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1、第八单元： 计算机网络Unit 8 Computer Network课文A： 网络基本原理Section A Network FundamentalsThe 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 transferred from machine to machine.在不同计算机之间共享信息和
2、资源的需要，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。In these networks, computer users can exchange messages and share resources - such as printing capabilities, software packages, and data storage facilities - that are scattered throughout the system.在网络中，计算机用户可以交换信息，并共享分散在整
3、个网络系统的资源，如打印能力、软件包以及数据存储设备。The underlying software 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.支持这类应用所需的基本软件，已经从简单的实用软件包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。In a s
4、ense, network software is evolving into a network wide operating system.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。一、网络分类Network ClassificationsA computer network is often classified as being either a local area network (LAN), a metropolitan area network (MAN), or a wide area network (WAN).计算机网络常常划分为局域网、城域网和广域网。A
5、 LAN normally consists of a collection of computers in a single building or building complex.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。For example, the computers on a university campus or those in a manufacturing plant might be connected by a LAN.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。A MAN is a network of intermediate si
6、ze, such as one spanning a local community.城域网属于中型网络，如一个覆盖某一社区的网络。A WAN links machines over a greater distance- perhaps in neighboring cities or on opposite sides of the world.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。Another means of classifying networks is based on whether the networks internal ope
7、ration 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.网络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。A network of the former type is called an open network; a network of the latter type
8、 is called a closed, or sometimes a proprietary, network.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。The Internet is an open system.因特网属于开放式系统。In particular, communication throughout the Internet is governed by an open collection of standards known as the TCP/IP protocol suite.尤其是，整个因特网的通信是由一组称为TCP/IP
9、协议组的开放标准来控制的。Anyone is free to use these standards without paying fees or signing license agreements.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。In 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 l
10、easing these products.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。Networks 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。Still another way of classifying networks is based on the topology of the network, which refers to the pattern in which the machines are conn
11、ected.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。The bus topology, ring topology, and star topology are three popular topologies.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。Of these, the star network is perhaps the oldest, having evolved from the paradigm of a large central computer serving many users.其中，星形网络或许是最古老的，这种网络
12、由一台大型中央计算机服务许多用户的范式演变而来。As the simple terminals employed by these users grew into small computers themselves, 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。A point to emphasize is that the connections between machines in a network do not need to be physical.需要强调的一点是，在一个网络中，计算机之间的连接并不一定是物
13、理连接。Wireless networks, using radio broadcast technology, are becoming quite common.使用无线广播技术的无线网络正在变得相当常见。Protocols二、协议 For a network to function reliably, it is important to establish rules by which network activities are conducted.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。Such rules are called protocols.这类规则称为协议。
14、By developing and adopting protocol standards, venders are able to build products for network applications that are compatible with products from other venders.通过开发和采用协议标准，不同厂商制造的网络应用产品能够相互兼容。Thus, the development of protocol standards is an indispensable process in the development of networking tec
15、hnologies.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。As an introduction to the protocol concept, let us consider the problem of coordinating the transmission of messages among computers in a network.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。Without rules governing this communication, all the computers might insist o
16、n transmitting messages at the same time or might fail to relay messages when that assistance is required.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。One approach to solving this problem is the token ring protocol, which was developed by IBM in the 1970s and continues to be a popular protocol in
17、 networks based on the ring topology.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。In this protocol, all the machines in the network transmit messages in only one common direction (Figure 8A-1), meaning that all messages sent over the network move around the ring in the same direc
18、tion by being forwarded from computer to computer.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（图8A-1）。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。When a message reaches its destination, the destination machine keeps a copy of it and forwards a copy on around the ring.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。When the
19、 forwarded copy reaches the originating computer, that machine knows that the message must have reached its destination and removes the message from the ring.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。Of course, this system depends on inter-machine cooperation.当然，这种系统的运行依靠计算机之间的合作。If a machine insist
20、s on constantly transmitting messages of its own rather than forwarding those of the other machines, nothing will be accomplished.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。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位模式。Posse
21、ssion of this token gives a machine the authority to transmit its own message; without the token, a machine is only allowed to forward messages.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。Normally, each machine merely relays the token in the same manner in which it relays messages.一般来说，每台计算机只是像传递报文那样传递令牌。
22、If, however, the machine receiving the token has messages of its own to introduce to the network, it transmits one message while holding the token. When this message has completed its cycle around the ring, the machine forwards the token to the next machine in the ring.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
23、会保存令牌，同时传输一份报文。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。Likewise, when the next machine receives the token, it can either forward the token immediately or transmit its own new message before sending the token on to the next machine.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。In this manner
24、, each machine in the network has equal opportunity to introduce messages of its own as the token circles around the ring.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。Another protocol for coordinating message transmission is used in bus topology networks that are based on the Ethernet protocol collection
25、.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。In 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（CSMA/CD）的协议控制。This protocol dictates that each message be broadcast to all t
26、he machines on the bus (Figure 8A-2).该协议规定，每一份报文必须向总线上的所有计算机广播（图8A-2）。Each machine monitors all the messages but keeps only those addressed to itself.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。To transmit a message, a machine waits until the bus is silent, and at this time it begins transmitting while continuing t
27、o 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。If another machine also begins transmitting, both machines detect the clash and pause for a brief 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。The result is a system similar to tha
28、t used by a small group of people in a conversation.结果是形成一种体制，这种体制类似于一小群人对话时所采用的。If two people start to talk at once, they both stop.如果两个人同时开始说话，他们都会停下来。The difference is that people might go through a series such as, “Im sorry,what were you going to say?”, “ No,no.不同的是，人们随后可能进行这样的对话：“抱歉，您想说什么？”，“不，
29、不，您先说。”You go first,” whereas under the CSMA/CD protocol each machine merely tries again.而根据CSMA/CD协议，每台计算机只是重新进行尝试。Interprocess Communication三、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)
30、must often communicate with each other to coordinate their actions and to perform their designated tasks.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。Such communication between processes is called interprocess communication.这种进程之间的通信称为进程间通信。A popular convention used for interpr
31、ocess communication is the client/server model.进程间通信使用的一种流行规约是客户机/服务器模型。This model defines the basic roles played by the processes as either a client, which makes requests of other processes, or a server, which satisfies 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。
32、An early application of the client/server model appeared in networks connecting all the computers in a cluster of offices.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中。In this situation, a single, high-quality printer was attached to the network where it was available to all the machines in the network
33、.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。In this case the printer played the role of a server (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。A
34、nother early application of the client/server model was used to reduce the cost of disk storage while also removing the need for duplicate copies of records.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。Here one machine in a network was equipped with a high-capacity mass storage system (usually a magnet
35、ic disk) that contained all of an organizations records.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。Other machines on the network then requested access to the records as they needed them.这样，网络中的其他计算机可根据需要请求访问这些记录。Thus the machine that actually contained the records played the role of a server (ca
36、lled 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。Today the client/server model is used extensively in network applications.今天，客户机/服务器模型在网络中广泛应用
37、。However, the client/server model is not the only means of interprocess communication.不过，客户机/服务器模型并非进程间通信的唯一方式。Another model is the peer-to-peer (often abbreviated P2P) model, whose properties provide insightful contrasts to the client/server model.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
38、成鲜明对照。Whereas the client/server model involves one process (the server) communicating with numerous others (clients), the peer-to-peer model involves two processes communicating as equals (Figure8A-3).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，而对等模型则涉及两个进程进行对等通信（图8A-3）。Moreover, whereas a server must execut
39、e continuously so that it is prepared to serve its clients at any time, the peer-to-peer model usually involves 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。For example, application of the peer-to-peer model include instant messaging in which two peopl
40、e carry on a written conversation over the Internet as well as situations in which people 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。The peer-to-peer model is also a popular means of sharing files such as music recordings and motion pictures via the I
41、nternet (sometimes with questionable legality).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）。In 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.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。Then,
42、 the items are transferred between the two parties using the peer-to-peer model.然后，双方之间就可以使用对等模型传输这些项目。This is in contrast to earlier approaches that applied the client/server model by establishing a central “distribution center” (the server) from which clients downloaded music recordings (or at lea
43、st found sources for those items).这与应用客户机/服务器模型的较早期方法截然不同，这些方法要建立一个“分发中心”（服务器），供客户机下载音乐录音（或者至少找到可以提供这些项目的地方）。The central server, however, proved to be a focal point at which the music industry could enforce copyright laws, leading to the eventual dismantling of these music distribution centers.然而，中心
44、服务器证明是音乐行业可以运用版权法的一个焦点对象，结果最终导致这些音乐分发中心的解体。In contrast, the lack of a centralized base of operation obtained via the peer-to-peer model makes legal efforts to enforce copyright laws more difficult.与此形成对照的是，对等模型缺乏一个操作中心，这使得运用版权法的努力难度增加。You might often read or hear the term peer-to-peer network, which
45、 is an example of how misuse of terminology can evolve when technical terms are adopted by the nontechnical community.你或许经常读到或听到“对等网络”这个说法，这是当技术术语被非科技界采用时可能发生术语误用的一个例子。The term peer-to-peer refers to a system by which two processes communicate over a network (or Internet). It is not a property of th
46、e network (or internet).“对等”指的是两个进程通过网络（或互联网）进行通信的一种体制，并不是网络（或互联网）的一种特性。A process might use the peer-to-peer model to communicate with another process and later use the client/server model to communicate with another process over the same network.一个进程可以通过同一个网络先是使用对等模型与另一个进程通信，然后又使用客户机/服务器模型与另一个进程通信。
47、Thus, it would be more accurate to speak of communicating by means of the peer-to-peer model rather than communicating over a peer-to-peer network.因此，比较准确的说法是，使用对等模型通信，而不是通过对等网络通信。I. Fill in the blanks with the information given in the text:1. MAN2. open; closed3. bus4. token5. Ethernet6. client/ser
48、ver7. equals; temporary8. networkII. Translate the following terms or phrases from English into Chinese and vice versa:1. file server 文件服务器2. carrier sense 载波检测，载波监听3. protocol suite 协议组，协议集4. peer-to-peer model 对等模型5. bus topology network 总线拓扑网络6. inter-machine cooperation 机器间合作，计算机间合作7. Ethernet protocol collection 以太网协议集8. proprietary network 专有网络9. utility package 实用软件包，公用程序包10. star network 星形网络11. 局域网 local area network (LAN)12. 令牌环 token ring13. 无线网络 wir