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1、Unit 8 Computer NetworkingText 1 Data CommunicationsThe end equipment can be computers, printers, keyboards, CRTs, and so on. They are either generates the digital information for transmission or uses the received digital data. This equipment generally operates digital information internally in word
2、 units.DTE is data terminal equipment. A station controller (STACO) is the corresponding unit at the secondaries. At one time, the DTE was the last piece of equipment that belonged to the subscriber in a data link system. Between the DTEs, first is the modem, and then is the communications equipment
3、. The communication equipment is owned and maintained by Telco. Data communications equipment (DCE) accepts the serial data stream from the DTE. And DCE converts it to some form of analog signal suitable for transmission on voice-grade lines. At the receive end, the DCE performs the reverse function
4、. It converts 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 be considered a form of analog-t
5、o-digital converter.参考译文数据通信终端设备可以是计算机、打印机、键盘、CRT等。它们可以生成要发送的数字信息，也可以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。DTE一般称为数据终端设备。辅助设备一方的相应设备是工作站控制器（STACO）。同时，DTE也是数据链路系统中属于用户端的最后一个设备。在两个DTE之间，先是调制解调器，接着是属于Telco电话公司维护的通信设备。数据通信设备（DCE）从DTE接收串行数据流，并将其转换成适合于在语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DC
6、E是调制解调器或数据传送机。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。Text 2 Architecture of Computer Networks Computer network is a complex unit, it consists of two or more connected computing units. It is used for the purpose of data communication and resource sharing. Design of a network and its logical structur
7、e should comply with a set of design principles. They include: the organization of functions, the description of data formats and procedure. In the following two sections we will discuss two important network architectures, the OSI reference model and the TCP/IP reference model.(1) the OSI reference
8、 model The OSI (open systems interconnection) model was created by the ISO. This model is a foundation for comparing protocols; However, more protocols deviate from the rules set up by the OSI model.The OSI model has seven layers. Note that the OSI model itself is not a network architecture. It just
9、 tells what each layer should do. However, ISO has also produced standards for all the layers. Each one has been published as a separate international standard.(2) the TCP/IP reference modelTCP/IP is a communication protocol; it provides many different networking services.The TCP/IP Internet protoco
10、l suite is formed from two standards: the TCP (Transmission Control Protocol) and the IP (Internet Protocol). TCP means it is on the transport layer. IP means it is on the network layer. On top of the transport layer is the application layer. It contains all the higher-level protocols. The early one
11、s included virtual terminal (TELNET), file transfer (FTP), electronic mail (SMTP) and domain name service (DNS).As we know, TCP/IP is the foundation of the Internet. These protocols are continually changing and evolving to support the needs of the user community.参考译文计算机网络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
12、系统，它用于数据通信和资源共享。网络设计及其逻辑结构应该遵循一套设计原则，其中包括：功能的组织以及数据格式和过程的说明。在下面的两小节里，我们将讨论两个重要的网络体系结构，即OSI参考模型和TCP/IP参考模型。(1) OSI参考模型OSI模型（开放系统互联参考模型）是由国际标准化组织（ISO）开发的一个建议。相对于其他协议来说，此种模式为基础模式。然而，更多的协议是背离OSI模型的。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，它只是说明每一层应该做什么。然而，ISO还是对各层制定了标准，每一层都是作为一个单独的国际标准来颁布的。(2) TCP/IP参考模型TCP/I
13、P参考模型是一种传输协议，提供了许多不同的网络服务。TCP/IP协议组由TCP（传输控制协议）和IP（网际协议）两部分组成。TCP表明是处于传输层，IP意为是在网络层。在传输层上面是应用层。应用层包括所有高层协议。早期的协议包括虚拟终端TELNET、文件传送协议FTP、电子邮件SMTP和域名服务DNS。现在我们知道，TCP/IP协议是因特网的基础。这些协议在用户们的支持下不断地改变和进化着。Text 3 Local Area NetworkA LAN (Local area data network) is a group of computers. The work devices conn
14、ected together usually within the same building. By definition, the connections must be high-speed and relatively inexpensive (e.g. token ring or Ethernet). For example, a LAN may be used to interconnect workstations distributed around offices within a single building or a group of buildings such as
15、 a university campus. LANs consist of carefully selected groups of components (hardware and software). They are configured for the specific requirements of the organization. A LAN is generally limited to the size of a department or an organization. And a LAN often consists of 2 to 100 devices. LANs
16、usually contain resources such as servers, printers, and connections to other networks through internetworking devices. The internetworking devices include switches and routers.In the next section we will discuss the structure of the different types of LAN.(1) TopologyMost wide area networks, such a
17、s 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 funct
18、ion as data communication subnetworks for the interconnection of local computer-based equipment is the hub topology. It is a variation of the bus and ring. (2) Transmission mediaTwisted pair, coaxial cable and optical fibre are the three main types of transmission medium used for LANs.(3) Medium acc
19、ess control methodsTwo techniques have been adopted for use of the medium access control in the LANs. One is carrier-sense-multiple-access with collision detection, and the other is control token.参考译文局域网局域数据网(局域网)由若干计算机组成。通常是一幢楼内的工作设备被连接在一起。通过定义可知，这种连接一定是高速并且相对廉价的（如：令牌网和以太网）。例如，一个局域网可以把一幢楼里或像大学校园内楼群
20、里的各个办公室的工作站连在一起。局域网是由精心挑选的各组设备（硬件和软件）构成的。它们根据组织的各种特殊需要来配置。局域网的大小一般限制在一个部门或者一个组织，由2100台电脑组成。局域网经常包含一些资源，如一些服务器和打印机，而且可以通过网络设备与其他的网络连接。这些网络设备包括交换机和路由器等。接下来，我们将讨论不同类型局域网的结构。(1) 拓扑结构大多数广域网，如公共电话交换网（PSTN），使用网状（有时称为网络）拓扑结构；而局域网由于用户数据终端设备相距很近，可采用简单的拓扑结构。常用的有星形、总线、环形和集线器等4种拓扑结构。应用最广的、用于互连本部门计算机设备以进行数据通信的局域网
21、拓扑结构是集线器拓扑结构。这种拓扑结构是总线和环形拓扑结构的变种。(2) 传输媒体双绞线、同轴电缆和光纤是局域网采用的3种主要传输媒体。(3) 媒体访问控制方法局域网中采用了两种媒体访问控制技术，一种是载波侦听多路访问冲突检测技术（CSMA/CD），另一种是令牌控制技术。Text 4 InternetThe Internet is used by millions of people across the world to communicate business and personal information. The Internet is huge. Then we will talk
22、 about the difference of “an internet” and “ the Internet”.An internet (note the lower case i) is a computer network. It allows computers with distinctive software and hardware to communicate. Many kinds of computers can be connected to an internet. Each computer can serve a specialized role. It off
23、ers a wide variety of services to its users. The Internet is specific kind of internet. In The Internet Passport, the Internet will be defined as the network of networks. It follows a set of rules known as the Internet Protocol (IP) suite.But what does this mean to you? It means that any computer th
24、at is connected to the Internet can communicate with any other Internet computer. From the users perspective, this works much like the telephone system works. You can dial from your phone to any other phone on the system, no matter what kind of telephone you have; you only need to know the phone num
25、ber of the person you want to reach.With an Internet connection you can get some of the basic services available are: E-mail. Its a fast, easy, and inexpensive way to communicate with other Internet users around the world. Telnet. It allows a user to log into a remote computer as though it were a lo
26、cal system. FTP. It allows a user to transfer virtually every kind of file that can be stored on a computer from one Internet-connected computer to another. Usenet news. Its a distributed bulletin board which offers a combination news and discussion service on thousands of topics. World Wide Web (WW
27、W). Its a hypertext interface to Internet information resources. Also, through an Internet connection, you can: access online library catalogs. copy computer files or software from archives. access databases for teaching or research. obtain free electronic books. use educational and information serv
28、ices. use directory services to find Internet users. access supercomputer sites.It was estimated that at this rapid growth everyone in the world would have an e-mail address in the near future.参考译文互联网因特网是世界上很多人用来进行商业贸易和个人信息交流的网络，它非常巨大。接下来，我们将讨论互联网及因特网。互联网(internet)（注意小写字母i）是一种计算机网络，该网络上的计算机在通信时可以使用不
29、同的软件和硬件。多种计算机都可以连入互联网，每台计算机都可以有一独特作用。一个互联网可以向它的用户提供各式各样的业务。因特网(Internet)是一种专用互联网。因特网在它的证书中定义为网络的网络，该网络使用了一组叫做互联网协议（IP）组的规则。但对你来说，这意味着什么呢？这表明连接到因特网上的计算机可以与因特网上任何其他计算机通信。从用户的角度来看，其工作方式很像电话系统的工作方式。在电话系统内，可以从你的电话机拨打任何其他电话，而不管你使用什么样的电话机，你只需知道对方的电话号码即可。你可以通过因特网得到的基本服务如下： 电子邮件。它是与世界范围内的因特网用户进行联系的一种快速、方便、廉价
30、的交流方式。 远程登录。允许用户连接到远程计算机上，就像这台远程计算机是本地机一样。 文件传输协议。该协议可以将存储在计算机上的各种文件，从因特网上的一台计算机传送给另一台计算机。 新闻组网络系统。一种分布式的电子公告牌，它能提供有关上千种话题的新闻和讨论服务。 万维网。一种因特网信息资源的超文本界面。 访问在线图书馆目录。 从计算机档案库存储器中拷贝文件或软件。 访问教学或科研数据库。 获取免费电子图书。 使用教育和信息服务。 使用目录服务以查找因特网用户。 访问超级计算机站点。估计以这样的高速发展，在不久的将来世界上每个人都将拥有至少一个电子邮件地址。Text 5 The World Wi
31、de WebThe World Wide Web (also known as WWW or Web) is one of the fastest-growing Internet software applications. It is an architectural framework. It linked documents spread out over thousands of machines for accessing all over the Internet. The World Wide Web ties the computers together into a vas
32、t collection of interactive multimedia resources. The WWW is a way of exchange information between computers on the Internet.The Web is built around hypertext and hypermedia. A hypertext document has certain keywords or phrases linked to other online documents. A person reading a hypertext document
33、about mobile phone, for example, might be able to select the highlighted word “Nokia 3310”, and he call up another document giving more information about that particular type. With documents intertwined by links into a web of information, you can select paths to browse online resources, a process of
34、ten referred to as surfing.Hypermedia extends the concept of hypertext to other forms of information. They include: images, sounds, and even video clips. If a person read a hypermedia document about mobile phones, then he might select a video show of a phone and hear the ring of it.The World Wide We
35、b also subsumes previous Internet information systems such as Gopher and FTP. These resources can still be accessed through the Web. But the Web offered by these more restricted connection methods. Now the Web provides a wealth of additional capabilities.Thousands of computers around the world are n
36、ow connected to the Web. They offer a huge variety of information and services to visitors. These online documents are generally referred to as pages. They are composed and supported by various people and organizations. Web pages are available for an amazing variety of tasks ranging from the playful
37、 to the serious. You can get many services access Web pages. For example, you can search database of mailing lists, you can see pictures of your favorite band and their concert schedule, or you can take a “tour” through a foreign country. Thousands of links to new services are added to the Web each
38、day, and its growth has been explosive.参考译文万维网万维网（又称WWW或Web）是因特网上发展最快的应用软件之一。它是一种结构化框架，用于访问遍布在因特网上的成千上万台机器中的链接文档。万维网把这些计算机连接成了一个巨大的交互式多媒体资源库，它是因特网上的计算机之间进行信息交流的一种方式。Web是用超文本和超媒体设计的。超文本文档中的一些关键词或短语被链接到了其他的在线文档中。例如，某人在阅读一篇有关移动电话的超文本文档时，如果他选择高亮显示的词“诺基亚3310”，就能链接到另一篇有关这一类型移动电话的文章，从而可以获取更多的信息。网络文章通过链接形成了
39、一个网络信息资源库，用户可以选择路径来浏览这些在线资源，这种行为通常被称为“网上冲浪”。超媒体将超文本的概念扩展到了其他的信息形式，其中包括图片、声音，甚至是录像剪辑。在超媒体文档中阅读有关移动电话的文章时，读者可以选择关于该电话的视频演示，还能听见铃声。万维网也包括了以前的因特网信息系统，如Gopher 和FTP。这些资源仍然可以通过网络访问，但是以前那些连接方式的局限性很大，现在的网络提供了许多先辈们没能提供的功能。现在世界上成千上万的计算机都连接到了网络上，而且为访问者提供了相当多的信息和服务。这些由不同的人和组织编写和支持的在线文档被称为网页。Web网页可以跨越从诙谐到严肃的各种风格。通过访问网页，用户可以查看邮件数据库，查阅自己喜爱的乐队的图片和他们的演出时间表，或去国外周游一圈。现在每天都会增加上千种新的服务链接，万维网已经飞速发展起来了。