Unit7-Computer Network

1、Section A Network Fundamentals课文课文A 网络基本原理网络基本原理Unit 7 Computer Network第第7单元单元 计算机网络计算机网络刘晓璐 讲师Unit7 Computer Network2 The need to share information and resources among different computers has led to linked computer systems, called networks, in which computers are connected so that data can be trans

2、ferred from machine to machine. 在不同计算机之间共享信息和资源的需在不同计算机之间共享信息和资源的需要，导致了相互连接的计算机系统的产生。这要，导致了相互连接的计算机系统的产生。这种相互连接的计算机系统被称为网络。在网络种相互连接的计算机系统被称为网络。在网络中，计算机连接在一起，从而数据可以从一台中，计算机连接在一起，从而数据可以从一台计算机传输到另一台计算机计算机传输到另一台计算机。Unit7 Computer Network3In these networks, computer users can exchange messages and share

3、resourcessuch as printing capabilities, software packages, and data storage facilitiesthat are scattered throughout the system. 在网络中，计算机用户可以在网络中，计算机用户可以交换信息交换信息，并，并共享共享分散在整个网络系统的分散在整个网络系统的资源资源，如打印，如打印能力、软件包以及数据存储设备。能力、软件包以及数据存储设备。插入语插入语定语从句，修饰定语从句，修饰resourcesUnit7 Computer Network4The underlying sof

4、tware required to support such applications has grown from simple utility packages into an expanding system of network software that provides a sophisticated network wide infrastructure. underlying,ndlaadj.潜在的;基础的;表面下的sophisticatedsfstketdadj.(机器、装置等)高级的,精密的;(方法)复杂的infrastructurenfrstrkt(r)n.基础设施；基础

5、建设支持这类应用所需的基本软件，已经从简单的支持这类应用所需的基本软件，已经从简单的实实用软件包用软件包发展成一个不断扩展的网络软件系统，发展成一个不断扩展的网络软件系统，该系统提供了一个复杂的网络范围的基础结构。该系统提供了一个复杂的网络范围的基础结构。Unit7 Computer Network5In a sense, network software is evolving into a network wide operating system.evolve into逐渐发展(进化、演变)成从某种意义上说，网络软件正在演变成一个网络范围的操作系统。Unit7 Computer Netw

6、ork6I. Network Classifications A computer network is often classified as being either a local area network (LAN), a metropolitan area network (MAN), or a wide area network (WAN). metropolitan,metrpltnadj.大都会的；大城市的；宗主国的；大主教教区的一、网络分类 计算机网络常常划分为局域网、城域网和广域网。Unit7 Computer Network7A LAN normally consists

7、 of a collection of computers in a single building or building complex. For example, the computers on a university campus or those in a manufacturing plant might be connected by a LAN. complex综合建筑群;综合大楼plant工厂;成套设备局域网通常由一幢建筑物或一个建筑群中的若干计算机组成。例如，大学校园里的计算机或制造工厂中的计算机可以用局域网连接。Unit7 Computer Network8A MAN

8、 is a network of intermediate size, such as one spanning a local community. A WAN links machines over a greater distanceperhaps in neighboring cities or on opposite sides of the world.spanspnvt.跨越;遍及communitykmju:ntn.社区;社会团体;群落城域网属于中型网络，如一个覆盖某一社区的网络。广域网连接地理范围更广的计算机，这些计算机或许在相邻的城市，或许在地球相反的两面。Unit7 Com

9、puter Network9 Another means of classifying networks is based on whether the networks internal operation is based on designs that are in the public domain or on innovations owned and controlled by a particular entity such as an individual or a corporation. domaindmenn.范围，领域；innovationnvenn. 革新，创新 网络

10、分类的另一种方式是根据网络的内部运行是基于无专利权保护的设计还是基于特定实体（如个人或公司）所拥有和控制的革新。宾语从句宾语从句Unit7 Computer Network10A network of the former type is called an open network; a network of the latter type is called a closed, or sometimes a proprietary prietaryprpratradj.专有的，专利的前一种类型的网络称为开放式网络，后一种类型的网络称为封闭式网络，有时也称为专有网络。U

11、nit7 Computer Network11 The Internet is an open system. In particular, communication throughout the Internet is governed by an open collection of standards known as the TCP/IP protocol suite. Anyone is free to use these standards without paying fees or signing license agreements. suiteswi:tn.计（软件的）套

12、件 因特网 属于开放式系统。尤其是，整个因特网的通信是由一组称为TCP/IP协议组的开放标准来控制的。任何人都可以自由地使用这些标准，而不需要付费或签署许可协议。 Unit7 Computer Network12In contrast, a company such as Novell Inc. might develop systems for which it chooses to maintain ownership rights, allowing the company to draw income from selling or leasing these products. In

13、c. (缩写)incorporated (责任有限)公司leaseli:svt.出租; 租借incomenkm n.收入，进款；相反，像Novell股份有限公司这样的公司可能选择为其开发的系统保留所有权，通过出售或出租这些产品获得收入。Unit7 Computer Network13Networks based on such systems are examples of closed networks.基于这类系统的网络属于封闭式网络的例子。Unit7 Computer Network14 Still another way of classifying networks is based

14、 on the topology of the network, which refers to the pattern in which the machines are connected. The bus topology, ring topology, and star topology are three popular topologies. topologytpldn.地质学; 拓扑结构 还有一种网络分类方法，它基于网络的拓扑结构，即计算机相互连接的模式。总线拓扑结构、环形拓扑结构和星形拓扑结构是3种流行的拓扑结构。Unit7 Computer Network15Of these

15、, the star network is perhaps the oldest, having evolved from the paradigm of a large central computer serving many users. paradigmprdam n.范例，样式，模范；其中，星形网络或许是最古老的，这种网络由一台大型中央计算机服务许多用户的范式演变而来。Unit7 Computer Network16As the simple terminals employed by these users grew into small computers themselves,

16、 a star network emerged. 随着这些用户所使用的简单终端本身发展成小型计算机，星形网络也就出现了。Unit7 Computer Network17A point to emphasize is that the connections between machines in a network do not need to be physical. Wireless networks, using radio broadcast technology, are becoming quite common.emphasizeemfsazvt.强调，着重需要强调的一点是，在一

17、个网络中，计算机之间的连接并不一定是物理连接。使用无线广播技术的无线网络正在变得相当常见。Unit7 Computer Network18Protocols协议 For a network to function reliably, it is important to establish rules by which network activities are conducted. Such rules are called protocols. functionfknvi.工作;运转;运行;发挥功能;reliablyrlabladv.可靠地，确实地conductkndktvt.&

18、 vi.引导; 带领; 控制 为了网络可靠运行，确立进行网络活动所遵循的规则很重要。这类规则称为协议。Unit7 Computer Network19By developing and adopting protocol standards, venders are able to build products for network applications that are compatible with products from other venders.vendervendn.卖主, 售卖者, 供应商compatiblekmptbladj.兼容的，相容的通过开发和采用协议标准，不同

19、厂商制造的网络应用产品能够相互兼容。Unit7 Computer Network20Thus, the development of protocol standards is an indispensable process in the development of networking technologies.indispensable,ndspensbladj.不可缺少的； 绝对必要的；因此，在联网技术的开发中，协议标准的开发是一个必不可少的过程。Unit7 Computer Network21 As an introduction to the protocol concept,

20、let us consider the problem of coordinating the transmission of messages among computers in a network. coordinatek:dnetvt.&vi.协调, 协同message报文 作为对协议概念的介绍，让我们考虑在一个网络的计算机之间协调报文传输的问题。Unit7 Computer Network22Without rules governing this communication, all the computers might insist on transmitting me

21、ssages at the same time or might fail to relay messages when that assistance is required.relayri:levt.转播，传达assistancesstnsn.帮助，援助如果没有控制这种通信的规则，所有的计算机就可能坚持同时传输报文，或者在需要传递报文时而未能传递。Unit7 Computer Network23 One approach to solving this problem is the token ring protocol, which was developed by IBM in the

22、 1970s and continues to be a popular protocol in networks based on the ring topology. tokentknn.象征; 令牌; 标记 解决这个问题的一种方法是令牌环协议，该协议由IBM公司20世纪70年代开发，今天在基于环形拓扑结构的网络中依然是一种很流行的协议。Unit7 Computer Network24In this protocol, all the machines in the network transmit messages in only one common direction, meanin

23、g that all messages sent over the network move around the ring in the same direction by being forwarded from computer to computer.forwardf:wdvt.促进, 助长; 转寄, 发送在这个协议里，网络中的所有计算机都只沿一个共同的方向传输报文。也就是说，通过网络发送的所有报文都沿一个相同的方向绕环形网络移动，依次经由各个计算机转发。Unit7 Computer Network25When a message reaches its destination, th

24、e destination machine keeps a copy of it and forwards a copy on around the ring. destination,destnenn.目的，目标； 目的地，终点；一份报文到达目的地后，目的地的计算机保留一个副本并绕环形网络转发一个副本。Unit7 Computer Network26When the forwarded copy reaches the originating computer, that machine knows that the message must have reached its destina

25、tion and removes the message from the ring. originaterdnetvt.&vi.发源; 来自; 产生; 创始; 开创当转发的副本到达始发计算机时，该计算机知道报文一定到达了目的地，并将其移出环形网络。Unit7 Computer Network27Of course, this system depends on inter-machine cooperation. If a machine insists on constantly transmitting messages of its own rather than forwar

26、ding those of the other machines, nothing will be accomplished.constantlyknstntliadv.不断地，时常地当然，这种系统的运行依靠计算机之间的合作。如果一台计算机坚持不断地传输自己的报文，而不转发其他计算机的报文，那么这个系统就什么也执行不了。Unit7 Computer Network28 To solve this problem, a unique bit pattern, called a token, is passed around the ring. 为了解决这个问题，在环形网络中传递一个称为令牌的独特

27、位模式。Unit7 Computer Network29Possession of this token gives a machine the authority to transmit its own message; without the token, a machine is only allowed to forward messages. authority:rtin.权威; 权力; 权限拥有令牌的计算机得到权利传输自己的报文，而没有令牌的计算机则只允许转发报文。Unit7 Computer Network30Normally, each machine merely relay

28、s the token in the same manner in which it relays messages.merelymili adv.仅仅，只不过； 只是； 纯粹； 全然一般来说，每台计算机只是像传递报文那样传递令牌。Unit7 Computer Network31Unit7 Computer Network32If, however, the machine receiving the token has messages of its own to introduce to the network, it transmits one message while holding

29、 the token.然而，如果收到令牌的计算机有自己的报文需要通过网络传输，它就会保存令牌，同时传输一份报文。Unit7 Computer Network33When this message has completed its cycle around the ring, the machine forwards the token to the next machine in the ring. 这份报文在环形网络中环绕一周后，该计算机就将令牌传递给环形网络中的下一台计算机。Unit7 Computer Network34Likewise, when the next machine r

30、eceives the token, it can either forward the token immediately or transmit its own new message before sending the token on to the next machine. likewiselakwazadv.同样地同样，当下一台计算机收到令牌时，它既可以立即传递令牌，也可以先传输自己的新报文，然后将令牌传递给下一台计算机。Unit7 Computer Network35In this manner, each machine in the network has equal op

31、portunity to introduce messages of its own as the token circles around the ring.circles:klvt.绕运转, 环绕, 盘旋这样，随着令牌绕着环形网络传递，网络中的每台计算机都有同等的机会传输自己的报文。Unit7 Computer Network36Unit7 Computer Network37 Another protocol for coordinating message transmission is used in bus topology networks that are based on t

32、he Ethernet protocol collection. Etherneti:netn.以太网 协调报文传输的另外一种协议用于基于以太网协议集的总线拓扑网络。Unit7 Computer Network38In an Ethernet system, the right to transmit messages is controlled by the protocol known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). 在以太网系统中，传输报文的权利由称为带有冲突检测的载波侦听多路访问(

33、CSMA/CD)的协议控制。Unit7 Computer Network39This protocol dictates that each message be broadcast to all the machines on the bus. Each machine monitors all the messages but keeps only those addressed to itself. addressed to寄往;寄给该协议规定，每一份报文必须向总线上的所有计算机广播。每台计算机监控所有的报文，但只保留发送给自己的那些报文。Unit7 Computer Network40

34、To transmit a message, a machine waits until the bus is silent, and at this time it begins transmitting while continuing to monitor the bus.一台计算机要等到总线静默时才可以传输报文。这时，它开始传输报文，同时继续监控总线。Unit7 Computer Network41If another machine also begins transmitting, both machines detect the clash and pause for a bri

35、ef random period of time before trying to transmit again.如果另一台计算机也开始传输报文，这两台计算机都会检测到冲突，并暂停短暂而随机的一段时间，然后再尝试传输。Unit7 Computer Network42The result is a system similar to that used by a small group of people in a conversation. If two people start to talk at once, they both stop.结果是形成一种体制，这种体制类似于一小群人对话时所

36、采用的。如果两个人同时开始说话，他们都会停下来。Unit7 Computer Network43The difference is that people might go through a series such as, “Im sorry, what were you going to say?”, “ No, no. You go first,” whereas under the CSMA/CD protocol each machine merely tries again.whereas,werzconj.鉴于; 然而; 反之不同的是，人们随后可能进行这样的对话：“抱歉，您想说什

37、么？”，“不，不，您先说。”而根据CSMA/CD协议，每台计算机只是重新进行尝试。Unit7 Computer Network44Interprocess Communication进程间通信 The various activities (or processes ) executing on the different computers within a network (or even executing on the same machine via time sharing) must often communicate with each other to coordinate

38、their actions and to perform their designated tasks.designatedeznetvt.指明，指出； 指派 在一个网络内不同计算机上（甚至使用分时方法在同一台计算机上）执行的各种活动（或进程）必须经常互相通信，以便协调行动，并完成指定的任务。Unit7 Computer Network45Such communication between processes is called interprocess communication.这种进程之间的通信称为进程间通信。Unit7 Computer Network46 A popular con

39、vention used for interprocess communication is the client/server model. conventionknvennn.习俗;惯例;常规;公约;协定;协约 进程间通信使用的一种流行规约是客户机/服务器模型。Unit7 Computer Network47This model defines the basic roles played by the processes as either a client, which makes requests of other processes, or a server, which sati

40、sfies the requests made by clients.这种模型将进程扮演的基本角色或定义为向其他进程提出请求的客户机，或定义为满足客户机所提请求的服务器。Unit7 Computer Network48 An early application of the client/server model appeared in networks connecting all the computers in a cluster of offices. clusterklst(r)n.丛； 簇，串； 群 客户机/服务器模型的一种早期应用，出现在将一组办公室里的所有计算机都连接起来的网络

41、中。Unit7 Computer Network49In this situation, a single, high-quality printer was attached to the network where it was available to all the machines in the network. attached to附着; 连接到; 附加到; 隶书于在这种应用中，一台高质量的打印机被连接到网络上，供其中的所有计算机使用。Unit7 Computer Network50In this case the printer played the role of a ser

42、ver (often called a print server), and the other machines were programmed to play the role of clients that sent print requests to the print server.在这种情况下，打印机扮演了服务器（常称为打印服务器）的角色，而其他计算机则通过程序设计而扮演了向打印服务器发送打印请求的客户机角色。Unit7 Computer Network51 Another early application of the client / server model was use

43、d to reduce the cost of disk storage while also removing the need for duplicate copies of records. duplicatedju:plikitadj.复制的,副本的,重复的 客户机/服务器模型的另外一种早期应用是为了降低磁盘存储费用，同时消除复制记录的需要。Unit7 Computer Network52Here one machine in a network was equipped with a high-capacity mass storage system (usually a magne

44、tic disk) that contained all of an organizations records. Other machines on the network then requested access to the records as they needed them. 在这种情况下，网络中的某一台计算机配备了大容量存储系统（通常是磁盘），存储一个机构的所有记录。这样，网络中的其他计算机可根据需要请求访问这些记录。Unit7 Computer Network53Thus the machine that actually contained the records play

45、ed the role of a server (called a file server), and the other machines played the role of clients that requested access to the files that were stored at the file server.因此，实际含有记录的计算机扮演了服务器（称为文件服务器）的角色，而其他计算机则扮演了请求访问文件服务器上存储的文件的客户机角色。Unit7 Computer Network54 Today the client/server model is used exte

46、nsively in network applications. However, the client/server model is not the only means of interprocess communication. extensivelyikstensivliadv.广大地，广泛地 今天，客户机/服务器模型在网络中广泛应用。不过，客户机/服务器模型并非进程间通信的唯一方式。Unit7 Computer Network55Another model is the peer-to-peer (often abbreviated P2P) model, whose proper

47、ties provide striking contrasts to the client/server model.strikingstraikiadj.显著的,引人注目的contrasts to 与形成对比，和相对照另外一种模型是对等（peer-to-peer，常缩写为P2P）模型，其特性与客户机/服务器模型的特性形成鲜明对照。Unit7 Computer Network56Whereas the client/server model involves one process (the server) communicating with numerous others (clients

48、), the peer-to-peer model involves two processes communicating as equals.反之，客户机/服务器模型涉及一个进程（服务器）与许多其他进程（客户机）通信，对等模型则涉及两个进程进行对等通信。Unit7 Computer Network57Moreover, whereas a server must execute continuously so that it is prepared to serve its clients at any time, the peer-to-peer model usually involv

49、es two processes that execute on a temporary basis. 而且，服务器必须持续运行，以准备好随时服务于客户机，而对等模型则通常涉及两个进程临时运行。Unit7 Computer Network58For example, application of the peer-to-peer model include instant messaging in which two people carry on a written conversation over the Internet as well as situations in which p

50、eople play games such as chess or checkers.例如，对等模型的应用包括两个人通过因特网进行书面对话的即时通信，以及人们参与下国际象棋或西洋跳棋等游戏的情况。Unit7 Computer Network59 The peer-to-peer model is also a popular means of sharing files such as music recordings and motion pictures via the Internet (sometimes with questionable legality). motion pict

51、ure电影； 影片questionablekwestnbladj.可疑的，有疑问的legalityli:gltin.合法性 对等模型还是通过因特网共享音乐录音、影片等文件的一种流行方法（其合法性有时值得怀疑）。Unit7 Computer Network60In this case, individuals who are seeking particular items broadcast their desire over the Internet and are contacted by those who own the items of interest. Then, the items are transferred between the two parties using the peer-to-peer model. 在这种情况下，寻求特定项目的个人在因特网上发布其需求，而拥有他所感兴趣项目的人则会与他取得联系。然后，双方之间就可以使用对等模型传输这些项目。Unit7 Computer Network61This is in contrast to earlier approaches that applied the client/server model by establishing a central “distribution