APS 审核复习 computer network

A computer network, also called as a network, is a collection of computers interconnected by communication channels that allow sharing of resources and information.The rules and data formats数据格式for exchanging information in a computer network are defined by communications protocols协议. Well-known communications protocols are Ethernet, a hardware and Link Layer standard in local area networks, and the Internet Protocol Suite, which defines a set of protocols for internetworking, i.e. for data communication between multiple networks, as well as host-to-host data transfer, and application-specific data transmission formats.PropertiesFacilitate communicationsUsing a network, people can communicate efficiently and easily via email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing. Permit sharing of files, data, and other types of informationIn a network environment, authorized users may access data and information stored on other computers on the network. The capability of providing access to data and information on shared storage devices is an important feature of many networks. Share network and computing resourcesIn a networked environment, each computer on a network may access and use resources provided by devices on the network, such as printing a document on a shared network printer. Distributed computing uses computing resources across a network to accomplish tasks.May be insecureA computer network may be used by computer hackers to deploy computer viruses or computer worms on devices connected to the network, or to prevent these devices from normally accessing the network (denial of service).May interfere冲突with other technologiesPower line communication strongly disturbs certain forms of radio communication, e.g., amateur radio.[5] It may also interfere with last mile access technologies such as ADSL and VDSL.May be difficult to set upA complex computer network may be difficult to set up. It may also be very costly to set up an effective computer network in a large organization or company.OSI ModelData unit Layer FunctionHost layers Data7.ApplicationNetwork process to application6.PresentationData representation, encryption anddecryption, convert machine dependent datato machine independent data5. SessionInterhost communication, managing sessionsbetween applicationsSegments段 4. TransportEnd-to-end connections, reliability and flowcontrolMedia layers Packet/Datagram 3. Network Path determination and logical addressing Frame 2. Data link Physical addressingBit 1. Physical Media, signal and binary transmissionA wide area network (WAN) is a telecommunication network that covers a broad area (links across national boundaries). People utilize WANs to transfer data from various geographical locations. In essence this mode of telecommunication allows a business to effectively carry out its daily function regardless of location.This is in contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.TCP/IPThe TCP/IP model (Transmission Control Protocol/Internet Protocol) is a descriptive framework描述的框架of computer network protocols. And Transmission Control Protocol/Internet Protocol is the most important protocols of the Internet Protocol Suite网络协议簇The TCP/IP model describes a set of guidelines and performances of specific networking protocols to enable computers to communicate over a network. TCP/IP specifies how data should be formatted格式化, addressed, transmitted, routed and received at the destination. There are a variety of different types of protocols for communication services between computers.The link layer makes the host connected directly by hardware components部分. The internet layer facilitates促进the interconnection of local networks. Host-to-host communication tasks are handled in the transport layer, transmit data between hosts using protocols like the Transmission Control Protocol and the User Datagram Protocol (UDP). Finally, the highest-level application layer contains allprotocols that are defined each specifically for the functioning运行of data communications services.pppIn networking, the Point-to-Point Protocol (PPP) is a data link protocol commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption, and compression. PPP is commonly used as a data link layer protocol for connection over synchronous and asynchronous circuits.PPP is used over many types of physical networks including serial cable, phone line, trunk line, cellular移动telephone, specialized radio links, and fiber optic光纤links such as SONET. PPP is also used over Internet access connections (now marketed as "broadband"). Internet service providers (ISPs) have used PPP for customer dial-up access to the Internet, since IP packets cannot be transmitted over a modem line on their own, without some data link protocol.FSC Frame Check SequenceCRC Cyclic redundancy checkIPThe Internet Protocol (IP) is the principal communications protocol used for relaying 广播datagrams (packets) across an internetwork using the Internet Protocol Suite. Responsible for routing packets across network boundaries, it is the primary protocol that establishes the Internet.IP is the primary protocol in the Internet Layer of the Internet Protocol Suite and has the task of delivering datagrams from the source host to the destination host solely based on their addresses. For this purpose, IP defines addressing methods and structures for datagram encapsulation.The first major version of IP, now referred to as Internet Protocol Version 4 (IPv4) is the dominant protocol of the Internet, although the successor, Internet Protocol Version 6 (IPv6) is in active, growing deployment worldwide.。