1、在网络中，计算机用户可以交换信息，并共享分散在整个网络系统的资源，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
2、提供了一个复杂的网络范围的基础结构。In a sense, 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）
3、.计算机网络常常划分为局域网、城域网和广域网。A 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 ne
4、twork of intermediate size, 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 t
5、he 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.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 ne
6、twork of the latter type 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.
7、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。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
8、income from selling or leasing 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 whi
9、ch the machines are connected.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 u
10、sers.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。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
11、点是，在一个网络中，计算机之间的连接并不一定是物理连接。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 ca
12、lled protocols.这类规则称为协议。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 devel
13、opment of networking technologies.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
14、computers might insist on 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
15、be a popular protocol in 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 th
16、e ring in the same direction 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
17、个副本并绕环形网络转发一个副本。When the 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
18、间的合作。If a machine insists 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
19、形网络中传递一个称为令牌的独特位模式。Possession 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.
20、一般来说，每台计算机只是像传递报文那样传递令牌。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
21、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。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
22、传递给下一台计算机。In this manner, 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 Ethe
23、rnet protocol collection.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 messa
24、ge be broadcast to all the 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 transmi
25、tting 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。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
26、 a system similar to that 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
27、能进行这样的对话：“抱歉，您想说什么？”，“不，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 ma
28、chine via time sharing） 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 con
29、vention used for interprocess 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
30、求的客户机，或定义为满足客户机所提请求的服务器。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.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 （o
