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1、DATE COMMUNICATIONSOne of the functions of the host computer is to store the applications programs for the various secondaries.The 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. Th

2、is equipment generally manipulates digital information internally in word unitsall 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 transmissi

3、on linesnot 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/

4、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 syste

5、m. 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

6、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. A

7、t 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 modul

8、ation （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 communic

9、ation. 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

10、（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.ARCHITECTURE OF COMPUTER NETWORKSComputer network is a complex consisti

11、ng 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 proce

12、dure. 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 mode

13、l 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 Interconn

14、ection） 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 becau

15、se 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 inter

16、national standard.2. The TCP/IP Reference Model The 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 used extensively and is

17、 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） ,

18、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 TCPs sequencing of fl

19、ows 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 th

20、e 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）. WIDE AREA NETWORKA wide area network, or WAN, spans a large geographical area, often a

21、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, jus

22、t 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 ma

23、ny 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

24、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

25、 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 network 2LEASED LINE NETWORKSThis is a dedicated network connected by leased lines. Lea

26、sed 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 t

27、imes are met with service.3. PACKET SWITCHING NETWORKS A 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 da

28、ta 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 pa

29、ckets 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 pub

30、lic 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

31、 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.LOCAL AREA NETWORK Local area data network , normally referred to simply as local area network or LANs, are used

32、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