.附件2：外⽂资料翻译译⽂Internet的历史起源——ARPAnetInternet是被美国政府作为⼀项⼯程进⾏开发的。
外文翻译 计算机科学与技术
毕业设计(论文)外文文献翻译专业计算机科学与技术学生姓名班级学号指导教师信息工程学院Mobile Malware and Smart Device Security:Trends,Challenges and SolutionsAbdullahi Arabo and Bernardi PranggonoThe Oxford Internet Institute(OII),Oxford University,Oxford,OX13JS,U.K. School of Engineering and Built Environment,Glasgow Caledonian University, Glasgow,G40BA,U.K.Abstract—This work is part of the research to study trends and challenges of cyber security to smart devices in smart homes.We have seen the development and demand for seamless interconnectivity of smart devices to provide various functionality and abilities to users.While these devices provide more features and functionality,they also introduce new risks and threats.Subsequently, current cyber security issues related to smart devices are discussed and analyzed. The paper begins with related background and motivation.We identified mobile malware as one of the main issue in the smart devices’security.In the near future,mobile smart device users can expect to see a striking increase in malware and notable advancements in malware-related attacks,particularly on the Android platform as the user base has grown exponentially.We discuss and analyzed mobile malware in details and identified challenges and future trends in this area.Then we propose and discuss an integrated security solution for cyber security in smart devices to tackle the issue.Index—Botnet,cyber security,mobile malware,security framework,smart device securityI.INTRODUCTIONThe Internet is one of the most remarkable developments to have happened to mankind in the last100years.The development of ubiquitous computing makes things even more interesting as it has given us the possibility to utilise devices and technology in unusual ways.We have seen the development and demand for seamless interconnectivity of smart devices to provide various functionalities and abilities to users.But we also know the vulnerabilities that exist within this ecosystem.However, these vulnerabilities are normally considered for larger infrastructures and little attention has been paid to the cyber security threats from the usage and power of smart devices as a result of the Internet of Things(IoT)technologies.In the IoT vision,every physical object has a virtual component that can produce and consume services.Smart spaces are becoming interconnected with powerful smart devices (smartphones,tablets,etc.).On the other hand,we also have the backbone,the power grid that powers our nations.These two phenomena are coming at the same time.The increased usage of smart meters in our homes or businesses provides an avenue ofconnectivity as well as powerful home services or interconnected powerful smart devices.The example of the smart grid also provides the means of controlling and monitoring smart grid infrastructures via the use of portable smart devices.The vulnerability of the connected home and developments within the energy industry’s new wireless smart grid are exposed to the wrong people;it will inevitably lead to lights out for everyone.This will eventually uncover the multitude of interconnected smart devices in the IoT as a hotbed for cyber-attacks or robot networks(botnets)and a security nightmare for smart space users and possibly for national infrastructures as a whole.The latest research has reported that on average people own three internet-connected smart devices such as smartphones and tablets.Therefore,as a result of the ubiquity of smart devices,and their evolution as computing platforms,as well as the powerful processors embedded in smart devices,has made them suitable objects for inclusion in a botnet.Botnets of mobile devices(also known as mobile botnets)are a group of compromised smart devices that are remotely controlled by bot-masters via command-and-control(C&C)channels.Mobile botnets have different characteristics in several aspects as compared to PC-based botnets,such as their C&C channels medium.PC-based botnets are seen as the most common platforms for security attacks, and mobile botnets are seen as less of a threat in comparison to their counterparts. This is so for different reasons,such as limited battery power,resource issues,and Internet access constraints,etc.Therefore,the efforts directed to both the manifestation of operating mobile botnets and corresponding research and development endeavours are not as wide as for PC-based botnets.However,this development could change with the recent surge in popularity and use of smart devices.Smart devices are now widely used by billions of users due to their enhanced computing ability,practicality and efficient Internet access,thanks to advancement in solid-state technologies.Moreover,smart devices typically contain a large amount of sensitive personal and corporate data and are often used in online payments and other sensitive transactions.The wide spread use of open-source smart device platforms such as Android and third-party applications made available to the public also provides more opportunities and attractions for malware creators.Therefore,for now and the near future smart devices will become one of the most lucrative targets for cybercriminals.The main focus of this paper is threefold:firstly to highlight the possible threats and vulnerability of smart devices,secondly to analyse the challenges involved in detecting mobile malware in smart devices and finally to propose a general security solution that will facilitate solving or addressing such threats.The rest of the paper isorganized as follows.In section II we provide a detailed analysis of the security threats on smart devices and their links with cyber security.We have identified mobile malware as one of the main issues and we discuss it in more detail in Section III. Section IV provides our proposed security solution that will be able to deter the problems of mobile malware.The paper is concluded in section V.II.SECURITY THREATS ON SMART DEVICESThe weakest link in any IT security chain is the user.The human factor is the most challenging aspect of mobile device security.Home users generally assume that everything will work just as it should,relying on a device’s default settings without referring to complex technical manuals.Therefore service content providers and hardware vendors need to be aware of their responsibilities in maintaining network security and content management on the devices they provide.Service providers might also have the opportunity to provide add-on security services to complement the weaknesses of the devices.The issue of cyber security is much closer to the home environment than has been usually understood;hence,the problem of cyber security extends beyond computers it is also a threat to portable devices.Many electronic devices used at home are practically as powerful as a computer-from mobile phones,video consoles,game consoles and car navigation systems.While these devices are portable,provide more features and functionality,they also introduce new risks.These devices previously considered as secure can be an easy target for assailants.The information stored and managed within such devices and home networks forms part of an individual’s Critical Information Infrastructure(CII)[2]as identified by the POSTnote on cyber security in the UK.For example,an attacker may be able to compromise a smart device with a virus,to access the data on the device.Not only do these activities have implications for personal information,but they could also have serious consequences if corporate information were also stored on the smart device.The use of mobile devices in healthcare is also more common these days,such as in mobile-health.A typical example is having a health device connected to the home network,which is capable of transmitting data wirelessly to hospitals and other relevant parties.Most of the manufacturers of these devices do not put much effort in trying to make sure that the devices are secure.If these devices are compromised not only will the information and privacy of the user of the device be compromised,but the attacker can even change the settings of the devices,which could lead to harmful consequences.It has been shown that it is possible to hack into a pacemaker and read the details of data stored in the device such as names and medical data without having direct access to the devices simply by standing nearby[3].Therefore,it is also possible to reconfigure the parameters of the device.This is not only applicable to medical devices,but also to any devices that are used within the home network for any purpose.According to the Juniper Networks report[4],76percent of mobile users depend on their mobile devices to access their most sensitive personal information,such as online banking or personal medical information.This trend is even more noticeable with those who also use their personal mobile devices for business purposes.Nearly nine in ten(89percent)business users report that they use their mobile device to access sensitive work-related information.Another more worrying impact is when cybercriminals use the vast resources of the network to turn it into a botnet and launch a cyber-attack on national critical infrastructures.There are some Android applications that when downloaded from a third party market(not the Android market)are capable of accessing the root functionality of devices(“rooted”)and turning them into botnet soldiers without the user’s explicit consent.People could easily and unwittingly download malware to their smart devices or fall prey to“man-in-the-middle”attacks where cyber-criminals pose as a legitimate body,intercept and harvest sensitive information for malicious use.In2011,there was a mix of Android applications removed from the Android Market because they contained malware.There were over50infected applications-these applications were copies of“legitimate”applications from legitimate publishers that were modified to include two root exploits and a rogue application downloader.The Juniper Networks Mobile Threat Centre(MTC)reported that in2011 there was an unparalleled increase in mobile malware attacks,with a155percent increase from the previous year across all platforms[5].It is also reported that Android malware experienced an increase of3,325percent in2011.Notable in these findings is a significant number of malware samples obtained from third-party applications which do not enjoy the benefit or protection Google Play Store scanning techniques.Previously,an Android developer could post an application to the official Android Market and have it available immediately,without inspection or vetting to block pirated or malicious applications.This increase in malware is mainly due to the combination of Google Android’s dominant market share in smartphone(68.8percent in2012)and the lack of security control over the applications appearing in the various Android application markets.It was reported recently that Google Play store,which has more than700,000apps just passed15billion downloads.Security firm Fortinet estimated that money-stealing malware has increased exponentially in2006-2011as shown in Figure1.Based on an estimation by Kaspersky Lab,cybercriminals who target smart devices likesmartphones earn from$1,000to$5,000per day per person.Mobile phone hacking is also getting more attractive with the rise of the Near-Field Communication technology(NFC),which expands the use of smart devices as e-wallet or helps people to read product information.In December2011alone,Kaspersky Lab discovered more than1,000new Trojans targeting smartphones.That is more than all the smartphone viruses spotted during2003-2010.This trend is continuing;in2012,the number of cyber-attacks targeting mobile devices increased exponentially during the first quarter,as reported by security firm Trend Micro[6].Their report identified approximately5,000new malicious Android applications in just the first three months of the year,mainly due to the increase of the Android user base.The research also pointed out a marked escalation in the number of active advanced persistent threat(APT)campaigns currently being mounted against companies and governments.APT is a cyber-attack launched by a group of sophisticated,determined,and coordinated attackers who systematically compromise the network of a specific target or entity for a prolonged period.Security researchers see APT in different ways,while some researchers regard APT as different type of attack;others just categorize it as a more organized botnet with more resources behind it.Malware developments that targets smart home devices have several known monetization factors.Most malwares are aimed at mobile pick pocketing(short message service(SMS)or call fraud)or the ability to charge premium bills via SMS or calls,as illustrated in Figure2.Some malware are used as part of botnet creations. Malwares like DreamDroid(or DroidDream)[7]have integrated thousands of mobile devices into extensive botnets.Some of the malwares are developed to exploit vulnerabilities on either the operating systems(OS),installed applications,or just to create trouble to user information.Home devices and general consumer electronics are progressively becoming more advanced and are capable of connecting with other devices over a network. While it may sound unreal,devices such as TVs,digital picture frames,smart meters and e-readers are quite vulnerable and absolutely capable of causing problems on your network.The next few years will provide opportunities for various types of malware developers to explore unlikely methods of achieving their goals.Smartphones are not invulnerable and Macs can get malware,such as the CVE-2012-0507vulnerability [8].Luigi Auriemma in[9]has uncovered a vulnerability in a Samsung D6000high definition(HD)TV that caused it to get stuck in an endless loop of restarts. Auriemma's report followed another denial-of-service(DoS)vulnerability in SonyBravia TVs uncovered by Gabriel Menezes Nunes[10]which stops users from changing the volume,channels or access any functions.In the2012first quarterly report from Trend Micro[11],it was pointed out that the large diffusion of mobile devices and the increase in awareness of the principal cyber threats have resulted in an increase in the interest of cybercrime in the mobile sector.Another significant interest is concentrated on the threat in terms of the rapid spread of botnets based on mobile devices,favored by the total almost absence of protection and the difficulty of tracing the agents composing the network.If these exploits are targeted by well-established hacker groups such as Anonymous,it will pose a bigger threat to organizations and smart environments that protect highly sensitive data,targeting companies and individuals for various political and financial reasons.III.MOBILE MALWAREOne of the major and most common problems in today’s Internet is malware. Among these malware,Botnets are considered as the biggest challenge.Botnets are used to send email spam,carry out distributed denial of services(DDoS)attacks,and for hosting phishing and malware sites.Botnets are slowly moving towards smart devices since those devices are now basically everywhere,powerful enough to run a bot and offer additional gains for a bot-master such as financial gains as discussed earlier.With PC-based botnets,cybercriminals often use zombies within botnets to launch DDoS attacks.Even though there have been no major mobile DDoS incidents, with current trends we can expect to see this in the near future.Botnets are maintained by malicious actors commonly referred to as “bot-masters”that can access and manage the botnet remotely or via bot proxy servers as illustrated in Error!Reference source not found..The bots are then programmed and instructed by the bot-master to perform a variety of cyber-attacks,including attacks involving the further distribution and installation of malware on other information systems.In PC-based botnets,botnet master controllers typically use http requests with normal port80to transmit and receive their messages.In mobile-based botnets,the bot-master also uses similar http techniques to distribute their commands but also exploits SMS,Bluetooth,etc.The bot-master exploits operating system and configuration vulnerabilities to compromise smart devices and to install the bot software.The first mobile malware,known as Cabir,was discovered in2004and was also known as the first mobile worm.The first mobile botnet was discovered around July 2009,when a security researcher found SymbOS.Yxes or SymbOS.Exy.C(aka Sexy Space)[12]targeting Symbian devices and using simple HTTP-basedCommand-and-Control(C&C).Later the same year,a security researcher discovered Ikee.B[13],which targets jailbroken iPhones using a similar mechanism to SymbOS.Yxes.Geinimi,which is considered to be the first Android botnet,was discovered in China in December2010. Geinimi also implements similar HTTP-based C&C with the added feature of encrypted communications.Geinimi steals the device’s international mobile equipment identity(IMEI),international mobile subscriber identity(IMSI),GPS coordinate,SMS,contact list,etc.and forwards it to the bot-master.Although advanced mobile botnets have not been observed in the main population of smartphones,we believe it is just a matter of time.As shown in[14], mobile botnets are obviously serious threats for both end users and cellular networks. Threats imposed by botnets will continue to increase.As more people use smart devices,it is essential to analyze and explore the mechanisms of mobile botnets and develop security solutions in regard to smart devices.The use of C&C for a mobile botnet stipulates additional challenges that differentiate it from well-known PC-based botnets.Some of these main challenges include,among others:computational power,seamless connectivity,inter-connectivity with other secure platforms networks,portability and amount of stored sensitive data, and computational power.PC-based botnets also use an IRC-channel as the main C&C communication channel.The impact of SMS-based C&C,IP-based C&C,and Bluetooth-based C&C has been addressed in detail in[15],while P2P-based C&C mobile botnets are analyzed and discussed in[16].As a result of the abilities of smart devices in terms of placing i.e.calls,use of SMS and MMS amongst others,the burdens for mobile botnets are very interesting and challenging as it opens the door for easy financial gain for a bot-master. Additionally,since mobile phones interact with operators and other networks,attacks against the critical infrastructure are also possible.Hence,it is possible to launch sophisticated cyber-attacks on the mobile phone network that will be very hard to prevent.Detecting and preventing malware is not a trivial task as malware developers adopt and invent new strategies to infiltrate mobile devices.Malware developers employ advanced techniques such as obfuscation and encryption to camouflage the signs of malware and thereby undermine anti-malware software.Some of the main reasons why mobile malware are an attractive point for viruses and malware developers are:1.The ubiquity of smart devices such as smartphones in general.2.The increasing computational powers of smart devices.Whose they arebecoming virtually as powerful as desktop systems.3.The lack of awareness of the threats and the risk attached to smartdevices from the end-user’s perspective.4.The growing uses of jailbreak/rooted devices both on iOS and Androiddevices.5.Each smart device really is an expression of the owner.It provides ameans to track the user’s activity,hence serves as a single gateway to our digital identity and activities.6.Most of the widely used smart devices operate on an open platform suchas Android,which encourages developers and download of applications from both trusted applications markets and third party markets.IV.POTENTIAL SECURITY SOLUTIONSConsidering the above threats and challenges,a new security solution is essential for cyber security for smart devices in smart homes.More specifically,several key research tasks are required:1)investigate new secure system architecture for smart devices in smart homes;2)re-evaluate and enhance security system architecture for smart devices in smart homes.Android OS has four layers:Linux kernel,libraries(+Android runtime), application solution and applications layers(see Figure4).So,basically Android runtime is a kind of“glue”between the Linux kernel and the applications.Figure4.Android OS layersThe main security features common to Android involve process and file system isolation;application or code signing;ROM,firmware,and factory restore;and kill switches.However,the main security issue with Android OS is it relies heavily to the end-user to decide whether an application is safe or not.Even though Google’s just adding one piece of the security layer by scanning an applications in the Google Play, the end users still needs to analyze and make the final decision themselves whether tocontinue with the installation or not.Until now,the end-users cannot rely on the operating system to protect themselves from malware.As part of Google’s marketing strategy to gain market share as big as possible by offering applications as many as possible,the Android application publishing process makes it easy for developers to develop Android applications,but also provides too much space for malicious application creators.Malicious applications have successfully infected Android market before,one example being a malware application called droid09which allowed users to carry out banking transactions.The application needs the user to provide the bank’s details and tricks the user by masquerading a legitimate login of a bank website(phishing).Malware applications have become more sophisticated these days;they find new ways and techniques to enter the system by exploiting software vulnerabilities or by just tricking the users.End-user:It is always essential for the end-user to be aware of the security measures of their mobile device.End-users should be aware of at least the following measures:•Install anti-virus and anti-malware solutions to protect the device against malware and viruses.Also ensure to turn on the automatic update.It is been shown that installing anti-virus and anti-malware is very effective to protect mobile devices from malicious applications[5,6,17].•Install a personal firewall to protect mobile device interfaces from direct attack and illegal access.The effectiveness of mobile firewalls to increase a mobile device’s security is shown in[18].•Install only applications from trusted sources that have legitimate contact information and a website.As the current Android Market(Google Play) does not adopt a certification process for applications,it is up to the end-user to make sure he/she only installs trusted applications from trusted developers.•Install only applications from the official and original developer(for example,if you are installing Instagram applications,make sure you download it from Instagram Inc.).•Check the permissions carefully when the application is prompting you during the installation phase.For example,when you install a wallpaper application,do you think it really needs full Internet access?•Ensure your OS and software's always up-to-date with the latest versions and security patches need to be installed.•Install remote locate,track,lock,wipe,backup and restore software to retrieve,protect or restore a lost or stolen mobile device and the personal data on the device.•Only install applications that have a high number of downloads and positive reviews.•Never view sensitive data over public wireless networks which have no passwords or encryption.•Should be alert to anomaly behaviours and activities in their devices.•Should be careful when clicking links on social network sites.Malicious links on social networks can be a very effective method to spread malware.Participants tend to trust such networks and are thus willing to click on links that are on“friends’”social networking sites.Mobile Network Operators(MNOs):MNO also has responsibility to create a more secure environment for their customers.MNOs need to install anti-virus and anti-malware software to scan outgoing and incoming SMS and MMS to the mobile network,as many malwares use SMS/MMS to propagate and contact the bot-master. MNO should also build a global partnership with related agencies such as other MNOs to prevent mobile malware propagation by exchanging information, knowledge,database and expertise.Apps Developers:Developers also need to take care of the security measures implemented in their application.They should ensure that private data is not being sent via an unencrypted channel;the data must be sent through HTTPS or TLS networks.Developers should minimize the use of built-in permissions in their applications, for example do not ask for full Internet access permission,INTERNET,unless it is essential for your applications to work properly.Android has about100built-in permissions that control operations such as dialing the phone(CALL_PHONE), sending shot message(SEND_SMS),etc.In Android,there are three main“security protection levels”for permission labels:a“normal”permission is granted to any application that requests it;a “dangerous”permission is only granted after user approval at install-time;and a “signature”permission is only granted to applications signed by the same developer key as the application defining the permission label.This“signature”protection level is integral in ensuring that third-party applications do not gain access affecting the Android’s trusted computing base (TCB)’s integrity.Furthermore,applications developers need only collect data which is essential and required for the application otherwise it will be tampered by the attackers.This is also useful to minimize repackaging attacks.Repackaging attacks are a very common approach,in which a malware developer downloads a legitimate application,modifies it to include malicious code and then republishes it to an application market ordownload site.It is shown that the repackaging technique is highly effective mainly because it is often difficult for end-users to tell the difference between a legitimate application and its malicious repackaged form.In fact,repackaging was the most prevalent type of social engineering attack used by Android malwaredevelopers in the first two quarters of2011[17].One of the characteristics of Android malware is typically it is specifically developed for a speci fic group of users.It is very unlikely for an Android user from Russia to be infected by Chinese malware for example.Android malware is typically created by cybercriminals with users in specific countries as their target, which is usually their own compatriot.Market Store:The store needs to vet and rigorously screen new mobile applications before they can be put in the market. Google(Google Play)recently made a significant improvement in their security by screening new applications before they were put in the market.Applications store providers also should consider certification for each application before it can be published in the marketplace.The effectiveness of such certification process is shown in[19].Applications should be rigorously reviewed to ensure that applications are safe from malicious codes,reliable,perform as expected,and are also free of explicit and offensive material.V.CONCLUSIONThe paper discussed a development of security solution to handle the challenges of cyber security to smart devices in smart homes.The IoT technologies may be able to extend anywhere computing to almost anything,but there are fundamental security issues that need to be properly addressed.In the near future,mobile smart device users can expect to see a striking increase in malware and notable advancements in malware-related attacks,particularly on the Android platform as the user base has grown exponentially.Today’s users utilize their mobile smart devices for everything from accessing emails to sensitive transactions such as online banking and payments. As users become more dependent on their mobile devices as digital wallets,this creates a very lucrative target for cybercriminals.Mobile smart device users can expect to see a significant malware increase on finance related applications,such as mobile Internet banking.Detecting and preventing malware in mobile device need comprehensive and multi-level approaches.This work is part of ongoing research to design and implement a security model for smart devices in the smart home environment.For the future work we plan to implement and assess the security solution proposed in the test-bed environment which includes a honeynet for mobile malware.。
计算机科学与技术专业 外文翻译 外文文献 英文文献 记录
外文文献原稿和译文原稿IntroductionThe creation and maintenance of records relating to the students of an institution are essential to:. managing the relationship between the institution and the student;. providing support and other services and facilities to the student;. controlling the student’s academic progress and measuring their achievement, both at the institution and subsequently;. providing support to the student after they leave the institution.In addition, student records contain data which the institution can aggregate and analyse to inform future strategy, planning and service provision.The number of students in HEIs has increased rapidly in the last twenty years. An institution’s relationship with an individual student has also become increasingly complex because of the range of support services institutions now provide to students and life long learning initiatives. Consequently, the volume and complexity of student records have also increased, as have the resources required to create, maintain, use, retain and dispose of them, irrespective of the format in which they are kept. Ensuring that the personal data contained in student records is controlled and managed in line with the principles of the Data Protection Act 1998 creates an additional complication.Institutions should, therefore, establish a policy on managing student records to ensure that they are handled consistently and effectively wherever they are held and whoever holds them. This policy should ensure that:. records relating to an individual student are complete, accurate and up to date;. duplication of student data is deliberate rather than uncontrolled and kept to the minimum needed to support effective administration;. records are held and stored securely to prevent unauthorised access to them;. records relating to the academic aspects of the student’s relationship with the institution are clearly segregated from those dealing with financial, disciplinary, social, support and contractual aspects of that relationship. This will enable differential retention periods to be applied to each of these to meet business and regulatory requirements.What are student records?Records are documents or other items which:. contain recorded information;. are produced or received in the initiation, conduct or completion of an activity;. are retained as evidence of that activity, or because they have other informational value.The recorded information may be in any form (e.g. text, image, sound) and the records may be in any medium or format.Student records –records associated with managing the relationship between an institution and its students –can be organised into three broad categories, each of which may be additionally divided:1. Records documenting the contractual relationship between the student and the institutione.g. records documenting admission and enrolment, payment of tuition fees, non-academic disciplinary proceedings.2. Records documenting the student as a learnere.g. records documenting programmes undertaken, academic progress and performance, awards.3. Records documenting the student as an individual and consumer of services provided by the institutione.g. records documenting use of accommodation services, counseling services, library and IT support services, careers and employment services.Most records in categories 1 and 3 have specific retention periods triggered by the formal end of a student’s direct relationship with an institution, although the information they contain may be aggregated and analyzed to provide data requested by third parties1 orto support the institution’s planning and development activities. An institution will need to retain some of the records in category 2 to provide confirmatory information to potential employers, professional bodies and associations, and to bodies which regulate entry to medical and other professions and which assess and maintain evidence of fitness to practice in those professions.Who is responsible for managing student records?HEI organizational structures vary considerably. As a result, it is difficult to specify exactly where these responsibilities should lie in any one institution.Responsibility for managing student records should be clearly defined and documented. It is important to define the responsibilities of staff involved in: . managing the institution’s general, contractual relationship with the student;. managing the institution’s relationship with the student as a learner;. providing technical and personal support services to the student;for creating, maintaining, using, retaining and disposing of records documenting those activities during the student’s time at the institution.Institutions should also designate one clear point of responsibility for maintaining complete, accurate and up to date records on every student, covering all aspects of the relationship. They should also define the minimum content of the core student record so that the institution can, if required:. demonstrate, within the provisions of limitation statutes, that its implied contract with the student has been fulfilled;. provide information on the student’s academic performance and award(s) to potential employers, to licensing/regulatory bodies (normally first registration only)which control entry to professions and to other organizations (e.g. those providing chartered status) as well as to the student;. provide information on the student as an individual as a means of enabling the institution, or others acting on its behalf, to analyse and aggregate student data for planning and developing its future programmes, recruitment activities and the facilities and services required to support future students.Where and how should student records be stored?The nature of student records and the personal information they contain demands that they should be stored in facilities and equipment (‘hard copy’ records) or electronic systems (digital records) which are, above all, secure and accessible only to authorized staff whose work requires them to have access. In addition, the facilities and equipment should provide: . adequate space for all the records which need to be produced and retained;. appropriate environmental conditions for the record media used.Storage facilities and systems should meet the same standards irrespective of where they are located and who is responsible for managing them.Authorized staff should maintain a record of:. the content, format and location of all student records;. the names and designations of all staff with access to student records, and any limitations on that access;. student records which have been transferred to another part of the institution, particularly after the student has left;. organizations, professional bodies, statutory regulators to whom personal data relating to the student has been provided.Student records should be stored and indexed so that they can be identified and retrieved quickly and easily.. Paper records should be housed in durable containers which carry only an impersonal code number related to a restricted-access list or index to prevent casual, unauthorised access. These containers should be stored in locked equipment or rooms when they are not being used to ensure that the personal data they contain is protected in line with the requirements of the Data Protection Act 1998.. Digital records should be uniquely identified and protected with passwords and other electronic security measures. In all cases, access should be limited to those staff who have ‘a need to know’. If ele ctronic systems are not centrally managed, designated staff should make back-up copies to prevent loss of records through accidental or intentional damage.Whatever its format, the ‘core student record’ shou ld be treated as a vital record and action taken to protect it from disaster or systems failure by copying and dispersal.Student records will become relatively inactive once the student leaves the institution.They may then be transferred to other storage facilities or systems. At this point, duplicates of records created for administrative convenience should be destroyed so that only the designated official records survive.Who should have access to student records?Institutions should tightly control access to student records to prevent unauthorised use, alteration, removal or destruction of the records themselves and unauthorised disclosure of the information they contain. Only those members of staff who need them to do their work should have access to student records and, their access should be restricted to records of the direct relationship and not to the content of the whole file.Student records contain personal data and are therefore subject to the provisions of the Data Protection Act 1998, including the provision that the student, as the data subject, should be given access to personal data held, whether in digital or hard copy form. In addition, the ‘core student record’ as defined by the KCL study includes personal data on the student’s parents which is also subject to the provisions of th e Act.How long should student records be kept?In general, student records should be kept only for as long as is necessary to:. fulfill and discharge the contractual obligations established between the institution and the student, including the completion of any non-academic disciplinary action;. provide information on the academic career and achievements of the student to employers, licensing/regulatory bodies and other organizations, as well as to the student as part of their lifelong learning record;. record the activities of the student as an individual and as a consumer of student support and other institutional services as a means of managing those services and planning and developing them in the future.The nature of the activities which give rise to these categories of records drives their retention.. The contractual relationship between the institution and the student is subject to the same statutory limitations on action as any other contract. This will include records of disciplinary action taken against the student. The records should be disposed of accordingly. The date at which the student leaves the institution normally provides the retention‘trigger’.. The records relating to the student as a learner need to be retained for longer than other student records. Institutions accept that they have an obligation, during a student’s working life, to provide factual information on what they have studied and achieved, i.e. a Transcript. The proposed lifelong learning record or progress file would also include additional data on relevant non-academic achievements and activities (e.g. voluntary work). The retention period for these records should reflect the need to fulfill this obligation over long periods of time, perhaps for the lifetime of the student. It is important to segregate these records from those relating to other aspects of the relationship so that non-academic records are not retained for unnecessarily long periods, consuming storage resources and creating potential breaches of the Data Protection Act 1998.. Records relating to the student as an individual and as a user of student support and institutional services are relatively short term and should be retained for a short finite period once the student leaves the institution. This period should be shorter than for records relating to the wider contractual arrangements.The KCL study proposed the development of a ‘core student record’ which would contain, in addition to the formal transcript, data relating to the background of the student, including parents’ address and occupation, schools attended, first employment, etc. In addition to providing academic information on the individual student, KCL suggested that the availability of this data facilitates its analysis for institutional business planning and development purposes, as well as supporting subsequent academic historical, sociological and demographic research.Individual institutions should decide whether they wish to retain this data for research purposes once immediate institutional business needs have been met. In doing so they will need to take account of:. the cost and technical difficulty of maintaining records, even in summary form, permanently;. the security and subject access implications of retaining personal data relating to named individuals;. the need to create and maintain finding aids so that individual records can be easilyand quickly retrieved when required, particularly to meet subject access requests.How should student records be destroyed?Student records should be destroyed in line with agreed retention periods. Destruction should be authorized by staff with appropriate authority and it should be carried out in accordance with the institution’s procedures for the destruction of redundant rec ords containing personal data.The authority for destruction and the date of destruction should be recorded and held by the section of the institution with final responsibility for the student record.译文介绍创建与维护和学生相关的记录对一个公共机构来说是十分重要的：处理机关和学生之间的关系；提供支持和其他服务以及便利给学生；在机关，控制学生学术进展和测量他们的成就；随后提供支持给学生，在他们离开机关之后。
计算机专业毕业设计论文外文文献中英文翻译(Object)
外文资料Object landscapes and lifetimesTechnically, OOP is just about abstract data typing, inheritance, and polymorphism, but other issues can be at least as important. The remainder of this section will cover these issues.One of the most important factors is the way objects are created and destroyed. Where is the data for an object and how is the lifetime of the object controlled? There are different philosophies at work here. C++ takes the approach that control of efficiency is the most important issue, so it gives the programmer a choice. For maximum run-time speed, the storage and lifetime can be determined while the program is being written, by placing the objects on the stack (these are sometimes called automatic or scoped variables) or in the static storage area. This places a priority on the speed of storage allocation and release, and control of these can be very valuable in some situations. However, you sacrifice flexibility because you must know the exact quantity, lifetime, and type of objects while you're writing the program. If you are trying to solve a more general problem such as computer-aided design, warehouse management, or air-traffic control, this is too restrictive.The second approach is to create objects dynamically in a pool of memory called the heap. In this approach, you don't know until run-time how many objects you need, what their lifetime is, or what their exact type is. Those are determined at the spur of the moment while the program is running. If you need a new object, you simply make it on the heap at the point that you need it. Because the storage is managed dynamically, at run-time, the amount of time required to allocate storage on the heap is significantly longer than the time to create storage on the stack. (Creating storage on the stack is often a single assembly instruction tomove the stack pointer down, and another to move it back up.) The dynamic approach makes the generally logical assumption that objects tend to be complicated, so the extra overhead of finding storage and releasing that storage will not have an important impact on the creation of an object. In addition, the greater flexibility is essential to solve the general programming problem.Java uses the second approach, exclusively]. Every time you want to create an object, you use the new keyword to build a dynamic instance of that object.There's another issue, however, and that's the lifetime of an object. With languages that allow objects to be created on the stack, the compiler determines how long the object lasts and can automatically destroy it. However, if you create it on the heap the compiler has no knowledge of its lifetime. In a language like C++, you must determine programmatically when to destroy the object, which can lead to memory leaks if you don’t do it correctly (and this is a common problem in C++ programs). Java provides a feature called a garbage collector that automatically discovers when an object is no longer in use and destroys it. A garbage collector is much more convenient because it reduces the number of issues that you must track and the code you must write. More important, the garbage collector provides a much higher level of insurance against the insidious problem of memory leaks (which has brought many a C++ project to its knees).The rest of this section looks at additional factors concerning object lifetimes and landscapes.1 Collections and iteratorsIf you don’t know how many objects you’re going to need to solve a particular problem, or how long they will last, you also don’t know how to store those objects. How can you know how much space to create for thoseobjects? You can’t, since that information isn’t known until run-time.The solution to most problems in object-oriented design seems flippant: you create another type of object. The new type of object that solves this particular problem holds references to other objects. Of course, you can do the same thing with an array, which is available in most languages. But there’s more. This new object, generally called a container(also called a collection, but the Java library uses that term in a different sense so this book will use “container”), will expand itself whenever necessary to accommodate everything you place inside it. So you don’t need to know how manyobjects you’re going to hold in a container. Just create a container object and let it take care of the details.Fortunately, a good OOP language comes with a set of containers as part of the package. In C++, it’s part of the Standard C++ Library and is sometimes called the Standard Template Library (STL). Object Pascal has containers in its Visual Component Library (VCL). Smalltalk has a very complete set of containers. Java also has containers in its standard library. In some libraries, a generic container is considered good enough for all needs, and in others (Java, for example) the library has different types of containers for different needs: a vector (called an ArrayListin Java) for consistent access to all elements, and a linked list for consistent insertion at all elements, for example, so you can choose the particular type that fits your needs. Container libraries may also include sets, queues, hash tables, trees, stacks, etc.All containers have some way to put things in and get things out; there are usually functions to add elements to a container, and others to fetch those elements back out. But fetching elements can be more problematic, because a single-selection function is restrictive. What if you want to manipulate or compare a set of elements in the container instead of just one?The solution is an iterator, which is an object whose job is to select the elements within a container and present them to the user of the iterator. As a class, it also provides a level of abstraction. This abstraction can be used to separate the details of the container from the code that’s accessing that container. The container, via the iterator, is abstracted to be simply a sequence. The iterator allows you to traverse that sequence without worrying about the underlying structure—that is, whether it’s an ArrayList, a LinkedList, a Stack, or something else. This gives you the flexibility to easily change the underlying data structure without disturbing the code in your program. Java began (in version 1.0 and 1.1) with a standard iterator, called Enumeration, for all of its container classes. Java 2 has added a much more complete container library that contains an iterator called Iterator that does more than the older Enumeration.From a design standpoint, all you really want is a sequence that can be manipulated to solve your problem. If a single type of sequence satisfied all of your needs, there’d be no reason to have different kinds. There are two reasons that you need a choice of containers. First, containers provide different types of interfaces and external behavior.A stack has a different interface and behavior than that of a queue, which is different from that of a set or a list. One of these might provide a more flexible solution to your problem than the other. Second, different containers have different efficiencies for certain operations. The best example is an ArrayList and a LinkedList. Both are simple sequences that can have identical interfaces and external behaviors. But certain operations can have radically different costs. Randomly accessing elements in an ArrayList is a constant-time operation; it takes the same amount of time regardless of the element you select. However, in a LinkedList it is expensive to move through the list to randomly selectan element, and it takes longer to find an element that is further down the list. On the other hand, if you want to insert an element in the middle of a sequence, it’s much cheaper in a LinkedList than in an ArrayList. These and other operations have different efficiencies depending on the underlying structure of the sequence. In the design phase, you might start with a LinkedList and, when tuning for performance, change to an ArrayList. Because of the abstraction via iterators, you can change from one to the other with minimal impact on your code.In the end, remember that a container is only a storage cabinet to put objects in. If that cabinet solves all of your needs, it doesn’t really matter how it is implemented (a basic concept with most types of objects). If you’re working in a programming environment that has built-in overhead due to other factors, then the cost difference between an ArrayList and a LinkedList might not matter. You might need only one type of sequence. You can even imagine the “perfect”container abstraction, which can automatically change its underlying implementation according to the way it is used.2 The singly rooted hierarchyOne of the issues in OOP that has become especially prominent since the introduction of C++ is whether all classes should ultimately be inherited from a single base class. In Java (as with virtually all other OOP languages) the answer is “yes”and the name of this ultimate base class is simply Object. It turns out that the benefits of the singly rooted hierarchy are many.All objects in a singly rooted hierarchy have an interface in common, so they are all ultimately the same type. The alternative (provided by C++) is that you don’t know that everything is the same fundamental type. From a backward-compatibility standpoint this fits the model of C better and can be thought of as less restrictive, but when you want to do full-onobject-oriented programming you must then build your own hierarchy to provide the same convenience that’s built into other OOP languages. And in any new class library you acquire, some other incompatible interface will be used. It requires effort (and possibly multiple inheritance) to work the new interface into your design. Is the extra “flexibility” of C++ worth it? If you need it—if you have a large investment in C—it’s quite valuable. If you’re starting from scratch, other alternatives such as Java can often be more productive.All objects in a singly rooted hierarchy (such as Java provides) can be guaranteed to have certain functionality. You know you can perform certain basic operations on every object in your system. A singly rooted hierarchy, along with creating all objects on the heap, greatly simplifies argument passing (one of the more complex topics in C++).A singly rooted hierarchy makes it much easier to implement a garbage collector (which is conveniently built into Java). The necessary support can be installed in the base class, and the garbage collector can thus send the appropriate messages to every object in the system. Without a singly rooted hierarchy and a system to manipulate an object via a reference, it is difficult to implement a garbage collector.Since run-time type information is guaranteed to be in all objects, you’ll never end up with an object whose type you cannot determine. This is especially important with system level operations, such as exception handling, and to allow greater flexibility in programming.3 Collection libraries and support for easy collection useBecause a container is a tool that you’ll use frequently, it makes sense to have a library of containers that are built in a reusable fashion, so you can take one off the shelf Because a container is a tool that you’ll use frequently, it makes sense to have a library of containers that are built in a reusable fashion, so you can take one off the shelf and plugit into your program. Java provides such a library, which should satisfy most needs.Downcasting vs. templates/genericsTo make these containers reusable, they hold the one universal type in Java that was previously mentioned: Object. The singly rooted hierarchy means that everything is an Object, so a container that holds Objects can hold anything. This makes containers easy to reuse.To use such a container, you simply add object references to it, and later ask for them back. But, since the container holds only Objects, when you add your object reference into the container it is upcast to Object, thus losing its identity. When you fetch it back, you get an Object reference, and not a reference to the type that you put in. So how do you turn it back into something that has the useful interface of the object that you put into the container?Here, the cast is used again, but this time you’re not casting up the inheritance hierarchy to a more general type, you cast down the hierarchy to a more specific type. This manner of casting is called downcasting. With upcasting, you know, for example, that a Circle is a type of Shape so it’s safe to upcast, but you don’t know that an Object is necessarily a Circle or a Shape so it’s hardly safe to downcast unless you know that’s what you’re dealing with.It’s not completely dangerous, however, because if you downcast to the wrong thing you’ll get a run-time error called an exception, which will be described shortly. When you fetch object references from a container, though, you must have some way to remember exactly what they are so you can perform a proper downcast.Downcasting and the run-time checks require extra time for the runningprogram, and extra effort from the programmer. Wouldn’t it make sense to somehow create the container so that it knows the types that it holds, eliminating the need for the downcast and a possible mistake? The solution is parameterized types, which are classes that the compiler can automatically customize to work with particular types. For example, with a parameterized container, the compiler could customize that container so that it would accept only Shapes and fetch only Shapes.Parameterized types are an important part of C++, partly because C++ has no singly rooted hierarchy. In C++, the keyword that implements parameterized types is “template.” Java currently has no parameterized types since it is possible for it to get by—however awkwardly—using the singly rooted hierarchy. However, a current proposal for parameterized types uses a syntax that is strikingly similar to C++ templates.译文对象的创建和存在时间从技术角度说，OOP（面向对象程序设计）只是涉及抽象的数据类型、继承以及多形性，但另一些问题也可能显得非常重要。
计算机专业毕业设计--英文文献(含译文)
外文文献原文THE TECHNIQUE DEVELOPMENT HISTORY OF JSPThe Java Server Pages( JSP) is a kind of according to web of the script plait distance technique, similar carries the script language of Java in the server of the Netscape company of server- side JavaScript( SSJS) and the Active Server Pages(ASP) of the Microsoft. JSP compares the SSJS and ASP to have better can expand sex, and it is no more exclusive than any factory or some one particular server of Web. Though the norm of JSP is to be draw up by the Sun company of, any factory can carry out the JSP on own system.The After Sun release the JSP( the Java Server Pages) formally, the this kind of new Web application development technique very quickly caused the people's concern. JSP provided a special development environment for the Web application that establishes the high dynamic state. According to the Sun parlance, the JSP can adapt to include the Apache WebServer, IIS4.0 on the market at inside of 85% server product.This chapter will introduce the related knowledge of JSP and Databases, and JavaBean related contents, is all certainly rougher introduction among them basic contents, say perhaps to is a Guide only, if the reader needs the more detailed information, pleasing the book of consult the homologous JSP.1.1 GENERALIZEThe JSP(Java Server Pages) is from the company of Sun Microsystems initiate, the many companies the participate to the build up the together of the a kind the of dynamic the state web the page technique standard, the it have the it in the construction the of the dynamic state the web page the strong but the do not the especially of the function. JSP and the technique of ASP of the Microsoft is very alike. Both all provide the ability that mixes with a certain procedure code and is explain by the language engine to carry out the procedure code in the code of HTML. Underneath we are simple of carry on the introduction to it.JSP pages are translated into servlets. So, fundamentally, any task JSP pages can perform could also be accomplished by servlets. However, this underlying equivalence does not mean that servlets and JSP pages are equally appropriate in all scenarios. The issue is not the power of the technology, it is the convenience, productivity, and maintainability of one or the other. After all, anything you can do on a particular computer platform in the Java programming language you could also do in assembly language. But it still matters which you choose.JSP provides the following benefits over servlets alone:• It is easier to write and maintain the HTML. Your static code is ordinary HTML: no extra backslashes, no double quotes, and no lurking Java syntax.• You can use standard Web-site development tools. Even HTML tools that know nothing about JSP can be used because they simply ignore the JSP tags.• You can divide up your development team. The Java programmers can work on the dynamic code. The Web developers can concentrate on the presentation layer. On large projects, this division is very important. Depending on the size of your team and the complexity of your project, you can enforce a weaker or stronger separation between the static HTML and the dynamic content.Now, this discussion is not to say that you should stop using servlets and use only JSP instead. By no means. Almost all projects will use both. For some requests in your project, you will use servlets. For others, you will use JSP. For still others, you will combine them with the MVC architecture . You want the appropriate tool for the job, and servlets, by themselves, do not complete your toolkit.1.2 SOURCE OF JSPThe technique of JSP of the company of Sun, making the page of Web develop the personnel can use the HTML perhaps marking of XML to design to turn the end page with format. Use the perhaps small script future life of marking of JSP becomes the dynamic state on the page contents.( the contents changes according to the claim of)The Java Servlet is a technical foundation of JSP, and the large Web applies the development of the procedure to need the Java Servlet to match with with the JSP and then can complete, this name of Servlet comes from the Applet, the local translation method of now is a lot of, this book in order not to misconstruction, decide the direct adoption Servlet but don't do any translation, if reader would like to, can call it as" small service procedure". The Servlet is similar to traditional CGI, ISAPI, NSAPI etc. Web procedure development the function of the tool in fact, at use the Java Servlet hereafter, the customer need not use again the lowly method of CGI of efficiency, also need not use only the ability come to born page of Web of dynamic state in the method of API that a certain fixed Web server terrace circulate. Many servers of Web all support the Servlet, even not support the Servlet server of Web directly and can also pass the additional applied server and the mold pieces to support the Servlet. Receive benefit in the characteristic of the Java cross-platform, the Servlet is also a terrace irrelevant, actually, as long as match the norm of Java Servlet, the Servlet is complete to have nothing to do with terrace and is to have nothing to do with server of Web. Because the Java Servlet is internal to provide the service by the line distance, need not start a progress to the each claimses, and make use of the multi-threadingmechanism can at the same time for several claim service, therefore the efficiency of Java Servlet is very high.But the Java Servlet also is not to has no weakness, similar to traditional CGI, ISAPI, the NSAPI method, the Java Servlet is to make use of to output the HTML language sentence to carry out the dynamic state web page of, if develop the whole website with the Java Servlet, the integration process of the dynamic state part and the static state page is an evil-foreboding dream simply. For solving this kind of weakness of the Java Servlet, the SUN released the JSP.A number of years ago, Marty was invited to attend a small 20-person industry roundtable discussion on software technology. Sitting in the seat next to Marty was James Gosling, inventor of the Java programming language. Sitting several seats away was a high-level manager from a very large software company in Redmond, Washington. During the discussion, the moderator brought up the subject of Jini, which at that time was a new Java technology. The moderator asked the manager what he thought of it, and the manager responded that it was too early to tell, but that it seemed to be an excellent idea. He went on to say that they would keep an eye on it, and if it seemed to be catching on, they would follow his company's usual "embrace and extend" strategy. At this point, Gosling lightheartedly interjected "You mean disgrace and distend."Now, the grievance that Gosling was airing was that he felt that this company would take technology from other companies and suborn it for their own purposes. But guess what? The shoe is on the other foot here. The Java community did not invent the idea of designing pages as a mixture of static HTML and dynamic code marked with special tags. For example, Cold Fusion did it years earlier. Even ASP (a product from the very software company of the aforementioned manager) popularized this approach before JSP came along and decided to jump on the bandwagon. In fact, JSP not only adopted the general idea, it even used many of the same special tags as ASP did.The JSP is an establishment at the model of Java servlets on of the expression layer technique, it makes the plait write the HTML to become more simple.Be like the SSJS, it also allows you carry the static state HTML contents and servers the script mix to put together the born dynamic state exportation. JSP the script language that the Java is the tacit approval, however, be like the ASP and can use other languages( such as JavaScript and VBScript), the norm of JSP also allows to use other languages.1.3 JSP CHARACTERISTICSIs a service according to the script language in some one language of the statures system this kind of discuss, the JSP should be see make is a kind of script language.However, be a kind of script language, the JSP seemed to be too strong again, almost can use all Javas in the JSP.Be a kind of according to text originally of, take manifestation as the central development technique, the JSP provided all advantages of the Java Servlet, and, when combine with a JavaBeans together, providing a kind of make contents and manifestation that simple way that logic separate. Separate the contents and advantage of logical manifestations is, the personnel who renews the page external appearance need not know the code of Java, and renew the JavaBeans personnel also need not be design the web page of expert in hand, can use to take the page of JavaBeans JSP to define the template of Web, to build up a from have the alike external appearance of the website that page constitute. JavaBeans completes the data to provide, having no code of Java in the template thus, this means that these templates can be written the personnel by a HTML plait to support. Certainly, can also make use of the Java Servlet to control the logic of the website, adjust through the Java Servlet to use the way of the document of JSP to separate website of logic and contents.Generally speaking, in actual engine of JSP, the page of JSP is the edit and translate type while carry out, not explain the type of. Explain the dynamic state web page development tool of the type, such as ASP, PHP3 etc., because speed etc. reason, have already can't satisfy current the large electronic commerce needs appliedly, traditional development techniques are all at to edit and translate the executive way change, such as the ASP → ASP+;PHP3 → PHP4.In the JSP norm book, did not request the procedure in the JSP code part( be called the Scriptlet) and must write with the Java definitely. Actually, have some engines of JSP are adoptive other script languages such as the EMAC- Script, etc., but actually this a few script languages also are to set up on the Java, edit and translate for the Servlet to carry out of. Write according to the norm of JSP, have no Scriptlet of relation with Java also is can of, however, mainly lie in the ability and JavaBeans, the Enterprise JavaBeanses because of the JSP strong function to work together, so even is the Scriptlet part not to use the Java, edit and translate of performance code also should is related with Java.1.4 JSP MECHANISMTo comprehend the JSP how unite the technical advantage that above various speak of, come to carry out various result easily, the customer must understand the differentiation of" the module develops for the web page of the center" and" the page develops for the web page of the center" first.The SSJS and ASP are all in several year ago to release, the network of that time is still very young, no one knows to still have in addition to making all business, datas and the expression logic enter the original web page entirely heap what better solvethe method. This kind of model that take page as the center studies and gets the very fast development easily. However, along with change of time, the people know that this kind of method is unwell in set up large, the Web that can upgrade applies the procedure. The expression logic write in the script environment was lock in the page, only passing to shear to slice and glue to stick then can drive heavy use. Express the logic to usually mix together with business and the data logics, when this makes be the procedure member to try to change an external appearance that applies the procedure but do not want to break with its llied business logic, apply the procedure of maintenance be like to walk the similar difficulty on the eggshell. In fact in the business enterprise, heavy use the application of the module already through very mature, no one would like to rewrite those logics for their applied procedure.HTML and sketch the designer handed over to the implement work of their design the Web plait the one who write, make they have to double work-Usually is the handicraft plait to write, because have no fit tool and can carry the script and the HTML contents knot to the server to put together. Chien but speech, apply the complexity of the procedure along with the Web to promote continuously, the development method that take page as the center limits sex to become to get up obviously.At the same time, the people always at look for the better method of build up the Web application procedure, the module spreads in customer's machine/ server the realm. JavaBeans and ActiveX were published the company to expand to apply the procedure developer for Java and Windows to use to come to develop the complicated procedure quickly by" the fast application procedure development"( RAD) tool. These techniques make the expert in the some realm be able to write the module for the perpendicular application plait in the skill area, but the developer can go fetch the usage directly but need not control the expertise of this realm.Be a kind of take module as the central development terrace, the JSP appeared. It with the JavaBeans and Enterprise JavaBeans( EJB) module includes the model of the business and the data logic for foundation, provide a great deal of label and a script terraces to use to come to show in the HTML page from the contents of JavaBeans creation or send a present in return. Because of the property that regards the module as the center of the JSP, it can drive Java and not the developer of Java uses equally. Not the developer of Java can pass the JSP label( Tags) to use the JavaBeans that the deluxe developer of Java establish. The developer of Java not only can establish and use the JavaBeans, but also can use the language of Java to come to control more accurately in the JSP page according to the expression logic of the first floor JavaBeans.See now how JSP is handle claim of HTTP. In basic claim model, a claimdirectly was send to JSP page in. The code of JSP controls to carry on hour of the logic processing and module of JavaBeanses' hand over with each other, and the manifestation result in dynamic state bornly, mixing with the HTML page of the static state HTML code. The Beans can be JavaBeans or module of EJBs. Moreover, the more complicated claim model can see make from is request other JSP pages of the page call sign or Java Servlets.The engine of JSP wants to chase the code of Java that the label of JSP, code of Java in the JSP page even all converts into the big piece together with the static state HTML contents actually. These codes piece was organized the Java Servlet that customer can not see to go to by the engine of JSP, then the Servlet edits and translate them automatically byte code of Java.Thus, the visitant that is the website requests a JSP page, under the condition of it is not knowing, an already born, the Servlet actual full general that prepared to edit and translate completes all works, very concealment but again and efficiently. The Servlet is to edit and translate of, so the code of JSP in the web page does not need when the every time requests that page is explain. The engine of JSP need to be edit and translate after Servlet the code end is modify only once, then this Servlet that editted and translate can be carry out. The in view of the fact JSP engine auto is born to edit and translate the Servlet also, need not procedure member begins to edit and translate the code, so the JSP can bring vivid sex that function and fast developments need that you are efficiently.Compared with the traditional CGI, the JSP has the equal advantage. First, on the speed, the traditional procedure of CGI needs to use the standard importation of the system to output the equipments to carry out the dynamic state web page born, but the JSP is direct is mutually the connection with server. And say for the CGI, each interview needs to add to add a progress to handle, the progress build up and destroy by burning constantly and will be a not small burden for calculator of be the server of Web. The next in order, the JSP is specialized to develop but design for the Web of, its purpose is for building up according to the Web applied procedure, included the norm and the tool of a the whole set. Use the technique of JSP can combine a lot of JSP pages to become a Web application procedure very expediently.JSP six built-in objectsrequest for:The object of the package of information submitted by users, by calling the object corresponding way to access the information package, namely the use of the target users can access the information.response object:The customer's request dynamic response to the client sent the data.session object1. What is the session: session object is a built-in objects JSP, it in the first JSP pages loaded automatically create, complete the conversation of management.From a customer to open a browser and connect to the server, to close the browser, leaving the end of this server, known as a conversation.When a customer visits a server, the server may be a few pages link between repeatedly, repeatedly refresh a page, the server should bethrough some kind of way to know this is the same client, which requires session object.2. session object ID: When a customer's first visit to a server on the JSP pages, JSP engines produce a session object, and assigned aString type of ID number, JSP engine at the same time, the ID number sent to the client, stored in Cookie, this session objects, and customers on the establishment of a one-to-one relationship. When a customer to connect to the server of the other pages, customers no longer allocated to the new session object, until, close your browser, the client-server object to cancel the session, and the conversation, and customer relationship disappeared. When a customer re-open the browser to connect to the server, the server for the customer to create a new session object.aplication target1. What is the application:Servers have launched after the application object, when a customer to visit the site between the various pages here, this application objects are the same, until the server is down. But with the session difference is that all customers of the application objects are the same, that is, all customers share this built-in application objects.2. application objects commonly used methods:(1) public void setAttribute (String key, Object obj): Object specified parameters will be the object obj added to the application object, and to add the subject of the designation of a keyword index.(2) public Object getAttribute (String key): access to application objects containing keywords for.out targetsout as a target output flow, used to client output data. out targets for the output data.Cookie1. What is Cookie:Cookie is stored in Web server on the user's hard drive section of the text. Cookie allow a Web site on the user's computer to store information on and then get back to it.For example, a Web site may be generated for each visitor a unique ID, and then to Cookie in the form of documents stored in each user's machine.If you use IE browser to visit Web, you will see all stored on your hard drive on the Cookie. They are most often stored in places: c: \ windows \ cookies (in Window2000 is in the C: \ Documents and Settings \ your user name \ Cookies).Cookie is "keyword key = value value" to preserve the format of the record.2. Targets the creation of a Cookie, Cookie object called the constructor can create a Cookie. Cookie object constructor has two string .parameters: Cookie Cookie name and value.Cookie c = new Cookie ( "username", "john");3. If the JSP in the package good Cookie object to send to the client, the use of the response addCookie () method.Format: response.addCookie (c)4. Save to read the client's Cookie, the use of the object request getCookies () method will be implemented in all client came to an array of Cookie objects in the form of order, to meet the need to remove the Cookie object, it is necessary to compare an array cycle Each target keywords.JSP的技术发展历史Java Server Pages(JSP)是一种基于web的脚本编程技术，类似于网景公司的服务器端Java脚本语言—— server-side JavaScript(SSJS)和微软的Active Server Pages(ASP)。
计算机专业外文翻译-----无线局域网技术
WIRELESS LANIn just the past few years, wireless LANs have come to occupy a significant niche in the local area network market. Increasingly, organizations are finding that wireless LANs are an indispensable adjunct to traditional wired LANs, as they satisfy requirements for mobility, relocation, ad hoc networking, and coverage of locationsdifficult to wire. As the name suggests, a wireless LAN is one that makes use of a wireless transmission medium. Until relatively recently, wireless LANs were little used; the reasons for this included high prices, low data rates, occupational safety concerns, and licensing requirements. As these problems have been addressed, the popularity of wireless LANs has grown rapidly.In this section, we first look at the requirements for and advantages of wireless LANs, and then preview the key approaches to wireless LAN implementation.Wireless LANs ApplicationsThere are four application areas for wireless LANs: LAN extension, crossbuilding interconnect, nomadic access, and ad hoc networks. Let us consider each of these in turn.LAN ExtensionEarly wireless LAN products, introduced in the late 1980s, were marketed as substitutes for traditional wired LANs. A wireless LAN saves the cost of the installation of LAN cabling and eases the task of relocation and other modifications to network structure. However, this motivation for wireless LANs was overtaken by events. First, as awareness of the need for LAN became greater, architects designed new buildings to include extensive prewiring for data applications. Second, with advances in data transmission technology, there has been an increasing reliance on twisted pair cabling for LANs and, in particular, Category 3 unshielded twisted pair. Most older building are already wired with an abundance of Category 3 cable. Thus, the use of a wireless LAN to replace wired LANs has not happened to any great extent.However, in a number of environments, there is a role for the wireless LAN as an alternative to a wired LAN. Examples include buildings with large open areas, such as manufacturing plants, stock exchange trading floors, and warehouses; historical buildings with insufficient twisted pair and in which drilling holes for new wiring is prohibited; and small offices where installation and maintenance of wired LANs is not economical. In all of these cases, a wireless LAN provides an effective and more attractive alternative. In most of these cases, an organization will also have a wired LAN to support servers and some stationary workstations. For example, a manufacturing facility typically has an office area that is separate from the factory floor but which must be linked to it for networking purposes. Therefore, typically, a wireless LAN will be linked into a wired LAN on the same premises. Thus, this application area is referred to as LAN extension.Cross-Building InterconnectAnother use of wireless LAN technology is to connect LANs in nearby buildings, be they wired or wireless LANs. In this case, a point-to-point wireless link is used between two buildings. The devices so connected are typically bridges or routers. This single point-to-point link is not a LAN per se, but it is usual to include this application under the heading of wireless LAN.Nomadic AccessNomadic access provides a wireless link between a LAN hub and a mobile data terminal equipped with an antenna, such as a laptop computer or notepad computer. One example of the utility of such a connection is to enable an employee returning from a trip to transfer data from a personalportable computer to a server in the office. Nomadic access is also useful in an extended environment such as a campus or a business operating out of a cluster of buildings. In both of these cases, users may move around with their portable computers and may wish access to the servers on a wired LAN from various locations.Ad Hoc NetworkingAn ad hoc network is a peer-to-peer network (no centralized server) set up temporarily to meet some immediate need. For example, a group of employees, each with a laptop or palmtop computer, may convene in a conference room for a business or classroom meeting. The employees link their computers in a temporary network just for the duration of the meeting.Wireless LAN RequirementsA wireless LAN must meet the same sort of requirements typical of any LAN, including high capacity, ability to cover short distances, full connectivity among attached stations, and broadcast capability. In addition, there are a number of requirements specific to the wireless LAN environment. The following are among the most important requirements for wireless LANs: Throughput. The medium access control protocol should make as efficient use as possible of the wireless medium to maximize capacity.Number of nodes. Wireless LANs may need to support hundreds of nodes across multiple cells. Connection to backbone LAN. In most cases, interconnection with stations on a wired backbone LAN is required. For infrastructure wireless LANs, this is easily accomplished through the use of control modules that connect to both types of LANs. There may also need to be accommodation for mobile users and ad hoc wireless networks.Service area. A typical coverage area for a wireless LAN may be up to a 300 to 1000 foot diameter.Battery power consumption. Mobile workers use battery-powered workstations that need to have a long battery life when used with wireless adapters. This suggests that a MAC protocol that requires mobile nodes to constantlymonitor access points or to engage in frequent handshakes with a base stationis inappropriate.Transmission robustness and security. Unless properly designed, a wireless LAN may be interference-prone and easily eavesdropped upon. The design of a wireless LAN must permit reliable transmission even in a noisy environment and should provide some level of security from eavesdropping.Collocated network operation. As wireless LANs become more popular, it is quite likely for two of them to operate in the same area or in some area where interference between the LANs is possible. Such interference may thwart the normal operation of a MAC algorithm and may allow unauthorized access to a particular LAN.License-free operation. Users would prefer to buy and operate wireless LAN products without having to secure a license for the frequency band used by the LAN.HandoWroaming. The MAC protocol used in the wireless LAN should enable mobile stations to move from one cell to another.Dynamic configuration. The MAC addressing and network management aspects of the LAN should permit dynamic and automated addition, deletion, and relocation of end systems without disruption to other users.Physical Medium SpecificationThree physical media are defined in the current 802.11 standard:Infrared at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950 nm.Direct-sequence spread spectrum operating in the 2.4-GHz ISM band. Up to 7 channels, each with a data rate of 1 Mbps or 2 Mbps, can be used.Frequency-hopping spread spectrum operating in the 2.4-GHz ISM band. The details of this option are for further study.Wireless LAN TechnologyWireless LANs are generally categorized according to the transmission techniquethat is used. All current wireless LAN products fall into one of the following categories:Infrared (IR) LANs. An individual cell of an IR LAN is limited to a single room, as infrared light does not penetrate opaque walls.Spread Spectrum LANs. This type of LAN makes use of spread spectrum transmission technology. In most cases, these LANs operate in the ISM (Industrial, Scientific, and Medical) bands, so that no FCC licensing is required for their use in the U.S.Narrowband Microwave. These LANs operate at microwave frequencies but do not use spread spectrum. Some of these products operate at frequencies that require FCC licensing, while others use one of the unlicensed ISM bands.A set of wireless LAN standards has been developed by the IEEE 802.11 committee. The terminology and some of the specific features of 802.11 are unique to this standard and are not reflected in all commercial products. However, it is useful to be familiar with the standard as its features are representative of required wireless LAN capabilities.The smallest building block of a wireless LAN is a basic service set (BSS), which consists of some number of stations executing the same MAC protocol and competing for access to the same shared medium. A basic service set may be isolated, or it may connect to a backbone distribution system through an access point. The access point functions as a bridge. The MAC protocol may be fully distributed or controlled by a central coordination function housed in the access point. The basic service set generally corresponds to what is referred to as a cell in the literature. An extended service set (ESS) consists of two or more basic service sets interconnected by a distribution system. Typically, the distribution system is a wired backbone LAN. The extended service set appears as a single logical LAN to the logical link control (LLC) level. The standard defines three types of stations, based on mobility:No-transition. A station of this type is either stationary or moves only within the direct communication range of the communicating stations of a single BSS.BSS-transition. This is defined as a station movement from one BSS to another BSS within the same ESS. In this case, delivery of data to the station requires that the addressing capability be able to recognize the new location of the station.ESS-transition. This is defined as a station movement from a BSS in one ESS to a BSS within another ESS. This case is supported only in the sense that the station can move. Maintenance of upper-layer connections supported by 802.11 cannot be guaranteed. In fact, disruption of service is likely to occur. details of this option are for further study.The 802.11 working group considered two types of proposals for a MAC algorithm: distributed-access protocols which, like CSMAICD, distributed the decision to transmit over all the nodes using a carrier-sense mechanism; and centralized access protocols, which involve regulation of transmission by a centralized decision maker. A distributed access protocol makes sense of an ad hoc network of peer workstations and may also be attractive in other wireless LANconfigurations that consist primarily of bursty traffic. A centralized access protocol is natural for configurations in which a number of wireless stations are interconnected with each other and with some sort of base station that attaches to a backbone wired LAN; it is especially useful if some of the data is time-sensitive or high priority.The end result of the 802.11 is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access-control mechanism with an optional centralized control built on top of that. Figure 13.20 illustrates the architecture. The lower sublayer of the MAC layer is the distributed coordination function (DCF). DCF uses a contention algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DCF. The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service. PCF is built on top of DCF and exploits features of DCF to assure access for its users. Let us consider these two sublayers in turn.Distributed Coordination FunctionThe DCF sublayer makes use of a simple CSMA algorithm. If a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit; otherwise, the station must wait until the current transmission is complete before transmitting. The DCF does not include a collision-detection function (i.e., CSMAICD) because collision detection is not practical on a wireless network. The dynamic range of the signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission. To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by considering a single delay known as an interframe space (IFS). In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail. Using an IFS, the rules for CSMA access are as follows:I. A station with a frame to transmit senses the medium. If the medium is idle, the station waits to see if the medium remains idle for a time equal to IFS, and, if this is so, the station may immediately transmit.2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over.3. Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off using a binary exponential backoff scheme and again senses the medium. If the medium is still idle, the station may transmit.Point Coordination FunctionPCF is an alternative access method implemented on top of the DCF. The operation consists of polling with the centralized polling master (point coordinator). The point coordinator makes use of PIFS when issuing polls. Because PIFS is smaller than DIFS, the point coordinator can seize the medium and lock out all asynchronous traffic while it issues polls and receives responses.As an extreme, consider the following possible scenario. A wireless network is configured so that a number of stations with time-sensitive traffic are controlled by the point coordinator while remaining traffic, using CSMA, contends for access.The point coordinator could issue polls in a round-robin fashion to all stations configured for polling. When a poll is issued, the polled station may respond using SIFS. If the point coordinator receives a response, it issues another poll using PIFS. If no response is received during theexpected turnaround time, the coordinator issues a poll. If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls. To prevent this situation, an interval known as the superframe is defined. During the first part of this interval, the point coordinator issues polls in a round-robin fashion to all stations configured for polling. The point coordinator then idles for the remainder of the superframe, allowing a contention period for asynchronous access.At the beginning of a superframe, the point coordinator may optionally seize control and issue polls fora give period of time. This interval varies because of the variable frame size issued by responding stations. The remainder of the superframe is available for contention-based access. At the end of the superframe interval, the point coordinator contends for access to the medium using PIFS. If the medium is idle, the point coordinator gains immediate access, and a full superframe period follows. However, the medium may be busy at the end of a superframe. In this case, the point coordinator must wait until the medium is idle to gain access; this results in a foreshortened superframe period for the next cycle.无线局域网技术最近几年，无线局域网开始在市场中独霸一方。
计算机专业中英文翻译外文翻译文献翻译
计算机专业中英文翻译外文翻译文献翻译英文参考文献及翻译Linux - Operating system of cybertimes Though for a lot of people , regard Linux as the main operating system to make up huge work station group, finish special effects of " Titanic " make , already can be regarded as and show talent fully. But for Linux, this only numerous news one of. Recently, the manufacturers concerned have announced that support the news of Linux to increase day by day, users' enthusiasm to Linux runs high unprecedentedly too. Then, Linux only have operating system not free more than on earth on 7 year this piece what glamour, get the favors of such numerous important software and hardware manufacturers as the masses of users and Orac le , Informix , HP , Sybase , Corel , Intel , Netscape , Dell ,etc. , OK?1.The background of Linux and characteristicLinux is a kind of " free (Free ) software ": What is called free, mean users can obtain the procedure and source code freely , and can use them freely , including revise or copy etc.. It is a result of cybertimes, numerous technical staff finish its research and development together through Inte rnet, countless user is it test and except fault , can add user expansion function that oneself make conveniently to participate in. As the most outstanding one in free software, Linux has characteristic of the following:(1)Totally follow POSLX standard, expand the network operatingsystem of supporting all AT&T and BSD Unix characteristic. Because of inheritting Unix outstanding design philosophy , and there are clean , stalwart , high-efficient and steady kernels, their all key codes are finished by Li nus T orvalds and otheroutstanding programmers, without any Unix code of AT&T or Berkeley, so Linu x is not Unix, but Linux and Unix are totally compatible.(2)Real many tasks, multi-user's system, the built-in networksupports, can be with such seamless links as NetWare , Windows NT , OS/2 ,Unix ,etc.. Network in various kinds of Unix it tests to be fastest in comparing and assess efficiency. Support such many kinds of files systems as FAT16 , FAT32 , NTFS , Ex t2FS , ISO9600 ,etc. at the same time .(3) Can operate it in many kinds of hardwares platform , including such processors as Alpha , SunSparc , PowerPC , MIPS ,etc., to various kinds of new-type peripheral hardwares, can from distribute on global numerous programmer there getting support rapidly too.(4) To that the hardware requires lower, can obtain very good performance on more low-grade machine , what deserves particular mention is Linux outstanding stability , permitted " year " count often its running times.2.Main application of Linux At present,Now, the application of Linux mainly includes:(1) Internet/Intranet: This is one that Linux was used most at present, it can offer and include Web server , all such Inter net services as Ftp server , Gopher server , SMTP/POP3 mail server , Proxy/Cache server , DNS server ,etc.. Linux kernel supports IPalias , PPP and IPtunneling, these functions can be used for setting up fictitious host computer , fictitious service , VPN (fictitious special-purpose network ) ,etc.. Operating Apache Web server on Linux mainly, the occupation rate of market in 1998 is 49%, far exceeds the sum of such several big companies asMicrosoft , Netscape ,etc..(2) Because Linux has outstanding networking ability , it can be usedin calculating distributedly large-scaly, for instance cartoon making , scientific caculation , database and file server ,etc..(3) As realization that is can under low platform fullness of Unix that operate , apply at all levels teaching and research work of universities and colleges extensively, if Mexico government announce middle and primary schools in the whole country dispose Linux and offer Internet service for student already.(4) Tabletop and handling official business appliedly. Application number of people of in this respect at present not so good as Windows of Microsoft far also, reason its lie in Lin ux quantity , desk-top of application software not so good as Windows application far not merely, because the characteristic of the freedom software makes it not almost have advertisement thatsupport (though the function of Star Office is not second to MS Office at the same time, but there are actually few people knowing).3.Can Linux become a kind of major operating system?In the face of the pressure of coming from users that is strengthened day by day, more and more commercial companies transplant its application to Linux platform, comparatively important incident was as follows, in 1998 ①Compaq and HP determine to put forward user of requirement truss up Linux at their servers , IBM and Dell promise to offer customized Linux system to user too. ②Lotus announce, Notes the next edition include one special-purpose edition in Linux. ③Corel Company transplants its famous WordPerfect to on Linux, and free issue.Corel also plans to move the other figure pattern process products to Linux platform completely.④Main database producer: Sybase , Informix , Oracle , CA , IBM have already been transplanted one's own database products to on Linux, or has finished Beta edition, among them Oracle and Informix also offer technical support to their products.4.The gratifying one is, some farsighted domestic corporations have begun to try hard to change this kind of current situation already. Stone Co. not long ago is it invest a huge sum of money to claim , regard Linux as platform develop a Internet/Intranet solution, regard this as the core and launch Stone's system integration business , plan to set up nationwide Linux technical support organization at the same time , take the lead to promote the freedom software application and development in China. In addition domestic computer Company , person who win of China , devoted to Linux relevant software and hardware application of system popularize too. Is it to intensification that Linux know , will have more and more enterprises accede to the ranks that Linux will be used with domestic every enterprise to believe, more software will be planted in Linux platform. Meanwhile, the domestic university should regard Linux as the original version and upgrade already existing Unix content of courses , start with analysing the source code and revising the kernel and train a large number of senior Linux talents, improve our country's own operating system. Having only really grasped the operating system, the software industry of our country could be got rid of and aped sedulously at present, the passive state led by the nose by others, create conditions for revitalizing the software industry of our country fundamentally.。
计算机毕业论文外文文献翻译中英文：IEEE802.11媒体接入控制
计算机毕业论⽂外⽂⽂献翻译中英⽂：IEEE802.11媒体接⼊控制英⽂资料与中⽂翻译IEEE 802.11 MEDIUM ACCESS CONTROLThe IEEE 802.11 MAC layer covers three functional areas：reliable data delivery, medium access control, and security. This section covers the first two topics.Reliable Data DeliveryAs with any wireless network, a wireless LAN using the IEEE 802.11 physical and MAC layers is subject to considerable unreliability. Noise, interference, and other propagation effects result in the loss of a significant number of frames. Even with error-correction codes, a number of MAC frames may not successfully be received. This situation can be dealt with by reliability mechanisms at a higher layer. such as TCP. However, timers used for retransmission at higher layers are typically on the order of seconds. It is therefore more efficient to deal with errors at the MAC level. For this purpose, IEEE 802.11 includes a frame exchange protocol. When a station receives a data frame from another station. It returns an acknowledgment (ACK) frame to the source station. This exchange is treated as an atomic unit, not to be interrupted by a transmission from any other station. If the source does not receive an ACK within a short period of time, either because its data frame was damaged or because the returning ACK was damaged, the source retransmits the frame.Thus, the basic data transfer mechanism in IEEE802.11 involves an exchange of two frames. To further enhance reliability, a four-frame exchange may be used. In this scheme, a source first issues a request to send (RTS) frame to the destination. The destination then responds with a clear to send (CTS). After receiving the CTS, the source transmits the data frame, and the destination responds with an ACK. The RTS alerts all stations that are within reception range of the source that an exchange is under way; these stations refrain from transmission in order to avoid a collision between two frames transmitted at the same time. Similarly, the CTS alerts all stations that are within reception range of the destination that an exchange is under way. The RTS/CTS portion of the exchange is a required function of the MAC but may be disabled.Medium Access ControlThe 802.11 working group considered two types of proposals for a MAC algorithm: distributed access protocols, which, like Ethernet, distribute the decision to transmit over all the nodes using a carrier-sense mechanism; and centralized access protocols, which involve regulation of transmission by a centralized decision maker. A distributed access protocol makes sense for an ad hoc network of peer workstations (typically an IBSS) and may also be attractive in other wireless LAN configurations that consist primarily of burst traffic. A centralized access protocol is natural for configurations in which a umber of wireless stations are interconnected with each other and some sort of base station that attaches to a backbone wired LAN: it is especially useful if some of the data is time sensitive or high priority.The end result for 802.11 is a MAC algorithm called DFWMAC (distributed foundation wireless MAC) that provides a distributed access control mechanism with an optional centralized control built on top of that. Figure 14.5 illustrates the architecture. The lower sub-layer of the MAC layer is the distributed coordination function (DCF). DCF uses a contention algorithm to provide access to all traffic. Ordinary asynchronous traffic directly uses DCE. The point coordination function (PCF) is a centralized MAC algorithm used to provide contention-free service. PCF is built on top of DCF and exploits features of DCF to assure access for its users. Let us consider these two sub-layers in turn.MAClayerFigure 14.5 IEEE 802.11 Protocol ArchitectureDistributed Coordination FunctionThe DCF sub-layer makes use of a simple CSMA (carrier sense multiple access) algorithm, which functions as follows. If a station has a MAC frame to transmit, it listens to the medium. If the medium is idle, the station may transmit; otherwise the station must wait until the current transmission is complete before transmitting. The DCF does not include a collision detection function (i.e. CSMA/CD) because collision detection is not practical on a wireless network. The dynamic range ofthe signals on the medium is very large, so that a transmitting station cannot effectively distinguish incoming weak signals from noise and the effects of its own transmission.To ensure the smooth and fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme. Let us start by considering a single delay known as an inter-frame space (IFS). In fact, there are three different IFS values, but the algorithm is best explained by initially ignoring this detail. Using an IFS, the rules for CSMA access are as follows (Figure 14.6):Figure 14.6 IEEE 802.11 Medium Access Control Logic1. A station with a frame to transmit senses the medium. If the medium is idle. It waits to see if the medium remains idle for a time equal to IFS. If so , the station may transmit immediately.2. If the medium is busy (either because the station initially finds the medium busy or because the medium becomes busy during the IFS idle time), the station defers transmission and continues to monitor the medium until the current transmission is over.3. Once the current transmission is over, the station delays another IFS. If the medium remains idle for this period, then the station backs off a random amount of time and again senses the medium. If the medium is still idle, the station may transmit. During the back-off time, if the medium becomes busy, the back-off timer is halted and resumes when the medium becomes idle.4. If the transmission is unsuccessful, which is determined by the absence of an acknowledgement, then it is assumed that a collision has occurred.To ensure that back-off maintains stability, a technique known as binary exponential back-off is used. A station will attempt to transmit repeatedly in the face of repeated collisions, but after each collision, the mean value of the random delay is doubled up to some maximum value. The binary exponential back-off provides a means of handling a heavy load. Repeated failed attempts to transmit result in longer and longer back-off times, which helps to smooth out the load. Without such a back-off, the following situation could occur. Two or more stations attempt to transmit at the same time, causing a collision. These stations then immediately attempt to retransmit, causing a new collision.The preceding scheme is refined for DCF to provide priority-based access by the simple expedient of using three values for IFS:●SIFS (short IFS):The shortest IFS, used for all immediate responseactions，as explained in the following discussion●PIFS (point coordination function IFS):A mid-length IFS, used by thecentralized controller in the PCF scheme when issuing polls●DIFS (distributed coordination function IFS): The longest IFS, used as aminimum delay for asynchronous frames contending for access Figure 14.7a illustrates the use of these time values. Consider first the SIFS.Any station using SIFS to determine transmission opportunity has, in effect, the highest priority, because it will always gain access in preference to a stationwaiting an amount of time equal to PIFS or DIFS. The SIFS is used in the following circumstances：●Acknowledgment (ACK): When a station receives a frame addressed onlyto itself (not multicast or broadcast) it responds with an ACK frame after, waiting on1y for an SIFS gap. This has two desirable effects. First, because collision detection IS not used, the likelihood of collisions is greater than with CSMA/CD, and the MAC-level ACK provides for efficient collision recovery. Second, the SIFS can be used to provide efficient delivery of an LLC protocol data unit (PDU) that requires multiple MAC frames. In this case, the following scenario occurs. A station with a multi-frame LLC PDU to transmit sends out the MAC frames one at a time. Each frame is acknowledged after SIFS by the recipient. When the source receives an ACK, it immediately (after SIFS) sends the next frame in the sequence. The result is that once a station has contended for the channel, it will maintain control of the channel until it has sent all of the fragments of an LLC PDU.●Clear to Send (CTS):A station can ensure that its data frame will getthrough by first issuing a small. Request to Send (RTS) frame. The station to which this frame is addressed should immediately respond with a CTS frame if it is ready to receive. All other stations receive the RTS and defer using the medium.●Poll response: This is explained in the following discussion of PCF.longer than DIFS(a) Basic access methoddefers(b) PCF super-frame constructionFigure 14.7 IEEE 802.11 MAC TimingThe next longest IFS interval is the: PIFS. This is used by the centralized controller in issuing polls and takes precedence over normal contention traffic. However, those frames transmitted using SIFS have precedence over a PCF poll.Finally, the DIFS interval is used for all ordinary asynchronous traffic.Point C00rdination Function PCF is an alternative access method implemented on top of the DCE. The operation consists of polling by the centralized polling master (point coordinator). The point coordinator makes use of PIFS when issuing polls. Because PI FS is smaller than DIFS, the point coordinator call seize the medium and lock out all asynchronous traffic while it issues polls and receives responses.As an extreme, consider the following possible scenario. A wireless network is configured so that a number of stations with time, sensitive traffic are controlled by the point coordinator while remaining traffic contends for access using CSMA. The point coordinator could issue polls in a round—robin fashion to all stations configured for polling. When a poll is issued, the polled station may respond using SIFS. If the point coordinator receives a response, it issues another poll using PIFS. If no response is received during the expected turnaround time, the coordinator issues a poll．If the discipline of the preceding paragraph were implemented, the point coordinator would lock out all asynchronous traffic by repeatedly issuing polls. To prevent this, an interval known as the super-frame is defined. During the first part of this interval, the point coordinator issues polls in a round, robin fashion to all stations configured for polling. The point coordinator then idles for the remainder of the super-frame, allowing a contention period for asynchronous access.Figure l4.7 b illustrates the use of the super-frame. At the beginning of a super-frame, the point coordinator may optionally seize control and issues polls for a give period of time. This interval varies because of the variable frame size issued by responding stations. The remainder of the super-frame is available for contention based access. At the end of the super-frame interval, the point coordinator contends for access to the medium using PIFS. If the medium is idle. the point coordinator gains immediate access and a full super-frame period follows. However, the medium may be busy at the end of a super-frame. In this case, the point coordinator must wait until the medium is idle to gain access: this result in a foreshortened super-frame period for the next cycle.OctetsFC=frame control SC=sequence controlD/I=duration/connection ID FCS=frame check sequence（a ） MAC frameBitsDS=distribution systemMD=more data MF=more fragmentsW=wired equivalent privacy RT=retryO=orderPM=power management (b) Frame control filedFigure 14.8 IEEE 802.11 MAC Frame FormatMAC FrameFigure 14.8a shows the 802.11 frame format when no security features are used. This general format is used for all data and control frames, but not all fields are used in all contexts. The fields are as follows:● Frame Control: Indicates the type of frame and provides contr01information, as explained presently.● Duration/Connection ID: If used as a duration field, indicates the time(in-microseconds) the channel will be allocated for successful transmission of a MAC frame. In some control frames, this field contains an association, or connection, identifier.●Addresses: The number and meaning of the 48-bit address fields dependon context. The transmitter address and receiver address are the MAC addresses of stations joined to the BSS that are transmitting and receiving frames over the wireless LAN. The service set ID (SSID) identifies the wireless LAN over which a frame is transmitted. For an IBSS, the SSID isa random number generated at the time the network is formed. For awireless LAN that is part of a larger configuration the SSID identifies the BSS over which the frame is transmitted: specifically, the SSID is the MAC-level address of the AP for this BSS (Figure 14.4). Finally the source address and destination address are the MAC addresses of stations, wireless or otherwise, that are the ultimate source and destination of this frame. The source address may be identical to the transmitter address and the destination address may be identical to the receiver address.●Sequence Control: Contains a 4-bit fragment number subfield used forfragmentation and reassembly, and a l2-bit sequence number used to number frames sent between a given transmitter and receiver.●Frame Body: Contains an MSDU or a fragment of an MSDU. The MSDUis a LLC protocol data unit or MAC control information.●Frame Check Sequence: A 32-bit cyclic redundancy check. The framecontrol filed, shown in Figure 14.8b, consists of the following fields.●Protocol Version: 802.11 version, current version 0.●Type: Identifies the frame as control, management, or data.●Subtype: Further identifies the function of frame. Table 14.4 defines thevalid combinations of type and subtype.●To DS: The MAC coordination sets this bit to 1 in a frame destined to thedistribution system.●From DS: The MAC coordination sets this bit to 1 in a frame leaving thedistribution system.●More Fragments: Set to 1 if more fragments follow this one.●Retry: Set to 1 if this is a retransmission of a previous frame.●Power Management: Set to]if the transmitting station is in a sleep mode.●More Data: Indicates that a station has additional data to send. Each blockof data may be sent as one frame or a group of fragments in multiple frames.●WEP：Set to 1 if the optional wired equivalent protocol is implemented.WEP is used in the exchange of encryption keys for secure data exchange.This bit also is set if the newer WPA security mechanism is employed, as described in Section 14.6.●Order：Set to 1 in any data frame sent using the Strictly Ordered service,which tells the receiving station that frames must be processed in order. We now look at the various MAC frame types. Control Frames Control frames assist in the reliable delivery of data frames. There are six control frame subtypes:●Power Save-Poll (PS-Poll): This frame is sent by any station to the stationthat includes the AP (access point). Its purpose is to request that the AP transmit a frame that has been buffered for this station while the station was in power saving mode.●Request to Send (RTS):This is the first frame in the four-way frameexchange discussed under the subsection on reliable data delivery at the beginning of Section 14.3.The station sending this message is alerting a potential destination, and all other stations within reception range, that it intends to send a data frame to that destination.●Clear to Send (CTS): This is the second frame in the four-way exchange.It is sent by the destination station to the source station to grant permission to send a data frame.●Acknowledgment:Provides an acknowledgment from the destination tothe source that the immediately preceding data, management, or PS-Poll frame was received correctly.●Contention-Free (CF)-End: Announces the end of a contention-freeperiod that is part of the point coordination function.●CF-End+CF-Ack:Acknowledges the CF-End. This frame ends thecontention-free period and releases stations from the restrictions associated with that period.Data Frames There are eight data frame subtypes, organized into two groups. The first four subtypes define frames that carry upper-level data from the source station to the destination station. The four data-carrying frames are as follows: ●Data: This is the simplest data frame. It may be used in both a contentionperiod and a contention-free period.●Data+CF-Ack: May only be sent during a contention-free period. Inaddition to carrying data, this frame acknowledges previously received data.●Data+CF-Poll: Used by a point coordinator to deliver data to a mobilestation and also to request that the mobile station send a data frame that it may have buffered.●Data+CF-Ack+CF-Poll: Combines the functions of the Data+CF-Ack andData+CF-Poll into a single frame.The remaining four subtypes of data frames do not in fact carry any user data. The Null Function data frame carries no data, polls, or acknowledgments. It is used only to carry the power management bit in the frame control field to the AP, to indicate that the station is changing to a low-power operating state. The remaining three frames (CF-Ack, CF-Poll,CF-Ack+CF-Poll) have the same functionality as the corresponding data frame subtypes in the preceding list (Data+CF-Ack, Data+CF-Poll,Data+CF-Ack+CF-Poll) but withotit the data. Management FramesManagement frames are used to manage communications between stations and APs. The following subtypes are included:●Association Request：Sent by a station to an AP to request an association,with this BSS. This frame includes capability information, such as whether encryption is to be used and whether this station is pollable.●Association Response:Returned by the AP to the station to indicatewhether it is accepting this association request.●Reassociation Request: Sent by a station when it moves from one BSS toanother and needs to make an association with tire AP in the new BSS. The station uses reassociation rather than simply association so that the new AP knows to negotiate with the old AP for the forwarding of data frames.●Reassociation Response:Returned by the AP to the station to indicatewhether it is accepting this reassociation request.●Probe Request: Used by a station to obtain information from anotherstation or AP. This frame is used to locate an IEEE 802.11 BSS.●Probe Response: Response to a probe request.●Beacon: Transmitted periodically to allow mobile stations to locate andidentify a BSS.●Announcement Traffic Indication Message: Sent by a mobile station toalert other mobile stations that may have been in low power mode that this station has frames buffered and waiting to be delivered to the station addressed in this frame.●Dissociation: Used by a station to terminate an association.●Authentication：Multiple authentication frames are used in an exchange toauthenticate one station to another.●Deauthentication:Sent by a station to another station or AP to indicatethat it is terminating secure communications.IEEE802.11 媒体接⼊控制IEEE 802．11 MAC层覆盖了三个功能区：可靠的数据传送、接⼊控制以及安全。
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毕业设计（论文）外文文献翻译
文献、资料中文题目：无线局域网
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专业：计算机科学与技术专业
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翻译日期： 2017.02.14
毕业设计(论文)外文资料翻译
外文出处：Chris Haseman. Android-essential
(用外文写)
s[M].London:Spring--Verlag,2008
.8-13.
附件： 1.外文资料翻译译文；2.外文原文。
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附件1：外文资料翻译译文
无线局域网
一、为何使用无线局域网络
对于局域网络管理主要工作之一，对于铺设电缆或是检查电缆是否断线这种耗时的工作，很容易令人烦躁，也不容易在短时间内找出断线所在。
再者，由于配合企业及应用环境不断的更新与发展，原有的企业网络必须配合重新布局，需要重新安装网络线路，虽然电缆本身并不贵，可是请技术人员来配线的成本很高，尤其是老旧的大楼，配线工程费用就更高了。
因此，架设无线局域网络就成为最佳解决方案。
二、什么情形需要无线局域网络
无线局域网络绝不是用来替代有限局域网络，而是用来弥补有线局域网络之不足，以达到网络延伸之目的，下列情形可能须要无线局域网络。
●无固定工作场所的使用者
●有线局域网络架设受环境限制
●作为有线局域网络的备用系统
三、无线局域网络存取技术
目前厂商在设计无线局域网络产品时，有相当多种存取设计方式，大致可分为三大类：窄频微波技术、展频(Spread Spectrum)技术、及红外线(Infrared)技术，每种技术皆有其优缺点、限制及比较，接下来是这些技术方法的详细探讨。
1.技术要求
由于无线局域网需要支持高速、突发的数据业务，在室内使用还需要解决多径衰落以及各子网间串扰等问题。
具体来说，无线局域网必须实现以下技术要求：
1)可靠性：无线局域网的系统分组丢失率应该低于10-5，误码率应该低
于10-8。
2)兼容性：对于室内使用的无线局域网，应尽可能使其跟现有的有线局
域网在网络操作系统和网络软件上相互兼容。
3)数据速率：为了满足局域网业务量的需要，无线局域网的数据传输速
率应该在1Mbps以上。
4)通信保密：由于数据通过无线介质在空中传播，无线局域网必须在不
同层次采取有效的措施以提高通信保密和数据安全性能。
5)移动性：支持全移动网络或半移动网络。
6)节能管理：当无数据收发时使站点机处于休眠状态，当有数据收发时
再激活，从而达到节省电力消耗的目的。
7)小型化、低价格：这是无线局域网得以普及的关键。
8)电磁环境：无线局域网应考虑电磁对人体和周边环境的影响问题。
Android是Google开发的基于Linux平台的开源手机操作系统（在华注册商标名为“安致”）。
它包括操作系统、用户界面和应用程序——移动电话工作所需的全部软件，而且不存在任何以往阻碍移动产业创新的专有权障碍，号称是首个为移动终端打造的真正开放和完整的移动软件。
Google与开放手机联盟合作开发了Android，这个联盟由包括中国移动、摩托罗拉、高通和T-Mobile在内的30多家技术和无线应用的领军企业组成。
Google通过与运营商、设备制造商、开发商和其他有关各方结成深层次的合作伙伴关系，希望借助建立标准化、开放式的移动电话软件平台，在移动产业内形成一个开放式的生态系统。
它采用了软件堆层(software stack，又名软件叠层)的架构，主要分为三部分：底层以Linux核心为基础，由c语言开发，只提供基本功能。
中间层包括函数库Library和虚拟机Virtual Machine，由c++语言开发。
最上层是各种应用软件，包括通话程序，短信程序等，应用软件则由各公司自行开发，以Java 编写。
为了推广此技术，Google和其它几十个手机公司建立了开放手机联盟（Open Handset Alliance）。
2.特性
●应用程序框架：支持组件的重用与替换
●Dalvik虚拟机：专门为移动设备做了优化
●内部集成浏览器：该浏览器基于开源的WebKit引擎
●优化的图形库：包括2D和3D图形库，3D图形库基于OpenGL ES 1.0
（硬件加速可选）
●#SQLite：用作结构化的数据存储
●多媒体支持：包括常见的音频、视频和静态印象文件格式（如MPEG4，
H.264，MP3，AAC，AMR，JPG，PNG，GIF）
●GSM电话（依赖于硬件）
●蓝牙Bluetooth，EDGE，3G和WiFi（依赖于硬件）
●照相机，GPS，指南针和加速度计（依赖于硬件）
●丰富的开发环境：包括设备模拟器，调试工具，内存及性能分析图表，
和Eclipse集成开发环境插件
3.应用程序
Android会同一个核心应用程序包一起发布，该应用程序包包括email客户端，SMS短消息程序，日历，地图，浏览器，联系人管理程序等。
所有的应用程序都是用Java编写的。
Android应用程序框架开发者也完全可以访问核心应用程序所使用的API 框架。
该应用程序架构用来简化组件软件的重用，任何一个应用程序都可以发布它的功能块并且任何其它的应用程序都可以使用其所发布的功能块（不过得遵循框架的安全性限制）。
该应用程序重用机制使得组建可以被用户替换。
以下所有的应用程序都由一系列的服务和系统组成，包括：
●一个可扩展的视图（Views）可以用来建应用程序，包括列表（lists），
网格（grids），文本框（text boxes），按钮（buttons），甚至包括一
个可嵌入的web浏览器
●内容管理器（Content Providers）使得应用程序可以访问另一个应用程
序的数据（如联系人数据库），或者共享它们自己的数据。
●一个资源管理器（Resource Manager）提供非代码资源的访问，如本地
字符串，图形，和分层文件（layout files）。
●一个通知管理器（Notification Manager）使得应用程序可以在状态栏中
显示客户通知信息。
●一个活动类管理器（Activity Manager）用来管理应用程序生命周期并
提供常用的导航回退功能。
4.点餐系统
点餐系统利用软件的全自动信息化手段，实现点菜、配菜、炒菜到传菜的全部管理过程；完成点菜单计算机管理、点菜单前端收银台与各厨房的同步打印；实现菜单无纸化操作；后台经理查询；仓库进销存管理等。
除此之外，点餐系统还可以对客户资料进行有效的管理，存档和备查，杜绝酒楼“漏单”、“跑单”现象；帮助酒楼利用计算机强大的数据处理能力和流程优化能力，实现自动化管理，简化酒楼的工作流程，减少浪费及人为管理的疏漏现象，重新优化配置企业资源，把经营成本降低到最低。
功能强大的点餐系统除支持一般的单机和局域网应用之外，还支持总店/分店多级架构的远程联网应用于分店使用POS系统实现前台销售收银、即时打印销售小票、销售日结、上报销售数据和接收新菜品信息等功能。
目前点餐系统共有三种实现模式：
1)触摸屏点菜模式：它利用了目前最为流行的触摸式电脑实现点菜过程，
即点菜员可以根据软件的画面提示，只需用手指点击画面就可以完成
整个点菜过程，方便快捷，这种模式适用于菜式和做法丰富大型的酒
楼、酒家和菜馆等。
2)无线PDA点菜模式：它利用了无线WiFi技术，通过PDA显示点菜界
面，使用触摸笔完成点菜过程，真正实现随时随地点菜实时响应，这
种模式适用于菜式和做法较为简单的西餐厅、特色菜馆和特具情调的
中高级餐厅等。
3)无线点菜宝模式：它使用了ISM频段，可以在有楼层或其它障碍物阻
挡的情况下无缝覆盖达10米之远，而信号依然稳定，这是其比之无线
PDA点菜模式最大的优势，这种模式适用于菜式和做法较为简单及其
它要求较少的快餐店、火锅店等。