计算机网络-chapter5
计算机网络 Chapter_5_V6.01
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datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link each link protocol provides different services e.g., may or may not provide rdt over link
datagram controller datagram controller
sending host
datagram
receiving host
frame
sending side: encapsulates datagram in frame adds error checking bits, rdt, flow control, etc.
• Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction
otherwise
Link Layer 5-11
transportation analogy:
trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne
tourist = datagram transport segment = communication link transportation mode = link layer protocol travel agent = routing algorithm
计算机网络技术第5章计算机网络基础精品PPT课件
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工作站
服务器
• 优点:最少的电缆,易于安装和扩展;成本较低。
• 缺点:信息传输依赖于每一节点的正常工作;信号 在总线上衰减使信号减弱,而且信息沿公用线路传 送,正确性和安全性较差;另外故障检测困难,可 能需要检查全网的连接。
环型结构
工作站
服务器 工作站
Token Ring
数据流在计算机之间单向传输
高速互联网络阶段
20世纪90年代中至今
计算机—终端网络阶段
数据处理和通信控制分开,专门用于负责与各 终端之间的通信控制
前端处理机 FEP(Front End Processor)
通信控制器CCU(Communication Control近U程线低n路速it)终端
远程高速线路 Modem
近线程路低速
集中器或复用器
Modem
终端
中心计算机
FEP或CUU
远程高速线路
Modem
Modem
近程低速 线路
集中器或复用器 近线程路低速
终端
终端
计算机—计算机网络阶段
多台主计算机不直接相连
由接口报文处理机IMP(Interface Message Processor)转接后通过通信线路互联
H H
IMP IMP
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备
校验
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接
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发
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收
串行到并行 的转换
并行到串行
的转换
5
送
设
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设 3
2
备
1
45678
2
备 1
同步与异步传输
保证数据的准确和完整传输,发送方和接收方 必须保持同步
异步传输
每个字符前后增加“起”、“止”符号
计算机网络英文题库(附答案)chapter5
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Chapter 5 The Link Layer and Local Area Network1.A ( ) protocol is used to move a datagram over an individual link.A application-layerB transport-layerC network-layerD link-layer2.The units of data exchanged by a link-layer protocol are called ( ).A datagramsB framesC segmentsD messages3.Which of the following protocols is not a link-layer protocol? ( )A EthernetB PPPC HDLCD IP4.In the following four descriptions, which one is not correct? ( )A link-layer protocol has the node-to-node job of moving network-layer datagrams over a single link in the path.B The services provided by the link-layer protocols may be different.C A datagram must be handled by the same link-layer protocols on the different links in the path.D The actions taken by a link-layer protocol when sending and receiving frames include error detection, flow control and random access.5.Which of the following services can not offered by a link-layer protocol? ( )A congestion controlB Link AccessC Error controlD Framing6.( ) protocol serves to coordinate the frame transmissions of the many nodes when multiple nodes share a single broadcast link.A ARPB MACC ICMPD DNS7.In the following four descriptions about the adapter, which one is not correct? ( )A The adapter is also called as NIC.B The adapter is a semi-autonomous unit.C The main components of an adapter are bus interface and the link interface.D The adapter can provide all the link-layer services.8.Consider CRC error checking approach, the four bit generator G is 1011, and suppose that the data D is 10101010, then the value of R is( ).A 010B 100C 011D 1109.In the following four descriptions about random access protocol, which one is not correct? ( )A I n slotted ALOHA, nodes can transmit at random time.B I n pure ALOHA, if a frame experiences a collision, the node will immediately retransmit it with probability p.C T he maximum efficiency of a slotted ALOHA is higher than a pure ALOHA.D I n CSMA/CD, one node listens to the channel before transmitting.10.In the following descriptions about MAC address, which one is not correct? ( )A T he MAC address is the address of one node’s adapter.B N o two adapters have the same MAC address.C T he MAC address doesn’t change no matter where the adapter goes.D M AC address has a hierarchical structure.11.The ARP protocol can translate ( ) into ( ). ( )A h ost name, IP addressB h ost name, MAC addressC I P address, MAC addressD broadcast address, IP address12.The value of Preamble field in Ethernet frame structure is ( )A 10101010 10101010……10101010 11111111B 10101011 10101011……10101011 10101011C 10101010 10101010……10101010 10101011D 10101010 10101010……10101010 1010101013.There are four steps in DHCP, the DHCP server can complete ( ).A DHCP server discoveryB DHCP server offersC DHCP requestD DHCP response14.In CSMA/CD, the adapter waits some time and then returns to sensing the channel. In the following four times, which one is impossible? ( )A 0 bit timesB 512 bit timesC 1024 bit timesD 1028 bit times15.The most common Ethernet technologies are 10BaseT and 100BaseT. “10” and “100” indicate( ).A the maximum length between two adaptersB the minimum length between two adaptersC the transmission rate of the channelD the transmission rate of the node16.The principal components of PPP include but not( ).A framingB physical-control protocolC link-layer protocolD network-layer protocol17.In the following four options, which service can not be provided by switch? ( )A filteringB self-learningC forwardingD optimal routing18.In the following four services, which one was be required in PPP? ( )A packet framingB error detectionC error correctionD multiple types of link19.The ability to determine the interfaces to which a frame should be directed, and then directing the frame to those interfaces is( ).A filteringB forwardingC self-learningD optimal routing20.In ( ) transmission(s), the nodes at both ends of a link may transmit packets at the same time.A full-duplexB half-duplexC single-duplexD both full-duplex and half-duplex21.Consider the data D is 01110010001, if use even parity checking approach, the parity bit is( ① ), if use odd parity checking approach, the parity bit is( ② ). ( )A ①0 ②1B ①0 ②0C ①1 ②1D ①1 ②022.In the following four descriptions about parity checks, which one is correct? ( )A Single-bit parity can detect all errors.B Single-bit parity can correct one errors.C Two-dimensional parity not only can detect a single bit error, but also can correct that error.D Two-dimensional parity not only can detect any combination of two errors, but also can correct them.23.MAC address is ( ) bits long.A 32B 48C 128D 6424.Wireless LAN using protocol ( ).A IEEE 802.3B IEEE 802.4C IEEE 802.5D IEEE 802.1125.The following protocols are belonging to multiple access protocols except for ( ).A channel partitioning protocolsB routing protocolsC random access protocolsD taking-turns protocols26.Which of the following is not belonging to channel partitioning protocols? ( )A CSMAB FDMC CDMAD TDM27.In the following four descriptions about CSMA/CD, which one is not correct? ( )A A node listens to the channel before transmitting.B If someone else begins talking at the same time, stop talking.C A transmitting node listens to the channel while it is transmitting.D With CSMA/CD, the collisions can be avoided completely.28.( ) provides a mechanism for nodes to translate IP addresses to link-layer address.A IPB ARPC RARPD DNS29.A MAC address is a ( )address.A physical-layerB application-layerC link-layerD network-layer30.Which of the following is correct? ( )A No two adapters have the same MAC address.B MAC broadcast address is FF-FF-FF-FF-FF-FF.C A portable computer with an Ethernet card always has the same MAC address, no matter where the computer goes.D All of the above31.In the following four descriptions, which one is not correct? ( )A ARP resolves an IP address to a MAC address.B DNS resolves hostnames to IP addresses.C DNS resolves hostnames for hosts anywhere in the Internet.D ARP resolves IP addresses for nodes anywhere in the Internet.32.In the LAN, ( )protocol dynamically assign IP addresses to hosts.A DNSB ARPC DHCPD IP33.DHCP protocol is a four-step process: ①DHCP request. ②DHCP ACK. ③DHCP server discovery. ④DHCP server offer(s). The correct sequence is ( )A ①②③④B ③②①④C ③④①②D ①④③②34.In the Ethernet frame structure, the CRC field is ( )bytes.A 2B 4C 8D 3235.In the Ethernet frame structure, the Data field carries the ( ).A IP datagramB segmentC frameD message36.In the following four descriptions, which one is not correct? ( )A Ethernet uses baseband transmission.B All of the Ethernet technologies provide connection-oriented reliable service to the network layer.C The Ethernet 10Base2 technology uses a thin coaxial cable for the bus.D The Ethernet 10BaseT technology uses a star topology.37.Ethernet’s multiple access protocol is ( ).A CDMAB CSMA/CDC slotted ALOHAD token-passing protocol38.In the following four descriptions about CSMA/CD, which one is not correct? ( )A An adapter may begin to transmit at any time.B An adapter never transmits a frame when it senses that some other adapter is transmitting.C A transmitting adapter aborts its transmission as soon as it detects that another adapter is also transmitting.D An adapter retransmits when it detects a collision.39.Which of the following descriptions about CSMA/CD is correct? ( )A No slots are used.B It uses carrier sensing.C It uses collision detection.D All of the above.40.The Ethernet 10BaseT technology uses( )as its physical media.A fiber opticsB twisted-pair copper wireC coaxial cableD satellite radio channel41.For 10BaseT, the maximum length of the connection between an adapter and the hub is ( )meters.A 100B 200C 500D 1042.A ( )is a physical-layer device that acts on individual bits rather than on frames.A switchB hubC routerD gateway43.A hub is a ( )device that acts on individual bits rather than on frames.A physical-layerB link-layerC network-layerD ransport-layer44.A switch is a( )device that acts on frame.A physical-layerB link-layerC network-layerD transport-layer45.In the following four descriptions, which one is not correct? ( )A Switches can interconnect different LAN technologies.B Hubs can interconnect different LAN technologies.C There is no limit to how large a LAN can be when switches are used to interconnect LAN segments.D There is restriction on the maximum allowable number of nodes in a collision domain when hubs are used to interconnect LAN segments.46.The ability to determine whether a frame should be forwarded to some interface or should just be dropped is ( ).A f ilteringB f orwardingC s elf-learningD o ptimal routing47.Which of the following devices is not a plug and play device? ( )A hubB routerC switchD repeater48.Which of the following devices is not cut-through device? ( )A hubB routerC switchD repeater49.In the following four descriptions, which one is not correct? ( )A Switches do not offer any protection against broadcast storms.B Routers provide firewall protection against layer-2 broadcast storms.C Both switches and routers are plug and play devices.D A router is a layer-3 packet switch, a switch is a layer-2 packet switch. 50.Which device has the same collision domain? ( )A HubB SwitchC RouterD Bridge51.IEEE802.2 protocol belong to ( )layerA networkB MACC LLCD physical52.IEEE802.11 protocol defines ( )rules.A Ethernet BusB wireless WANC wireless LAND Token Bus53.In data link-layer, which protocol is used to share bandwidth? ( )A SMTPB ICMPC ARPD CSMA/CD54.When two or more nodes on the LAN segments transmit at the same time, there will be a collision and all of the transmitting nodes well enter exponential back-off, that is all of the LAN segments belong to the same( ).A collision domainB switchC bridgeD hub55.( )allows different nodes to transmit simultaneously and yet have their respective receivers correctly receive a sender’s encoded data bits.A CDMAB CSMAC CSMA/CDD CSMA/CA56.Because there are both network-layer addresses (for example, Internet IP addresses) and link-layer addresses (that is, LAN addresses), there is a need totranslate between them. For the Internet, this is the job of ( ).A RIPB OSPFC ARPD IP57.PPP defines a special control escape byte, ( ). If the flag sequence, 01111110 appears anywhere in the frame, except in the flag field, PPP precedes that instance of the flag pattern with the control escape byte.A 01111110B 01111101C 10011001D 1011111058.The device ( ) can isolate collision domains for each of the LAN segment.A modemB switchC hubD NIC59.In the following four descriptions about PPP, which one is not correct? ( )A PPP is required to detect and correct errors.B PPP is not required to deliver frames to the link receiver in the same order in which they were sent by the link sender.C PPP need only operate over links that have a single sender and a single receiver.D PPP is not required to provide flow control.60.In the PPP data frame, the( ) field tells the PPP receivers the upper-layer protocol to which the received encapsulated data belongs.A flagB controlC protocolD checksum61.PPP’s link-control protocols (LCP) accomplish ( ).A initializing the PPP linkB maintaining the PPP linkC taking down the PPP linkD all of the above62.The PPP link always begins in the ( ) state and ends in the ( ) state. ( )A open, terminatingB open, deadC dead, deadD dead, terminating63.For( ) links that have a single sender at one end of the link and a single receiver at the other end of the link.A point-to-pointB broadcastC multicastD all of the above64.With ( )transmission, the nodes at both ends of a link may transmit packets at the same time.A half-duplexB full-duplexC simplex(单工)D synchronous65.With ( ) transmission, a node can not both transmit and receive at the same time.A half-duplexB full-duplexC simplex(单工)D synchronous66.Which of the following functions can’t be implemented in the NIC? ( )A encapsulation and decapsulationB error detectionC multiple access protocolD routing67.Which of the following four descriptions is wrong? ( )A The bus interface of an adapter is responsible for communication with the adapter’s parent node.B The link interface of an adapter is responsible for implementing the link-layer protocol.C The bus interface may provide error detection, random access functions.D The main components of an adapter are the bus interface and the link interface. 68.For odd parity schemes, which of the following is correct? ( )A 011010001B 111000110C 110101110D 00011011069.( )divides time into time frames and further divides each time frame into N time slots.A FDMB TMDC CDMAD CSMA70.With CDMA, each node is assigned a different ( )A codeB time slotC frequencyD link71.Which of the following four descriptions about random access protocol is not correct? ( )A A transmission node transmits at the full rate of the channelB When a collision happens, each node involved in the collision retransmits at once.C Both slotted ALOHA and CSMA/CD are random access protocols.D With random access protocol, there may be empty slots.72.PPP defines a special control escape byte 01111101. If the data is b1b201111110b3b4b5, the value is( )after byte stuffing.A b1b20111110101111110b3b4b5B b1b20111111001111101b3b4b5C b5b4b30111111001111101b2b1D b5b4b30111110101111110b2b173.MAC address is in ( ) of the computer.A RAMB NICC hard diskD cache74.Which of the following is wrong? ( )A ARP table is configured by a system administratorB ARP table is built automaticallyC ARP table is dynamicD ARP table maps IP addresses to MAC addresses75.NIC works in ( )layer.A physicalB linkC networkD transport76.In LAN, if UTP is used, the common connector is( ).A AUIB BNCC RJ-45D NNI77.The modem’s function(s) is(are) ( ).A translates digital signal into analog signalB translates analog signal into digital signalC both translates analog signal into digital signal and translates digital signal into analog signalD translates one kind of digital signal into another digital signal78.( )defines Token-Ring protocol.A IEEE 802.3B IEEE 802.4C IEEE 802.5D IEEE 802.279.( )defines Token-Bus protocol.A IEEE 802.3B IEEE 802.4C IEEE 802.5D IEEE 802.280.( ) defines CSMA/CD protocol.A IEEE 802.3B IEEE 802.4C IEEE 802.5D IEEE 802.281.The computer network that concentrated in a geographical area, such as in a building or on a university campus, is ( )A a LANB a MANC a WAND the Internet82.The MAC address is ( ) bits long.A 32B 48C 128D 25683.Which of the following four descriptions about MAC addresses is wrong? ( )A a MAC address is burned into the adapter’s ROMB No two adapters have the same addressC An adapter’s MAC address is dynamicD A MAC address is a link-layer address84.Which of the following four descriptions about DHCP is correct? ( )A DHCP is C/S architectureB DHCP uses TCP as its underlying transport protocolC The IP address offered by a DHCP server is valid foreverD The DHCP server will offer the same IP address to a host when the host requests an IP address85.The ( )field permits Ethernet to multiplex network-layer protocols.A preambleB typeC CRCD destination MAC address86.For 10BaseT, the maximum length of the connection between an adapter and the hub is ( ) meters.A 50B 100C 200D 50087.An entry in the switch table contains the following information excepts for ( )A the MAC address of a nodeB the switch interface that leads towards the nodeC the time at which the entry for the node was placed in the tableD the IP address of a node88.Consider the 4-bit generator , G is 1001, and suppose that D has the value 101110000. What is the value of R?89.Consider the following graph of the network. Suppose Host A will send a datagram to Host B, Host A run OICQ on port 4000, Host B run OICQ on port 8000. All of ARP tables are up to date. Enumerate all the steps when message “Hello” is sent from host A to host B.。
计算机网络基础(第五章)ppt课件
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非对等结构网络操作系统
网络服务器
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工作站
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网络服务
网络操作系统除了应具有一般操作系统 的进程管理,存储管理,文件管理和设 备管理等功能外,还可提供高效可靠的 通信能力和多种网络服务功能。
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常见的网络服务
文件服务(file service) 打印服务(print service) 数据库服务(database service) 通信服务(communication service) 信息服务(message service) 分布式服务(distributed service) 网络管理服务(network management service) Internet/Intranet服务(Internet/Intranet service)
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典型的网络操作系统
Microsoft公司的Windows系列操作系统 UNIX Linux Novell公司的NetWare
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Windows系列操作系统
Windows NT是Microsoft公司最具有代表 性的网络操作系统。
Windows 2000又称为Windows NT 5.0, 包括:
通用型网络操作系统又分为两类:
变形级系统:在原来的单机操作系统上增加网 络服务功能;
基础级系统:以计算机硬件为基础,根据网络 服务的特殊要求,直接利用计算机硬件与少量 软件资源专门设计的网络操作系统。
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网络操作系统的分类
从网络操作系统的结构分: 对等结构:所有结点平等; 特点:
结构简单,任何结点间均能直接通信; 每一结点既是工作站,又是服务器, 加重计算机负荷,适于小规模系统; 不便于整体管理,不安全。
计算机网络_第5章习题答案
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第五章练习题答案5.1 网络互连有何实际意义?进行网络互连时,有哪些共同的问题需要解决?答:网络互连使得相互连接的网络中的计算机之间可以进行通信,也就是说从功能上和逻辑上看,这些相互连接的计算机网络组成了一个大型的计算机网络。
网络互连可以使处于不同地理位置的计算机进行通信,方便了信息交流,促成了当今的信息世界。
需要解决的问题有:不同的寻址方案;不同的最大分组长度;不同的网络介入机制;不同的超时控制;不同的差错恢复方法;不同的状态报告方法;不同的路由选择技术;不同的用户接入控制;不同的服务(面向连接服务和无连接服务);不同的管理与控制方式;等等。
注:网络互连使不同结构的网络、不同类型的机器之间互相连通,实现更大范围和更广泛意义上的资源共享。
5.2 转发器、网桥和路由器都有何区别?答:1)转发器、网桥、路由器、和网关所在的层次不同。
转发器是物理层的中继系统。
网桥是数据链路层的中继系统。
路由器是网络层的中继系统。
在网络层以上的中继系统为网关。
2)当中继系统是转发器或网桥时,一般并不称之为网络互连,因为仍然是一个网络。
路由器其实是一台专用计算机,用来在互连网中进行路由选择。
一般讨论的互连网都是指用路由器进行互连的互连网络。
5.3 试简单说明IP、ARP、RARP和ICMP协议的作用。
答:IP:网际协议,TCP/IP 体系中两个最重要的协议之一,IP 使互连起来的许多计算机网络能够进行通信。
无连接的数据报传输. 数据报路由。
ARP(地址解析协议)实现地址转换,将IP地址映射成物理地址。
RARP(逆向地址解析协议)将物理地址映射成IP 地址。
ICMP:Internet 控制消息协议,进行差错控制和传输控制,减少分组的丢失。
注:ICMP 协议帮助主机完成某些网络参数测试,允许主机或路由器报告差错和提供有关异常情况报告,但它没有办法减少分组丢失,这是高层协议应该完成的事情。
IP 协议只是尽最大可能交付,至于交付是否成功,它自己无法控制。
《计算机网络》第五章
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第五章网络层网络层——端到端数据传输的最低层●网络层负责把源计算机发出的信息分组经过适当的路径送到目的计算机。
●网络层需要了解通信子网的拓扑结构,选择合适的传输路径。
●网络层要预防和控制通信子网中超量的信息分组造成的拥塞。
●网络层还要处理不同网络中源端和目的端之间的差异。
§5.1 网络层设计的有关问题●对网络层所提供的服务存在着两种观点:–面向连接的服务,复杂的功能放在通信子网中。
–无连接服务,复杂的功能放在主机中。
网络层的内部结构●网络层提供的服务是否可靠与有无连接并没有关系。
理论上存在四种组合,但最重要的只是其中的两种组合:–可靠的面向连接服务。
–不可靠的无连接服务。
●针对两种服务,网络层有两种不同的工作方式:–虚电路(virtual circuit)方式。
当建立连接时,从信源到信宿的路由就作为连接建立的一部分加以保存,此路由用于传输连接上的所有数据,当释放连接时,虚电路也随之撤消。
–数据报(datagrams)方式。
即使提供面向连接的服务也不预先选择路由,发出的每个分组所选择的路由独立于其前面发出的分组,后续的分组可以走不同的路由,比虚电路方式更健壮,更容易处理传送失败和拥塞。
§5.2 路由选择算法●路由选择是网络层的主要功能,负责为分组选择合适的转发路径。
●路由选择算法应具有以下几种特征:正确性(correctness)、简单性(simplicity)、健壮性(robustness)、稳定性(stability)、公平性(fairness)和最优性(optimality)。
●一个好的路由选择算法是兼顾某几种重要的性能指标。
路由选择算法的分类●分为两大类:–非自适应算法(non-adaptive algorithm):也叫静态路由选择(staticrouting),预先离线计算好路由表,在网络启动时装入到路由器中,在网络运行过程中不会根据网络流量和拓扑结构的变化而改变。
计算机网络课件-第5章
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无线局域网的组成 无线局域网由无线网卡、接入点 (AP)和网络管理软件等组成。
无线局域网的协议标准 无线局域网的协议标准包括IEEE 802.11系列标准,其中最常用的 是IEEE 802.11n和IEEE 802.11ac。
虚拟局域网技术
虚拟局域网技术概述
虚拟局域网(VLAN)是一种将物理局域 网逻辑划分为多个子网的技术,可以实现
功能
每个层次都有特定的功能,如数据传 输、数据封装、路由选择等,各层次 间通过协议进行通信。
结构
OSI参考模型分为七个层次,从下到上 分别是物理层、数据链路层、网络层、 传输层、会话层、表示层和应用层。
TCP/IP模型
定义
TCP/IP(Transmission Control Protocol/Internet Protocol) 模型是一个用于描述计算机网络 协议的分层体系结构的标准。
04 广域网技术
公共交换电话网
概述
01
公共交换电话网(PSTN)是一种基于模拟信号的广域网技术,
通过电话线路进行数据传输。
工作原理
02
PSTN使用模拟信号传输语音,通过调制解调器将数字信号转换
为模拟信号进行传输,在接收端再还原为数字信号。
特点
03
PSTN具有覆盖范围广、接入便利等优点,但传输速率较慢,不
传输方式
传输方式包括串行传输和并行传 输两种,其中串行传输是指数据 一位一位地顺序传输,而并行传 输是指数据多位同时传输。
03 局域网技术
以太网技术
以太网技术概述
以太网帧结构
以太网是一种基于CSMA/CD(载波监听多 路访问/冲突检测)技术的局域网标准,具有 高带宽和低成本的特点。
05计算机网络第五章
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《网络技术基础》课件
5.3.2 应用服务器
《网络技术基础》课件
5.3 网络服务器的种类
在计算机网络中,随着网络规模的扩 大,可能需要多台服务器来处理客户机的 服务请求,且有可能每台服务器完成的任 务各不相同。因此,根据服务器所完成的 任务不同,可以将服务器分为以下几类: 文件服务器、应用服务器和特殊服务器。
《网络技术基础》课件
5.3.1 文件服务器
通常,在客户机和应用服务器上都运 行有应用程序。客户机运行本地的程序, 向服务器发出服务请求,要求服务器对某 个数据进行处理,而服务器会将处理后的 信息返送给客户机。通过这种方法,客户 机几乎不处理信息,所有的任务都由服务 器来处理。
《网络技术基础》课件
5.3.3 特殊服务器
在计算机网络中,还会有一些具有特殊用途的 服务器,例如: 邮件服务器 Web服务器 通信服务器 视频服务器
《网络技术基础》课件
5.4.4
Linux操作系统
1. Linux操作系统的发展
Linux操作系统虽然与UNIX操作系统类似,但它并不 是UNIX操作系统的变种。Torvalds从开始编写内核代码时 就仿效UNIX,几乎所有UNIX的工具与外壳都可以运行在 Linux上。因此,熟悉UNIX操作系统的人能很容易掌握 Linux。Torvalds将源代码放在芬兰最大的FTP站点上,他 们认为这套系统是“Linus”的“Minix”,因此就建了一个 Linux子目录来存放这些源代码,结果Linux这个名字就此 使用起来。
计算机网络第五单元讲义
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Computer Networks
5.6
运输层为相互通信的应用进程提供了逻辑通信
5AP1 AP2 4 3 2 1
应用进程
应用进程
端口 运输层提供应用进程间的逻辑通信 端口
IP 层
AP3 AP45 4 3 2 1
主机 A AP1 AP2
Computer Networks
路由器 1
路由器 2
LAN1
WAN
LAN2
IP 协议的作用范围 运输层协议 TCP 和 UDP 的作用范围
5.7
主机 B
AP3 AP4
应用进程之间的通信
Computer Networks
5.4
5.1 运输层协议概述
5.1.1 进程之间的通信 5.1.2 运输层的两个主要协议 5.1.3 运输层的端口
ቤተ መጻሕፍቲ ባይዱ
Computer Networks
5.5
5.1.1 进程之间的通信
运输层的作用
运输层为相互通信的应用进程提供了逻辑通信 从通信和信息处理的角度看,运输层向它上面的应用层提供 通信服务,它属于面向通信部分的最高层,同时也是用户功 能中的最低层。
Computer Networks
5.13
服务类型
UDP提供无连接服务 UDP 在传送数据之前不需要先建立连接 对方的运输层在收到 UDP 报文后,不需要给出任何确认 UDP 不提供可靠交付,但在某些情况下 UDP 很有效。
TCP 则提供面向连接的服务 在传送数据之前必须建立连接,传送结束后要释放连接 TCP 不提供广播或多播服务 开销大 由于 TCP 要提供可靠的、面向连接的运输服务,因此不可避 免地增加了许多的开销。这不仅使协议数据单元的首部增大 很多,还要占用许多的处理机资源。
计算机网络chapter5
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sending host
datagram
receiving host
frame
sending side: encapsulates datagram in frame adds error checking bits, rdt, flow control, etc.
receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side
5: DataLink Layer 5-9
Link Layer
host bus (e.g., PCI)
attaches into host’s
system buses combination of hardware, software, firmware
network adapter card
5: DataLink Layer
5-8
Adaptors Communicating
instantiation and implementation of various link
layer technologies
5: DataLink Layer
5-2
Link Layer
5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet
计算机网络第五章.
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?
端口的概念:
是应用层的各种协议进程与传输层实体间进行
交互的依据(一种逻辑地址),用来标识不同 的应用进程; 端口是用一个16bit的端口号标识,端口号只有 本地意义,只标识本计算机上的应用层进程。 因特网中不同计算机的相同端口号是没有联系 的。
11
6.1 概述(续7)
端口号的分类:
熟知端口(或系统端口):TCP/IP最重要的一些应用服务
每一条TCP连接唯一地被通信两端的两个端点所确定。两
个连接的任何一端的端点不同,则表示两个不同的连接。
单一主机内的多个进程可以通过不同的端口号同时使用
TCP通信服务,某个TCP端口号可以被多个连接所共享, 如服务器上的各熟知端口。
22
6.4 可靠传输协议工作原理
基于以下条件考虑问题: 信道不是理想的,传输过程中会产生差错; 同时需要对数据的发送端进行流量控制,接收方接 收数据时的状况是复杂的。
都是可靠的全双工逻辑通道;
前者是在传输层建立的端到端的抽象逻辑信道,路由器
并不知道其中传输的信息,是否建立连接等;而虚电路 的是在网络层建立的,需要在路由器中记录其相关状态 信息,用来传输IP数据报;
网络层连接的改变不会影响TCP连接。
10
6.1 概述(续6)
如何标识通信双方的进程(必须与特定OS无关)
3
2 1
主机 A
主机 B 路由器 1 LAN1 WAN IP 协议的作用范围 运输层协议 TCP 和 UDP 的作用范围
4
AP1 AP2
路由器 2 LAN2
AP3
AP4
6.1 概述(续1)
主机对主机的通信和进程对进程的通信
两主机通信实际是主机中的应用进程互相通信; 应用进程之间的通信又称为端到端的通信。 IP协议用于实现计算机级的逻辑通信(主机-主机
计算机网络第五章
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计算机网络第五章第五章网络层5.1网络层设计问题网络层任务:存储转发数据包互换,向传输层提供更多服务。
紧紧围绕这一任务,在设计网络层时,internet社团与电话公司为代表的社团进行了争议,internet社团指出:网络必须就是不可信的,错误掌控与流量掌控均必须由主机顺利完成,电话公司阵营任务:网络必须就是可信的,面向相连接的服务。
并无相连接服务的同时实现:ip(internetprotocol)就是整个internet的基础,每个数据包随身携带一个目标的ip地址,路由器采用该地址去单独留言每一个数据包。
面向连接服务的实现:存在一个重要过程标签交换(labelswitching)。
例子:多协议标签交换(mpls,multiprotocollabelswitching),主要用于internet的isp网络,mpls往往对客户是隐藏的,isp用它来为超大流量建立长期的连接。
备注:没绝对一说道,中庸之道就是很存有道理的,什么东东都会存有用武之地,不要那么极端嘛!5.2路由算法路由算法就是网络层的重中之重,(在书中的第一部分就说道了,本章高度关注的就是算法,所以我还买了一本《算法导论》),它就是网络层软件的一部分,它负责管理确认一个入境数据包必须被发送到哪个输出线路上,因此路由和留言这就是两个功能。
路由算法必须满足用户的特性:正确性,直观性,鲁棒性,稳定性,公平性和有效性!(这里面存有相互矛盾的目标)。
算法分类:(1)非自适应性算法(nonadaptivealgorithm),也表示静态路由(staticrouting),适应环境于路由挑选非常确切的场合。
(2)自适应性算法(adaptivealgorithm),也表示动态路由(dynamicrouting)。
下面的路由算法均就是基于流形水平结构展开路由决策的。
最短路算法:这里涉及到每一条边的权重,这个衡量的因素很多,包括:跳数,物理距离,距离,带宽,平均流量,通信成本,平均延迟等。
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Three main ideas provide much of the motivation for data communications and help define the scope
The first point is relevant considering the popularity of multimedia applications:
the information from each source must be identified and data from all the sources must be intermixed for transmission
14
Figure 5.3 A conceptual framework for a data communications system. Multiple sources send to multiple destinations through an underlying physical channel
information is not restricted to bits that have been stored in a computer information can also be derived from the physical world, including audio and video various sources and forms of information available and one form can be transformed into another
11
5.4 The Conceptual Pieces of a Communication System
Imagine a communication system that has multiple sources of information and allows each source to send to a separate destination How does such a system work?
Information can arrive from many types of sources
the techniques used to handle multiple sources vary
Before it can be sent, information must be digitized Extra data must be added to protect against channel errors If privacy is a concern, the information may need to be encrypted To send multiple streams of information across a shared media
Goal is to develop practical ways to design and build transmission systems
5.2 The Essence of Data Communications
5.3
1. 2. 3.
5.1
Introduction
This chapter
provides an overview of data communications explains how the conceptual pieces form a complete communication system
Each of the boxes in Figure 5.3 corresponds to one subtopic of data communications. Let us look closer: Information Sources
The source of information can be either analog or digital Important concepts include characteristics of signals, such as amplitude, frequency, and phase, and classification as either periodic or aperiodic The subtopic focuses on the conversion between analog and digital representations of information
10
5.3
Motivation and Scope of the Subject
The third point suggests that sharing is fundamental Sharing plays a fundamental role in most computer networks
Successive chapters
each explain one concept in detail
5.2 The Essence of Data Communications
What does data communications entail? As Figure 5.1 illustrates, the subject is a combination of ideas and approaches from three disciplines Involves the transmission of information over physical media The subject draws on ideas about electric current, light, and other forms of electromagnetic radiation Information is digitized and digital data is transmitted Data communications uses mathematics and includes various forms of analysis
16
Figure 5.3
Processing at the transmitter/sender end
Figure 5.3 Processing at the receiver end
5.5 The Subtopics of Data Communications
Chapter 5 Overview of Data Communications
Topics Covered
5.1 Introduction 5.2 The Essence of Data Communications 5.3 Motivation and Scope of the Subject 5.4 The Conceptual Pieces of a Communication System 5.5 The Subtopics of Data Communications
5.3
Motivation and Scope of the Subject
such as electricity and electromagnetic radiation, to transmit information
The second point suggests that we must use natural phenomena It is important to understand the following issues:
Motivation and Scope of the Subject
The sources of information can be of arbitrary types Transmission uses a physical system Multiple sources of information can share the underlying medium
Figure 5.2 illustrates the simplistic view
5.4 The Conceptual Pieces of a Communication Sons is much more complex than the simplistic diagram in Figure 5.2 suggests, such as
the types of media that are available the properties of each media how physical phenomena can be used to transmit information over each medium the relationship between data communications and the underlying transmission the limits of physical systems the problems that can arise during transmission techniques that can be used to detect or solve the problems
计算机网络及应用技术
计算中心 林龙新
2012-12-21
暨南大学计算中心
1
主要参考书目
1. 计算机网络与因特网(第五版)机械工业出版社 2. 计算机网络与因特网(第四版)机械工业出版社 3. Internetworking with TCP/IP Volume I (Principles,Protocols and Architecture ) er 清华大学出版社 4. Interconnections Second Edition Bridges, routers,Switches and Internetworking Protocols Radia Perlman 清华大学出版社 5. computer networks (the fourth edition) Andrew S.Tanenbaum 清华大学出版社