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1、 外文原文：COMPUTER NETWORKSDATE 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 ,keyboar

2、ds, CRTs, and so on. This 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 ei

3、ght pairs of transmission 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

4、 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 mul

5、tipoint 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 da

6、ta 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/demo

7、dulator) 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 qua

8、drature 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 use

9、d 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

10、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.ARCHITECTURE OF COMPUTER NETWORKSComputer networ

11、k 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

12、 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 Refe

13、rence 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

14、 (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 net

15、work 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 publis

16、hed as a separate international 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

17、used extensively 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 (Transmissi

18、on 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 wan

19、t TCPs 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

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

21、raphical 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

22、 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 show

23、n 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 switch

24、ed.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

25、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 network 2LEASED LINE NETWORKSThis is a dedicated network connect

26、ed 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

27、. Very short response times 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 d

28、evice. 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 examin

29、e 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 a

30、n 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

31、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.LOCAL AREA NETWORK Local area data network , normally referred to simply as local area net

32、work 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

33、 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

34、 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 l

35、ower 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 co

36、nsidered. 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. Topology Most wide area networks, such as the PSTN, use a mesh (sometimes referred to as a network) topology. With LANs, however, the limited

37、 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 th

38、e 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 adop

39、ted 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”

40、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 transmis

41、sion 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 trans

42、mitted the frame, it passed the token on to allow another DTE to access the transmission medium.中文译文：计算机网络14.1 数据通信图14-1是数据通信链路的简化框图。虽然图中只画了一个从设备，但它代表了多点系统中的典型从设备。主计算机的功能之一是存储供各种设备使用的应用程序。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。这种设备通常在内部以字为单位处理数字信息在一个实际设备中构成一个字的所有位是并行传递的。数字数据在发送一个8对传输线，这是不

43、经济的。那些在发送、接收数据分别完成并-串和串-并转换并进行数据链路管理的所有电路，一般称为数据终端设备（DTE）。UART（通用异步接收/发送器）和USART（通用同步/异步接收/发送器）是实现并-串和串-并转换的设备。主DTE包括控制多点数据链路系统中信息流的线路控制部件（LCU或LinCo）。从设备一方的相应设备是工作站控制器（STACO）。如果DTE有与LCU相关的软件，则称为前端处理机（FEP）。同时，DTE也是数据链路系统中属于用户的最后一个设备。在两个DTE之间，先是调制解调器，接着是属于并有电话公司维护的通信设备。最近的调整结果是调制解调器已不属于电话公司独家管理的设备了。数据

44、通信设备（DCE）从DTE接收串行数据流，并将其转化成适合在话音线路上发送的某种模拟信号形式。在接收端，DCE完成相反的功能，把接收到的模拟信号转换成串行数字数据流。最简单的DCE是调制解调器或数传机。在发送端，调制解调器可以视为一种数/模转换器，而在接收端则可视为一种模/数转换器。调制解调器最常用的几种调制方式是移频键控（FSK）、移相键控（PSK）和正交调幅（QAM）。这是一种典型的使用模拟电话线路传输数据的方式。如果使用数字通信（有时称为T-载波）传输数据，则必须使用脉码调制（PCM）设备。也可以用微波传输系统进行的数据通信，当然还可以用卫星通信系统去传输数据。如果链接DTE和DCE的电

45、缆和信号电平都是未调整的，则所产生的偏差就是可能与生产厂家的数量成正比。电子工业协会（EIA）（由关心建立行业标准的厂家组成的组织）同意将RS-232C作为DTE和调制解调器间的标准接口。它是一个25针电缆，每一针都规定了功能及信号电平，预计RS-232C将被修订的标准所取代。 14.2 计算机网络的体系结构计算机网络是由两个或多个计算机设备互连而成的一种复合系统，用于数据通信和资源共享。网络设计及逻辑结构应该遵循一套设计原则，包括功能的组织与数据格式和过程的描述，这就是网络的体系结构。由于这种体系结构是分层次的，故又称为一组层次和协议。下面两节将讨论两个重要的网络体系结构，即OSI参考模型和

46、TCP/IP参考模型。1. OSI参考模型OSI模型如图14-2所示（物理介质未画出）。这一模型是基于国际标准化组织（ISO）提出的一个建议，作为用于各层次的国际标准协议的第一步。该模型也称为ISO开放系统互连参考模型，因为它涉及开放系统的互连，即于其他系统的通信是开放的，通常简称为OSI模型。OSI模型有7层。应该注意的是，OSI模型本身并不是一种网络体系结构，因为它不具体规定每一层所使用的确切的服务和协议。它只是说明每一层应该做什么。然而，ISO还是对各层制定出了标准，尽管这些标准不是参考模型本身的一部分。每一层都作为一个单独的国际标准来颁布。2. TCP/IP参考模型TCP/IP参考模型

47、是美国国防部大约于1978年设计的一种早期的运输协议。该协议通常被视为OSI无连接工作方式的源头。TCP/IP仍在广泛的使用，并且被称为互联网事实上的行业标准。TCP/IP有两部分：TCP和IP。TCP表明是处于运输层，IP意为是网络层。（1）运输层有两个端到端协议，其中之一是传输协议（TCP），另一个是用户数据报协议（UDP）。TCP是一种面向连接的协议，它可以在互联网上无差错地将字节流从一台机器发送出去。UDP是一种不可靠的无连接协议，用于不希望那种TCP流控制序列，而希望只提供自身功能的场合。（2）网络层定义了正式的分组格式及称为互联网协议的IP。该网络层的任务是将IP分组发送到指定的地

48、方。图14-3所示为TCP/IP参考模型。在运输层上面是应用层。应用层包括所有高层协议。早期的协议包括虚拟终IELNET、文件传送协议FTP、电子邮件（SMTP）和域名服务DNS。14.3 广域网广域网跨越地域很大，通常是一个国家或一个大陆。这种网络中汇集了运行各种用户程序（即应用程序）的计算机。本文还是沿用传统的称呼把它们叫做主机，在一些文献里有时也称其为端系统。这些主机是通过通信子网（有时简称子网）连接起来的。子网的任务是把报文从一个主机传送到另一个主机，就像电话系统把话音从讲话人传到收听者一样。把网络（子网）的纯通信任务与主机的应用任务分开以后，整个网络的设计工作就得到很大的简化。图14

49、-4展示了主机与通信子网之间的关系。广域网的分类方法很多，其中一种方法是按传输设备中的信息流分类。如果使用这种方法，可将广域网分成3种基本类型：电路交换、专线和分组交换。1. 电路交换网电路交换网是最常用的并且是几乎所有用户都要用的网络类型。然而，公用电话交换网不限于电话公司。任何单位只要购买了合适的交换设备，就可以建立自己的内部电路交换网，而且如果需要，还可配置一至多个与公用交换网互连的接口，在公用网和他们的专用内部网之间话音和数据传输。2. 专线网这是一种通过租用线连接的专用网。租用线是由租用客户专用的一条通信线，不须进行交换机间的交换。租用或私人线路是用户专用的。专线的优点是终端或计算机一直连在租用线上，服务的响应时间极短。3. 分组交换网分组网络是用分组装拆设备、分组路由的设备，以及将分组从源传到目的地的设备构成的。某些类型的数据终端设备DTE可以生成他们自己的分组，而其他一些DTE