通信英语The - Principle - of - PCM译文

图解:译文“蓝色”

UNIT 1

The Principle of PCM

PCM原理

Pcm is dependent on three separate operations, sampling, quantizing, and coding. Many different schemes for performing these three functions have evolved during recent years, and we shall describe the main ones. In these descriptions we shall see how a speech channel of telephone quality may be conveyed as a series of amplitude values, each value being represented, that is, coded as a sequence of 8 binary digits. Furthermore, we shall prove that a minimum theoretical sampling frequency of order 6.8 kilohertz(khz) is required to convey a voice channel occupying the range 300 HZ to 3.4 Khz. Practical equipments, however, normally use3 a sampling rate of 8 khz, and if 8-digits per sample value are used, the voice channel becomes represented by a stream of pulses with a repetition rate of 64khz. Fig .1-1 illustrates the sampling, quantizing, and coding processes.

PCM的构成依赖于三个环节，即采样、量化和编码。近年来，人们对这三个环节的实现提出了许多不同的方案，我们将对其中的一些主要的方案进行讨论。在这些讨论中，我们会看到话路中的语声信号是如何转换成幅值序列的，而每个幅值又被编码，即以8位二进制数的序列表示。而且，我们将证明，为了转换频率范围为300HZ—3.4KHZ的话路信号，理论上最小采样频率须为6.8khz。但是，实际设备通常用8khz的采样速率，而如果每个样值用8位码的话，则话路是由一个重复速率为64khz的脉冲流来表示的。图1-1表示了采样、量化、编码的过程。

Reexamination of our simple example shows us that the speech signal of maximum frequency 3.4khz has been represented by a signal of frequency 64khz. However, if only 4-digits per sample value had been used, the quality of transmission would drop, and the repetition rate of the pulses would be reduced to 32khz. Thus the quality of transmission is dependent on the pulse repetition rate, and for digital communication systems these two variables may be interchanged most efficiently.

让我们再研究一下上面提到的简单例子。可以看出，最高频率为3.4khz的话音信号适用64khz的（脉冲流）信号来表示的。但是，如果每个样值中用4位（码）表示，则传输质量会下降，而脉冲的重复速率也将减小到32khz。因而传输质量是取决于脉冲重复速率的。对于数字通信系统，这两个量之间极明显的互相影响着。

Digital transmission provides a powerful method for overcoming noisy environments. Noise can be introduced into transmission patch in many different ways : perhaps via a nearby lightning strike, the sparking of a car ignition system, or the thermal low-level noise within the communication equipment itself. It is the relationship of the true signal to the noise signal, known as the signal-to-noise ratio, which is of the most interest to the communication engineer. Basically, if the signal is very large compared to the noise level, then a perfect message can take place; however, this is not always the case. For example, the signal received from a satellite, located in far outer space, is very weak and is at a level only slightly above that of the noise. Alternative examples may be found within terrestrial systems where, although the message signal is strong, so is the noise power.

数字传输对于克服噪声环境的影响提供了一个强有力的手段。噪声可以以多种不同方式

进入传输信道，比如说因为附近的闪电、汽车点火系统的打火或因通信设备本身低电平的热噪声所致。正是这种被称为信噪比的东西，即真实信号与噪声的关系引起了通信工程师的极大的兴趣。从本质上讲，如果信号比噪声电平大得多，则信息的传输是完美的。但是，实际情况并不总是这样，例如，从位于遥远太空中的卫星接收到的信号极其微弱，其电平仅比噪声稍高一点。地面系统则是另一类例子，尽管信号很强，噪声也很强。

If we consider binary transmission, the complete information about a particular message will always be obtained by simply detecting the presence or absence of the pulse. By comparison, most other forms of transmission systems convey the message information using the shape, or level of the transmitted signal; parameters that are most easily affected by the noise and attenuation introduced by the transmission path. Consequently there is an inherent advantage for overcoming noisy environments by choosing digital transmission.

研究二进制信号的传输可见，只要简单的区判别脉冲的有和无，完美就获得了一条消息的全部信息。相比之下，许多其他形式的传输系统是利用被传信号的波形或电平的高低来传送信息的，而这些参数又极易受到传输途径中的噪声和衰耗的影响。因此选择数字传输系统在克服噪声环境的影响方面有其固有的优势。

So far in this discussion we have assumed that each voice channel has a separate coder, the unit that converts sampled amplitude values to a set of pulses; and decoder, the unit that performs the reverse operation. This need not be so, and systems are in operation where a single codec (i.e., coder and its associated decoder) is shared between 24, 30, or even 120 separate channels. A high-sped electronic switch is used to present the analog information signal of each channel, taken in turn, to the codec. The codec is then arranged to sequentially sample the amplitude value, and code this value into the 8-digit sequence. Thus the output to the codec may be seen as a sequence of 8 pulses relating to channel 1, then channel 2, and so on. This unit is called a time division multiplexer (TDM), and is illustrated in fig. 1-2. The multiplexing principle that is used is known as work interleaving. Since the words, or 8-digit sequences, are interleaved in time.

到目前为止，在这个讨论中，我们一直假定每个话路各有一个编码器和解码器。前者是将幅度采样值变换成脉冲，而后者则施行相反的变换，这种设置并非必须。在实际运行的PCM系统中，一个编、译码器为24路、30路，甚至120路所共用（注：在当代的PCM设备中，编、译码器系分路设备，即每个话路各有一套。）一个高速的电子开关被用来将每一话路的模拟信号依次的送往编、译码器。然后编、译码器再顺序采样幅值并把这个幅值编成8位码序列。这样，编解码器输出的8位码序列就分别对应于话路1、话路2，等等。这种设备称为时分复用（TDM）,如图1-2所示。由于8位码的码字序列按时间顺序插接在一起，所以上面所用的复用原则称为码字插接。

At the receive terminal a demultiplexer is arranged to separate the 8-digit sequences into the appropriate channels. The reader may ask, how does the demultiplexer know which group of 8-digits relates to channel 1,2, and so on? Clearly this is important! The problem is easily overcome by specifying a frame format, where at the start of each frame a unique sequence of pulses called the frame code, or synchronization word, is placed so at to identify the start of the frame. A circuit of the demultiplexer is arranged to detect the synchronization word, and thereby it knows that the next group of 8-digits corresponds to channel 1. The synchronization word reoccurs once again after the last channel has been received.

接收端设置了分路设备将8位码序列分配到相应的话路中。读者也许会问，分路设备怎么会知道哪一组8位码对应于第1路、第2路及其他各路呢？显然这是很重要的。这个问题

是很容易解决的。我们只要制定一个帧格式，即在每一帧的开始放置一个被称作帧码或同步字的独特码序列以标志每帧的起始，而用分路设备的一个电路去检测同步字，从而就知道下一个8位码组对应于话路1。当最后一个话路的码字收到之后，同步码又再次出现。

UNIT 2

Asynchronous Serial Data Transmission

异步串行数据传输

By far the most popular serial interface between a computer and its CRT terminal is the asynchronous serial interface. This interface is so called because the transmitted data and the received data are not synchronized over any extended period and therefore no special means of synchronizing the clocks at the transmitter and receiver is necessary. In fact ,the asynchronous serial data link is a very old form of data transmission system and has its origin in the era of teleprinter.

在计算机及其显示器之间最为常见的串行接口是异步串行接口。这个接口之所以如此称呼，是因为无论在多长的时间区间里发送的数据和接收的数据是不同步的，因而没有必要采用特殊的手段使发送器和接收器的时钟同步。实际上，异步串行数据链路是一种古老的数据传输方式，它起源于电传打字机的时代。

Serial data transmission systems have been around for a long time and are found in the telephone (human speech), Morse code, semaphore, and even the smoke signals one used by native Americans. The fundamental problem encountered by all serial data transmission systems is how to split the incoming data steam into individual units (i.e., bits) and how to group these units into characters. For example, in Morse code the dots and dashes of a character are separated by an intersymbol space, while the individual characters are separated by an intercharacter space, which is three times the duration of an intersymbol space.

串行数据传输系统已有很长的历史了，电话（人类语音）、莫尔斯电码、旗语，甚至土著美洲人从前用过的烟火信号都可以视为传些数据传输。所有的串行数据传输系统面临的首要的问题都是如何把如数的数据流分开为单独的码元（即比特），以及怎样把这些码元组合成字符。例如，在莫尔斯电码中，字的点、划是由符号间的空格来分开的，而字符之间又是由字符间的空格分开的，字符间空格的时长是点、划间空格的三倍。

First we examine how the data stream is divided into individual bits and the bits grouped into characters in an asynchronous serial data link. The key to the operation of this type of link is both simple and ingenious. Fig.2-1 gives the format of data transmitted over such a link.

首先我们研究一下在异步串行数据链路中数据流是怎样分成单独码元的，以及码元是如何组成字符的。这类系统运行的核心原理既简单有精巧。图2-1绘出了在这个链路中传送的数据格式。

An asynchronous serial data link is said to be character-oriented, as information is transmitted in the form of groups of bits called characters. These characters are invariable units comprising 7 or 8 bits of ―information‖ plus 2 to 4 control bits and frequently correspond to ASCII-encoded characters. Initially ,when no information is being transmitted, the line is in an

idle state. Traditionally, the idle state is referred to as the mark level. By convention this corresponds to a logical 1 level.

异步串行数据链路被称为面向字符的，因为信息是以被称作字符的比特组的形式传送的。这些字符是一些故此能够的单元，每个单元都包含7或8个信息比特加上2-4个控制比特，并通常与ASCII码的字符一致。当传输开始，无信息发送时，线路处于空闲状态，而空闲状态习惯上被称为信号电平。通常它对应于逻辑1电平。

When the transmitter wishes to send data, it first places the line in a space level (i.e., the complement of a mark ) for one element period. This element is called the start bit and has a duration of T seconds. The transmitter then sends the character, 1 bit at a time, by placing each successive bit on the line for a duration of T seconds, until all bits have been transmitted. Then a single parity bit is calculated by the transmitter and sent after the data bits. Finally, the transmitter sends a stop bit at a mark level (i.e., the same level at the idle state) for one or two bit periods. Now the transmitter may send another character whenever it wishes.

当发送器想要发送数据时，它首先将线路置成空号电平（即信号的反码），此电平持续一个单元（码元）的间隔时间。此（空号）单元称为起始位，持续时间为T秒。然后发送器发送字符，一次一个比特的将相继的码元送上线路。每个码元持续T秒，直到所有码元发完为止。此后发送器计算得出一个奇偶校验位并将它在数据码元之后发出。最后，发送器送出一个停止位，其电平为信号电平（于空闲状态电平相同），时长为1个或2个比特宽度。如果发送器需要，它又可发送另一个字符。

At the receiving end of an asynchronous serial data link, the receiver continually monitors the link looking for a start bit. Once the start bit has been detected, the receiver waits until the end of the start bit and then samples the next N bits at their centers, using a clock generated locally by the receiver. As each incoming bit is sampled, it is used to construct a new character. When the received character has been assembled, its parity is calculated and compared with the received parity bit following the character. If they are not equal, a parity error flag is set to indicate a transmission error.

在异步串行数据链路系统的接收端，接收器持续监视着线路。搜索着起始位。一旦检测到起始位并等到它结束，接收器就对随后的N个码元抽样，抽样点选择这些码元的中心处。抽样所用的时钟是由接收器本地产生的。当每一个输入的码元被抽样后，就用这些样值构成一个新的字符。当接收到的字符汇齐后，它的奇偶校验位就由计算得出并与接收到的奇偶校验位进行比较。如果它们不等，则将奇偶校验错误标志置位，以标明传输错误。

The most critical aspect of the system is the receiver timing. The falling edge of the start bit triggers the receiver’s local clock, which samples each incoming bit at its nominal center. Suppose the receiver clock waits T/2 seconds from the falling edge of the start bit and samples the incoming data every T seconds thereafter until the stop bit has been sampled. Fig .2-2 shows this situation. As the receiver’s clock is not synchronized with the transmitter clock, the sampling is not exact.

对系统来说，最关键的问题是接收器的定时。接收器的本地时钟由起始位的下降沿启动，然后在码元的标称中心处对每个输入比特进行抽样。接收器的时钟自起始位的下降沿开始等待T/2，而后每隔T秒抽样输入数据，直至抽样到停止位。图2-2标明的这种情况。如果接收器时钟与发送器时钟不同步，抽样则是不准确的。

The most obvious disadvantage of asynchronous data transmission is the need for a start, parity, and stop bit for each transmitted character. If 7 bit characters are used, the overall efficiency is only 70%. A less obvious disadvantage is due to the character-oriented nature of the

data link. Whenever the data link connects a CRT terminal to a computer, few problems arise, as the terminal is itself character oriented. However, if the data link is being used to, say, dump binary data to a magnetic tape ,problems arise.

对于每一个传送的字符，异步数据传输都需要起始位、奇偶校验位和停止位，这是它最明显的缺点。如果采用7比特字符，则总效率仅为70％。一个不太明显的缺点是由于数据链路面向字符的特性造成的。在数据链路中无论何时将CRT终端连接到计算机上都不会出现什么问题，因为终端本身也是面向字符的。但是如果数据链路由于别处，比方说，将大量二进制数据转储到磁带上，则会产生麻烦。

UNIT 3

The ISO Networking Standards

ISO联网标准

The early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980s saw tremendous increases in the numbers and sizes of networks. As companies realized they could save money and gain productivity by using networking technology, they added networks and expanded existing networks almost as rapidly as new network technologies and products could be introduced.

局域网、城域网和广域网的早期发展，在许多方面是混乱无序的。80年代初期，网络的数量和规模发展极快。由于公司意识到，利用联网技术，它们可以省钱和提高生产率，因而只要能引入新的网络技术和产品，它们就要扩充现有的网络和添加网络。

By the mid-1980s, these companies began to experience growing pains from all the expansions they made. It became harder for networks that used different specifications and implementations to communicate with each other. They realized that they needed to move away from proprietary networking systems.

到了80年代中期，这些公司开始经历由于他们扩张网络所带来的不断的麻烦。采用不同规范和实现的网络不能够相互通信。他们认识到，需要离开专用的联网系统。

Proprietary systems are privately developed, owned, and controlled. In the computer industry, proprietary is the opposite of open. Proprietary means that one or a small group of companies controls all usage and evolution of the technology. Open means that free usage of the technology is available to the public.

专用系统是指那些被私人开发、拥有和控制的网络。在计算机行业，专用是开放的反义词。专用意味着一个公司或一小群公司完全控制着技术的适用和发展。而开放却意味着技术可为整个公众所利用。

To address the problem of different network systems being incompatible and incapable of communicating with each other, the International Organization for Standardization (ISO) researched network schemes, such as DECnet, SNA, and TCP/IP, to find a set of rules. As a result of this research, the ISO created a network model that could help vendors create networks that would be compatible with, and interoperate with, other networks.

为了弄清不同网络系统不能兼容和不能互相通信的问题，国际标准化组织（ISO）研究了网络体系，比如DEC网、SNA和TCP/IP，以便寻找一些规律。结果，国际标准化组织创造了一个网络模型，它能帮助供应商创建可以与其他网络互相兼容和互操作的网络。

The OSI reference model, released in 1984, was the descriptive scheme they created. It provided vendors with a set of standards that could enable greater compatibility and interoperability between the various types of network technologies that were produced by many companies around the world.

OSI参考模型发布于1984年，是由其发明的可描述体系。它向供应商提供了一组标准，该标准能使在全世界由许多公司生产的各种各样的网络技术间极好的兼容和相互操作。

The OSI reference model is the primary model for network communications. A primary objective of the OSI reference model is to accelerate the development of future networking products. Although there are other models in existence, most network vendors today relate their products to the OSI reference model, especially when they want to educate users on the sue of their products. They consider it the best tool available to teach people about sending and receiving data on a network.

OSI参考模型是网络通信的基本模型。OSI 参考模型的主要目的是加快未来联网产品的开放。尽管存在其他的模型，今天大多数网络供应商将他们的产品和OSI参考模型联系在一起，尤其当他们想教育用户使用他们产品的时候更是这样。他们认为，要教会人们在网上发送和接收数据，OSI参考模型是最好的有效的工具。

The OSI reference model allows you to view the network functions that occur at each layer. More importantly, the OSI reference model is a framework you can use to understand how information travels throughout a network. In addition, the OSI reference model can be used to visualize how information, or data packets, travels from application programs, through a network medium, to other application programs that are located in another computer on a network, even if the sender and the receiver have different types of network media.

OSI参考模型使人看到在每一层上的网络功能。更为重要的是，OSI参考模型是这样一种结构，它使人理解信息如何通过网络传送。此外，OSI参考模型可被用来想象信息，或数据包是如何有应用程序通过网络截至被传送到位于网络傻瓜等另一个计算机上的，即使发送机和接收机具有不同类型的网络介质亦是如此。

In the OSI reference model, there are seven numbered layers. Each layer illustrates a particular network function. This separation of networking functions is called layering. Dividing the network into these seven layers provides the following advantages:

在OSI参考模型中有7层，每层表示了一个特定的网络功能。这种联网功能的分解被称为分层。将网络分成7层提供了下述优点：

? It breaks network communication into smaller, simpler parts that are easier to develop. ? It facilitates standardization of network components to allow multiplevendor

development and support.

? It allows different types of network hardware and software to communicate with each

other.

? It prevents changes in one layer from affecting the other layers, so that they can develop

more quickly.

? It breaks network communication into smaller parts to make learning it easier to

understand.

? 它将网络通信分成更小的、更简单的部分，这些部分更容易开发。 ? 它简化了网络部件的标准化，以容许多个供应商开发与支持。 ? 它容许不同类型的网络硬、软件互相通信。

? 它防止了一层的变动对其它层的影响，从而可以更快地进行开发。

? 它将网络通信分解成更小的部分，从而更易理解。

The process of moving information between computers is divided into seven smaller and more manageable steps in the OSI reference model. Each of the seven smaller problems is represented by its own layer in the model. The seven layers of the OSI reference model are

在OSI参考模型中，计算机之间传送信息的过程被分为7个较小的和交易处理的步骤。这每一个较小的问题由模型中相应的一层来表示。OSI参考模型的7层是：

? Layer7: the application layer ? Layer6: the presentation layer ? Layer5: the session layer ? Layer4: the transport layer ? Layer3: the network layer ? Layer2: the data link layer ? Layer1: the physical layer ? 第7层：应用层 ? 第6层：表示层 ? 第5层：会话层 ? 第4层：传输层 ? 第3层：网络层 ? 第2层：数据链路层 ? 第1层：物理层

Now let’s take a look at each layer in the OSI reference model, as shown in figure3-1. 现在，就让我妈来看看OSI参考模型中的每一层。该模型如图3-1所示。 The upper layers

The three upper layers of the OSI reference model are referred to as the application layers. Figure 3-2 shows the upper layers and provides information on their functionality with some examples.

较高层

OSI参考模型中的3个较高的层被称为应用诸层。图3-2显示了这些较高层，提供了他们的功能信息并给出了例子。

Layer7: the application layer

The application layer is the OSI layer that is closest to the user. It provides network services, such as file access and printing, to the user’s applications. It differs form the other layers in that is does not provide services to any other OSI layer, but rather, only to applications outside the OSI model. The application layer establishes the availability of intended communication partners. It also synchronizes and establishes an agreement on procedures for error recovery and control of data integrity. If you want to remember Layer 7 in as few words as possible, think of the browsers.

第7层：应用层

应用层是OSI参考模型中离用户最近的层。它为用户的应用提供了网络服务，诸如文件接入和打印等。与其它层的区别在于，它不向OSI任何其他层提供服务，相反的，它仅向OSI参考模型之外提供服务。应用层使对多方通信成为可能。它还对纠错和数据整合的过程进行同步并达成协议。如果想用寥寥数语记住第七层，请记住浏览器就行了。

Layer6: the presentation layer

The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation

layer translates between multiple data formats by using a common format. This layer also is responsible for compression and encryption. If you want to remember Layer 6 in as few words as possible, think of a common data format.

第六层：表示层

表示层保证由一个系统的应用层所发出的信息，另一个系统的应用层可以读懂。如果有必要，表示层可利用一种公共格式在多种数据格式之间进行转换。这一层还负责数据压缩和加密。如果想用寥寥数语记住第六层，请记住公共数据格式。

Layer5: the session layer

As its name implies, the session layer establishes, manages, and terminates sessions between two communication hosts. The session layer provides its services to the presentation layer. It also synchronizes dialogue between the two hosts’ presentation layers and manages their data exchange. In addition to session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems. If you want to remember layer 5 in as few words as possible, think of dialogues and conversations.

第五层：会话层

正如其名称所示，会话层建立、控制并终结在两个通信主机之间的对话。会话层向表示层提供服务。它还对两个主机表示层之间的对话进行同步，并对他们数据的交换进行管理。除了会话管理外，会话层还提供高效的数据传输、服务的分类以及对会话层、表示层、应用层的意外问题给出报告。如果想用寥寥数语记住第五层，请记住对话或谈话。

The lower layers

The four lower layers of the OSI model define how data is transferred across a physical wire through internetworking devices, to the desired end station, and finally to the application figure 3-3summarizes the basic function of these four layers.

较低层

OSI参考模型中的4个较低层定义了数据如果通过联网部件，经过物理线路传送到目的站，并最终到达应用。图3-3总结了这4层的基本功能。

Layer4: the transport layer

The transport layer segments data from the sending host’s system and reassembles the data into a data stream on the receiving host’s system. The boundary between the transport layer and the session layer can be thought of as the boundary between application protocols and data-flow protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower four layers are concerned with data transport issues.

第四层：传输层

传输层将发方主机系统送来的数据进行分段。并将数据重新组装成数据流送到收方主机系统上。传输层和会话层之间的交界可以认为是应用协议和数据流协议的交界。应用层、表示层和会话层涉及应用问题，而较低层却涉及数据传输问题。

The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, such issue as how reliable transport between two hosts is accomplished in the concern of the transport layer. In providing communication service, the transport layer establishes, maintains, and properly terminates connection-oriented circuits. In providing reliable service, transport error detection-and –recovery and information flow control are used. If you want to remember layer 4 in as few works as possible, think of the flow control and reliability.

传输层试图提供数据传送服务而将较高层在传输实现细节方面屏蔽。尤其应指出，在两个主机之间数据传送是否可靠之类的问题是由传输层来考虑实现的。在提供通信服务时，传输层建立、维持和终结面向连接的电路。在提供可靠服务方面，传输层采用了传输误码检测和恢复。以及信息流控制技术。如果想用寥寥数语记住第四层，请记住流量控制和可靠性。

Layer3: the network layer

The network layer is a complex layer that provides connectivity and path selection between two host systems that might be located on geographically separated networks. If you want to remember layer 3 in as few words as possible, think of path selection, routing, and logical addressing.

第三层：网络层

网络层是一个复杂的层。它在两个主机系统之间提供连接和路由选择，而这两主机系统可能分属在地理位置上两个不同的网络。如果想用寥寥数语记住第三层，请记住路由选择和逻辑选址。

Layer2: the data link layer

The data link layer provides the transit of data across a physical link. In so doing ,the data link layer is concerned with physical (as opposed to logical) addressing, network (sometimes called logical) topology, network media access, and error detection. If you want to remember layer 2 in as few words as possible, think of frames and media access control.

第二层：数据链路层

数据链路层负责数据通过物理链路的传送。此时，数据链路层涉及物理（而不是逻辑）选址、网络（有时称逻辑）拓扑、网络媒体接入以及误码检测。如果想用寥寥数语记住第二层，请记住帧和媒体接入控制。

Layer1: the physical layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications. If you want to remember layer 1 in as few words as possible, think of signals and media.

第一层：物理层

物理层定义了在端系统之间为了建立、维持和撤销物理链路所需的电气的、机械的、过程的和功能的特性。这些特性，例如电压电平、电压改变的定时、物理数据的速率、最大传输距离、物理连接器件以及其他类似的特征，都由物理层的说明来定义。如果想用寥寥数语记住第一层，请记住信号和媒体。

UNIT4

The TCP/IP Reference Model TCP/IP参考模型

Although the OSI reference model is recognized universally, the historical and technical open standard of the Internet is Transmission Control Protocol/Internet Protocol (TCP/IP). The TCP/IP reference model and the TCP/IP protocol stack make data communication possible between any two computers, anywhere in the world. The TCP/IP model has historical importance, just like the standards that allowed the telephone, electrical power, railroad, television, and videotape

industries to flourish.

尽管OSI参考模型被全球认可，但互联网历史的技术上的开放标准却是传输控制协议/互联协议(TCP/IP)。TCP/IP参考模型和TCP/IP协议簇使世界上任何地点、任何两个计算机之间的数据通信成为可能。TCP/IP模型具有历史上的重要性，正如那些曾让电话、电力、铁道、电视和录像带行业繁荣昌盛的标准一样。

The Layers of the TCP/IP Reference Model

The U.S. Department of defense (DOD) created the TCP/IP reference mode because it wanted a network that could survive any condition, even a nuclear war. To illustrate further, imagine a world at war, crisscrossed by different kinds of connections: wires, microwaves, optical fibers, and satellite links. Then imagine that you need information/data(in the form of packets)to flow, regardless of the condition of any particular node or network on the inter network (which, in This case, might have been destroyed by the war). The DOD wants its packets to get through every time, under any condition, from any one point to any other point. It was this very difficult design problem that brought about the creation of the TCP/IP reference model, which is the standard on which the internet has grown.

TCP/IP参考模型的分层

因为美国国防部曾想建立一种能在任何条件下，甚至在核战争条件下存活的网络，所以它创建联网TCP/IP参考模型。为了进一步说明问题，让我们想象在战争中的世界，被各种各样连接弄的杂乱无章：电线、微波、光纤，以及卫星链路。然后设想你需要信息或数据（以分组的形式）流动，而不管在哪个互联网络中任何一个节点或任何一个网络的条件（这些节点或网络，在该情况下，兴许已被战争摧毁了）。国防部期望在任何条件下，从任一点到另一点，每次都能获得其分组。正是这个极为困难的设计问题创造了TCP/IP参考模型，而这个标准却是互联网发展起来的基础。

As you read about the TCP/IP model layers, keep in mind the original intent of the Internet; it can help explain why certain things are as they are. The TCP/IP model has for layers: the application layer, the transport layer, the internet layer, and the network access layer.

当你在学习TCP/IP模型的各层时，请记住互联网的原始意图，它能够帮助说明为什么某些事情会是那样的。TCP/IP模型具有四层:应用层、传输层、互联层、网络接入层。

Application layer

The designers of TCP/IP felt the higher-level protocols should include the session and presentation layer details. They simply created an application layer that handles high-level protocols, issues of representation, encoding, and dialog control. The TCP/IP combines all application-related issues into one layer, and ensures this data is properly packaged for the next layer. This is also referred to as the process layer.

应用层

TCP/IP的设计者们认为，较高层协议应包括会话层和表示层的细节。他们简化了应用层，使之处理了高层的协议、表示的事宜、编码和会话控制。TCP/IP将所有与应用层相关的问题都综合进了一层，并确保这个数据被打包到下一层。该层亦被称为处理层。

Transport layer

The transport layer typically deals with the issues of the reliability, flow control, and retransmission. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well flowing, network communications. TCP is a connection-oriented protocol. It supports dialogues between source and destination while packaging application layer information into units called segments. Connection-oriented does not

mean that a physical circuit exists between the communication computers (that would be circuit switching). It does mean that a logical connection must be set up before data can be sent to let layer 4 segments travel back and forth. This layer is also sometimes called the host-to-host layer.

传输层

典型地，传输层涉及可靠性、流量控制和重传问题。其协议之一，传输控制协议（TCP）提供了完美的和灵活的方法以产生可靠、通畅的网络通信。TCP是一个面向连接的协议。它将应用层的信息打包，成为一种被人们称为字段的单元，以此支持在信源和信宿间的对话。面向连接并不意味着在通信的计算机之间存在着物理的电路。它意味着在两个主机之间必须来回传送第四层的字段，以便在可传送数据之前建立逻辑连接。这一层有时也被称为主机到主机层。

Internet layer

The purpose of the internet layer is to send source packets from any netwok on the internetwork and have them arrive at the destination independent of the path an networks they took to get there. The specific protocol that governs this layer is called the Internet Protocol (IP). Best path determination and packet switching occur at this layer. Think of it in terms os the postal system. When you mail a letter, you do not know how it gets there (there are various possible routes), but you do care that it arrives.

互联层

互联层的作用是由互联网络上任意网络发出源数据包并将他们送至信宿，而与到信宿所走的路由和网络无关。主宰这一层的特定协议被称为互联协议（IP）。最佳路由确定和分组交换都发生在这一层。可联想一些邮政系统，如果想寄一封信，你并不知道它是怎样到达对方的（可能会有多条路由），但你会关心它是否到达。

Network access layer

The name of this layer is broad and somewhat confusing. It also is called the host-to-network layer. Sometimes, it’s shown at tow layers, as in the OSI model. The network access layer is concerned with all the issues that an IP packet requires to actually cross a physical link from one device to a directly connected one. It includes the LAN and WAN technology details, and all the details in the OSI physical and data link layers.

网络接入层

这一层的名字是宽泛的，在某种程度上令人有点迷惑不解。它还被人们称为主机到网络层。有时它就像OSI模型中的那样，被用两层表示。网络接入层涉及到一个IP分组从一个部件实际穿过一条物理链路而达到直接相连的一个部件的所有问题。它包括局域网和广域网的技术细节，以及在OSI模型中物理层和数据链路层的所有细节。

TCP/IP Protocol Graph

The diagram shown in fig4-1 is called a protocol graph. At the application layer, you see different network tasks you might not recognize but, as a user of the Internet, probably use ever day. These applications include the following:

? FTP---file transfer protocol

? HTTP---hypertext transfer protocol ? SMTP---simple mail transfer protocol ? DNS---domain name system

? TFTP---trivial file transfer protocol TCP/IP协议图

图4-1所示的图形被称为协议图。在应用层，你看到你可能还不了解的许多不同的网络

任务，然而作为互联网用户，你每天兴许都在使用他们。这些应用包括下述部分： ? FTP---文件传送协议 ? HTTP---超文本传输协议 ? SMTP---简单邮件传输协议 ? DNS---域名系统

? TFTP---简单文件传输协议

The TCP/IP model emphasizes maximum flexibility, at the application layer, for developers of software. The transport layer supports two protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The lowest layer, the network access layer, refers to the particular LAN or WAN technology that is being used.

TCP/IP模型强调在应用层给软件开发者最大的灵活性。传输层支持两个协议：传输控制协议（TCP）和用户数据报协议（UDP）。最底层，网络接入层，是指在被使用的特定的LAN技术或WAN技术。

In the TCP/IP model, regardless of which application requesting network services, and regardless of which transport protocol used, there is only one network protocol: Internet Protocol (IP). This is a deliberate design decision. IP serves as a universal protocol that allows any computer, anywhere, to communicate at any time.

在TCP/IP模型中，不论那种应用需要网络服务，也不管使用那种传输协议，网络协议只有一种：互联协议（IP）.这种设计选择是深思熟虑的。IP作为一种通用的标准，它允许任何地方任何计算机在任何时间进行通信。

If you compare the OSI model and the TCP/IP model, you can notice that they have similarities and differences (see fig4-2). Examples include the following：

如果将OSI模型与TCP/IP模型作一比较，你会注意到，他们既有相似之处，又有一些不同。（见图4-2）。例如下面所列的几点：

Similarities:

? Both have layers.

? Both have application layers, although they include very different services. ? Both have comparable transport and network layers.

? Packet-switched (not circuit-switched) technology is assumed. ? Networking professionals need to know both models. 相似点：

? 两个模型都由多层组成。

? 两个模型都有应用层，尽管他们包括的服务极为不同。 ? 两个模型都有类似的传输层和网络层。

? 都以分组交换（而不是电路交换）技术为背景。 ? 联网专业人员对两个模型都应了解 Differences:

? TCP/IP combines presentation and session layer functions into its application layer. ? TCP/IP combines the OSI data link and physical layers into one layer.

? TCP/IP appears simpler because it has fewer layers; however, this is a misconception.

The OSI reference model, with its less complex and multiple layers, is simpler to develop and troubleshoot.

? TCP/IP protocols are the standards around which the Internet was developed, so the

TCP/IP model gains credibility just because of its protocols. In contrast, networks

typically aren’t built with the OSI protocols, although the OSI model is used as a guide. 不同点：

? TCP/IP将表示层和会话层的功能全部并入到它网的应用层中。 ? TCP/IP将OSI的数据链路层和物理层合成了一层。

? TCP/IP显得更为简单，因为其层数少，但是这只是一种错觉。OSI参考模型具有

多层但却不复杂，因而在开发和故障诊断方面更为简单。

? 由于TCP/IP协议是当时开发建设互联网所基于的标准，因而TCP/IP模型由于其

协议获得了广泛的信任。与之相反，尽管OSI模型被用来引导入门，但以OSI协议所建设的网络却不典型。

Many networking professionals have different opinions on which model to use. You should become familiar with both. You can use the OSI model as the microscope through which to analyze networks, but you also can use the TCP/IP protocols in common situation. Remember that there is a difference between a model (that is, layers interfaces, and protocol specifications) and an actual protocol that is used in networking.

许多互联网专家对采用那种模型存在意见分歧。你应对这两者都熟悉为好。可将OSI模型用作显微镜，通过它去分析网络，但在通常情况下，也能用TCP/IP协议来讨论问题。应记住，在模型（即分层、接口和协议规范）和在联网中采用的实际协议之间，是有区别的。

UNIT 5

Local-Area Networks (1) 局域网（1）

The focus of this chapter is local-area networks (LANs). 本章的重点是局域网。

As you know, LANs are high-speed, low-error data networks that cover a relatively small geographic area (up to a few thousand meters). LANs connect workstations, peripherals, and other devices in a single building or other geographically limited area.

我们知道，局域网是一个覆盖在地理上相对面积不大（不超过几千米）的高速的、低误码的数据网络。局域网连接工作站、外部设备和其他设施，这些设施分布在一座大楼，或其他有限面积的地段。

Topology defines the structure of the network. The topology definition contains two parts: the physical topology, which is the actual layout of the wire (media), and the logical topology, which defines how the media is accessed by the hosts. The physical topologies that are used commonly are bus, ring, star, extended star, hierarchical, and mesh (see figure 5-1).

拓扑定义网络的结构。拓扑的定义含有两个部分：物理拓扑，它是线路（介质）的实际安排；逻辑拓扑，它定义了媒体如何接入主机。通常使用的物理拓扑有总线型、环形、星型、扩展式星型、分层和网络结构（如图5-1示）

The commonly used physical topologies are:

? Bus topology—Uses a single backbone segment (length of cable) to which all the hosts

directly connect.

? Ring topology—Connects one host to the next and the last host to the first. This creates

a physical ring of cable.

? Star topology—Connects all cables to a central point of concentration. This point is

usually a hub or a switch.

? Extended star topology—Uses the star topology. It links individual stars together by

linking the hubs/switches. This extends the length and the size of the network.

? Hierarchical topology—Similar to an extended star topology, but instead of linking the

hubs/switches together, each secondary (subordinate) system is linked to a primary computer that controls the traffic on the topology.

? Mesh topology—Used when there can be absolutely no break in communication. As you

can see in figure5-1, with a full mesh topology, each host has its own connections to all other hosts. A partial mesh reflects the design of the Internet, which has multiple paths to any one location, although not a connection from every host to every other host. 常用的拓扑是：

? 总线拓扑－采用单一的主干线路段（即电缆长度），所有的主机都直接接到这段骨

干线上。

? 环型拓扑—将一个主机连接到下一个主机，而最后一个主机又连到第一个主机上。

这样就产生了一个物理的电缆环。

? 星型拓扑－将所有的电缆都连接到一个中心点上集中，这个点通常是一个集线器

或交换器。

? 扩展式星拓扑－使用星型拓扑。它采用连接集中器或交换器的办法将各个星型连

接到一起。这样就扩展了网络的长度和大小。

? 分层拓扑－类似于扩展式星型拓扑，但不是将集中器或交换器连接到一起。每个

第二级（下级）系统被连到一个主机上，由该主机控制拓扑中的流量。

? 网状拓扑――在要求绝对没有通信中断的条件下，使用这种拓扑。正如图4-1所示

的那样，在完全往事的拓扑中，每一个主机都有连线到任何其他主机。互联网（internet）的设计表现为部分网状拓扑。在互联网中，到任意一点具有多条路径，但不是每个主机到每个其他的主机都有连线。

Figure 5-2 shows many topologies. It shows a LAN of moderate complexity that is typical of a school or a small business. It has many symbols, and it depicts many networking concepts that you will learn.

图5-2表示了多种拓扑。它画出了一个适度复杂的局域网，这种局域网典型的用于学校或小的机关。图中有多种符号，它表示了许多将要学到的联网知识。

Devices that connect directly to a network segment often are referred to as hosts. These hosts include computers (both clients and servers), printers scanners, and many other user devices. The host devices can exist without a network, but its capabilities are greatly limited.

直接连到一个网络段的部件常被称为主机。这些主机包括计算机（无论客户机或服务器）、打印机、扫描仪和许多其他用户设备。主机设备可以离开网络单独存在，但其能力将大打折扣。

Host devices are not part of any layer. They have a physical connection to the network media by having a network interface card (NIC) and the functions of the other OSI layers are performed in software inside the hose. This means that they operate at all seven layers of the OSI model. Host devices perform the entire process of the encapsulation and de-encapsulation to do their job of sending e-mails, printing reports, scanning pictures, or accessing databases. For those who are familiar with the inner workings of PCs, think of the PC itself as a tiny network that connects the bus and the expansion slots to the CPU, the RAM, and the ROM.

主机设备并不属于某一层。他们到网络介质的物理连接有网络接口卡（NIC）来完成，

而OSI其他层的功能由主机中的软件来完成。这就意味这他们运行于OSI模型的整个7层中。主机设备实现着打包和拆包的全过程，以完成诸如发送电子邮件、打印报告、扫描图片或接入数据库等工作。对熟悉个人计算机内部工作原理的人来说，可将PC机本身看成一个小型的网络，它将总线和扩展口连接到CPU，RAM和ROM.

There are no standardized symbols within the networking industry for hosts, but they usually are fairly easy to figure out. As shown in figure 5-2, they look like the real device so you are constantly reminded of that device.

在网络业内部，并没有标准化的符号表示主机，但它们通常极易识别。如图5-2所示，它们很像实际设备，因而记住它们是不成问题的。

Some important devices depicted in Figure 5-2 are explanted as following. 在图5-2中描绘了一些重要部件可说明如下。 NICs 网络接口卡

So far in this chapter, you deal with layer 1 devices and concepts. Starting with the network interface card (NIC), the discussion moves to layer two, the data link layer, of the OSI model. NICs are considered layer 2 devices because each individual NIC throughout the world carries a unique code, called a Media Access Control (MAC) address. This address is used to control data communication for the host on the network.

本章到现在为止，仅涉及到第一层的部件和概念。由网络接口卡开始，讨论进入第二层，即OSI模型的数据链路层。网络接口被认为是第二层的部件是由于世界上每一个网络接口卡都携带有一个独特的码，即媒体接入控制（MAC）地址。这个地址被用来控制网上主机的数据通信。

In terms of physical appearance, a NIC is a printed circuit board, that fits into the expansion slot of a bus on a computer’s motherboard or peripheral device. It is also called a network adapter. Its function is to adapt the host device to the network medium.

就物理外形来看，网络接口卡是一个印刷电路板，它可插入计算机主板或外设的总线扩展槽里。它亦被称为网络适配器，其功能是使主机适应网络介质。

The NIC is the basic hardware component of network communications. It translates the parallel signal produced by the computer into the serial format that is sent over the network cable. The 1s and 0s of binary communication are turned into electrical impulse, pulses of light , radio waves , or whatever signaling scheme he network media uses.

网络接口卡是网络通信的基本硬件。它将由计算机产生的并行信号转换成串行格式，此串行信号才通过网络电缆发送。二进制通信的1和0被转换为电脉冲、光脉冲、无线电波或网络介质使用的不管何种信号体系。

NICs have no standardized symbol. It is implied that, whenever you see networking devices attached to network media, there is some sort of NIC or NIC-like device present (although it is generally not shown). Wherever you see a dot on a topology, there is either a NIC or an interface (port) that acts like at least part of a NIC.

网络接口卡没有标准化的符号。这就意味着，无论何时看到网络器件被连到网络介质上，就有某种网络接口卡或类似网络接口卡的部件存在（尽管通常不显示出来）。无论在哪个拓扑图中看到一个圆点，则要么有网络接口卡，要么就有一个接口，它至少部分起着网络接口卡的作用。

Media 介质

The symbols for media vary, as shown in figure 5-3. For example, the Ethernet symbol is typically a straight line with perpendicular lines projecting from it ; the Token Ring network

symbol is a circle with hosts attached to it ; and for FDDI, the symbol is two concentric circles with attached devices.

介质的符号有多种，如图5-3示。例如，以太网的典型符号是一条直线，并由它射出许多垂直线。令牌环网的符号是一个圆圈，许多主机与之相连。对FDDI，符号是两个同心圆并连有许多部件。

The basic functions of media are to carry a flow of information, in the form of bits, through a LAN. Other than wireless LANs (that use the atmosphere, or space, as the medium) and the new PANs (personal-area networks, which used the human body as a networking medium!), networking media confine network signals to wire, cable, or fiber. Networking media are considered Layer 1 components of LANs.

介质的基本功能以比特的形式通过局域网传送信息流。与无线局域网（它采用大气或空间作为媒体）和新的人体网（个人网络――它利用人体作为联网介质）不同，网络介质将网络信号局限于导线、电缆或光纤上。联网介质被认为是局域网的第一层部件。

You can build computer networks with many different media types. Each media has advantages and disadvantages. What is an advantage for one media (Category 5 cost) might be a disadvantage for another (fiber optic cost). Some of the possible advantages or disadvantages include the following:

你可用许多不同的介质类型构建计算机网络。每种介质都有有缺点。一种媒体的优点（如五类线的价格）可能是一种媒体的缺点（如光纤的价格）。一些可能的优缺点常包括：

? Cost

? Ease of installation

? Maximum cable length ? 价格

? 安装的难易 ? 最大电缆长度

Coaxial cable, optical fiber, and even free space can carry network signals. However, the principal medium you will study is called Category 5 unshielded twisted-pair (CAT 5 UTP) cable because it is the most-used medium in network installations.

同轴电缆、光纤，甚至自由空间都能传送网络信号。但是，你将学习的主要介质被叫做五类非屏蔽双绞线（CAT 5 UTP）, 因为它是网络安装中最常使用的介质。

Repeaters

As mentioned in the ―media‖ section, many types of media exist and each type has advantages and disadvantages. One of the disadvantages of the type of cable that we primarily use, CAT 5 UTP, is cable length. The maximum length for UTP cable in a network is 100 meters (approximately 333 feet). If you need to extend beyond the network limit, as shown in figure 5-4, you must add a device to your network. This device is called a repeater.

中继器

正如在介质一节所提及，存在着许多类型的介质，且每种都有其优缺点。我们主要使用的电缆，即CAT 5 UTP, 其缺点是电缆长度。在一个网络中UTP电缆长度的最大长度是100米（大约333英尺）。如果想要将网络延伸超过这个界限，如图5-4所示，就必须向网络添加设备，该设备被称为中继器。

The term repeater comes from the early days of visual communication, when a man situated on a hill received a signal from a person on the hill to his left, and then repeated the signal to the person on the hill to his right. It also comes from telegraph, telephone, microwave, and optical

communications, all of which use repeaters to strengthen their signals over long distances to prevent the signals from fading or dying out.

中继器一词源于视频通信的早期，当时在一个山头的人收到左边山头发来的信号，然后再生（中继）这一信号送到右边山头上。该词也源于电报、电话、微波和光通信，它们在长途通信中都使用中继器以增强信号，防止信号衰减或消失。

Like networking media, repeaters are networking devices that exist at layer 1, the physical layer, of the OSI reference model. To begin understanding how a repeater works , it is important to understand first that as data leaves a source and goes out over the network, it is transformed into either electrical or light pulses that pass along the networking media. These pulses are referred to as signals. When signals first leave a transmitting station, they are clean and easily recognizable. However, the longer the cable length , the weaker and more deteriorated the signals become as they pass along the networking media. For example, specifications for Category 5 twisted-pair Ethernet cable establish the maximum distance that signals can travel along a network as 100 meters. If a signal travels beyond that distance, there is no guarantee that a NIC can read the signal. A repeater can provide a simple solution if this problem exists.

和联网介质一样，中继器也是OSI参考模型第一层，即物理层的联网部件。要懂得中继器如何工作，重要的是首先要得知，当数据离开信号源并且通过网络传输时，它被转换成要么电脉冲，要么光脉冲再通过联网介质传送。这些脉冲被称为信号。当信号最初离开一个发送站，它们是清晰的、容易识别的。但是当它们通过联网介质传送的时候，电缆的长度越长，信号就越弱，信号质量就越差。例如，五类双绞以太网电缆的技术标准设定的最大距离是信号能够经网络传输100米。如果信号传送超过了这一距离，则不能保证网络接口卡能读这一信号。如果存在这样的问题。一个简单的解决办法即使采用中继器。

UNIT 6

Local-Area Networks (2) 局域网（2）

Hubs

Generally speaking, the term hub is used instead of repeater when referring to the device that serves as the center of a network, as shown in Figure6-1. Although a hub operates in physical star topology, it creates the same contention environment as a bus. This is because when one device transmits, all other devices hear is and the contention creates a logical bus.

集线器

一般的讲，当我们需要一个部件作为网络中心时，就要用集线器来代替中继器，这种情况如图6-1所示。尽管集线器运行于物理的星型拓扑之中，它却造成了像总线一样的竞争机制。这是因为当一个部件发送时，所有其他部件侦听，竞争产生了一个逻辑总线。

The purpose of a hub is to regenerate and retime network signals. This is done at the bit level for a large number of hosts (for example, 4,8,or eren24). This is known as concentration. The following are the most important properties of hubs:

集线器的作用是对网络信号进行再生和再定时。这一工作完成在比特层面上而且是对多个主机（例如，4，8，甚至24个主机）进行的。这被称为“集中”。集线器最重要的性质如下述：

? They regenerate and repeat signals.

? They propagate signals through the network.

? They cannot filter network traffic. ? They cannot determine the best path.

? They are used as network concentration points. ? 它们对信号进行再生和重复。 ? 它们通过网络传递信号

? 它们不能对网络流量进行过滤 ? 它们不能决定最佳路径

? 它们被用来作为网络的集中点。

You will notice the characteristics of a hub are similar to the repeater’s, which is why a hub is also known as multi-port repeater. The difference is the number of cables that connect to the device. A repeater typically has only two ports, but a hub generally has from four to twenty or more ports. Whereas a repeater receives on one port and repeats on the other, a hub receives on one port and transmits on all other ports.

你将会看到，集线器的特点与中继器极为相似，这就是集线器也被称为多端口中继器的原因。它们的不同点是与设备连接的电缆数量。典型地，中继器只有两个端口，而集中器的端口通常为4个，甚至到20个或更多。中继器从一个端口上接收，重发到另一个端口上；而集中器从一个端口上 接收后，却将它送传到其他所有的端口上。

Two reasons to use hubs are to create a central connection point for the wiring media, and to increase the reliability of the network. The reliability of the network is increased by allowing any single cable to fail without disrupting the entire network. This differs from the bus topology where having one cable fail disrupts the entire network. Hubs are considered layer 1 devices because they only regenerate the signal and repeat it out all of their ports.

使用集线器有两个原因，一是对所铺设的媒体设置一个中央连接点，二是想增加网络的可靠性。网络可靠性的增加是因为它允许任何一个电缆失效而又不影响整个网络。这不同于总线拓扑，因为在总线拓扑中一根电缆失效将影响整个网络。集线器被认为是第一层的部件，因为它仅仅再生信号并重发到它的所有端口上。

There are different classifications of hubs in networking. The first classification is passive or active hubs. Most modern hubs are active; they take energy from a power supply to regenerate network signals. Some hubs are called passive devices because they merely split the signal for multiple users, like using a ―Y‖ cord on a CD player to use more than one set of headphones Passive hubs do not regenerate bits, so they do not extend a cable’s length, they allow only two or more hosts to connect to the same cable segment. As a result, passive hubs weaken the signal.

在联网中对集线器有许多不同的分类。第一种分类是无源或有源的集线器。大多数现代集线器都是有源的，它们需要电能供应以便再生网络信号。某些集线器被称为无源的，因为它们仅仅将信号分到多个用户，就像CD播放器使用“Y”状插口可以使用多个耳机。无源集线器不能再生比特，因而不能加大电缆的长度，它们只允许两个或多个主机连接到同一个电缆段上。结果，无源集线器削弱了信号。

Active hubs also are called multiple port repeaters because they have multiple ports and they regenerate the signal coming into one port before sending it back out the other ports ( like a repeater). Active hubs require electrical power. The intelligent hub is a special type of active hub. It not only regenerates the signal, it also has an onboard processor that enables you to perform diagnostics and to detect whether there is a problem with a particular port.

有源集线器也被称为多端口中继器，因为它们具有多个端口并且将进入一个端口的信号再生到其他端口上。有源集线器休要电能。智能型集线器是有源集线器的一个特例。它不

仅再生信号，而且还具有一个处理器，该处理器能实现故障诊断以及检测是否在某个端口存在问题。

Bridges

A bridge is a layer 2 device designed to create two or more LAN segments, each of which is a separate collision domain. That is , they were designed to create more useable bandwidth. The purpose of a bridge is to filter traffic on a LAN- to keep local traffic local- yet allow connectivity to other pares ( segments) of the LAN for traffic that is directed there. You might wonder, then, how the bridge knows which traffic is local and which is not. The answer is the same one the postal service uses when asked how it knows which mail is local. It looks at the local address. Every networking device has a unique MAC address on the NIC. The bridge keeps track of MAC addresses on each side of the bridge and makes its decisions based on this MAC address list.

网桥

网桥是第二层部件，用以产生两个或多个局域网段，其中每段都是一个独立的冲突域。这就是说，它们被设计来产生更多的可用的带宽。网桥的作用是要过滤局域网上的流量――将本地的流量保持在本地－而有对要送往局域网的其他部分（段）的流量建立连接。那么你可能会奇怪，网桥是怎么知道那些流量是本地的，而那些有不是。其答案与邮政服务－问它怎么知道那封信是本地的－是一个道理。它看的是本地的地址。每一个联网部件的网络接口卡上都具有惟一的MAC地址。网桥注视着在其每边的每个MAC地址，并以这个MAC地址表来做出决策。

Bridges filter network traffic by looking only at the MAC address. Therefore , they can rapidly forward traffic representing any network layer protocol. Because bridges look only at MAC addresses, they are not concerned with network layer protocols. Consequently, bridges are concerned only with passing or not passing frames, based on their destination MAC addresses. The following are the important properties of bridges:

网桥根据MAC地址来过滤网络流量。因此，它们能够迅速地将代表着任何网络层协议的流量向前传送。由于网桥只看MAC地址，它们就不涉及网络层的协议。因而，网桥只根据帧的目的MAC地址，关心是通过还是不通过。以下是网桥的一些重要特征:

? They are more intelligent than hubs—that is , they can analyze incoming frames and

forward (or drop) them based on addressing information.

? They collect and pass packets between two or more LAN segments.

? They create more collision domains, allowing more than one device to transmit

simultaneously without causing a collision. ? They maintain address tables.

? 比起集线器，它们更富有智能－即它们能对流入的帧进行分析，并基于地址信息

将它们继续传送或滤除。

? 它们在两个或更多个LAN段之间收集和传送分组。

? 它们产生更多个冲突域，允许两个以上的部件同时发送且不会产生冲突。 ? 它们保持着地址表。 Switches

A switch is a layer 2 device just as a bridge is. In fact, a switch is sometimes called a multi-port bridge, just like a hub is called a multi-port repeater. The difference between the hub and the switch is the same as the difference between a repeater and a bridge: switches make decisions based on MAC address and hubs don’t make decisions at all.

交换器

交换器是第二层部件，正像网桥一样。事实上，交换器有时被称为多端口网桥，就像集线器被叫做多口中继器一样。集线器和交换器之间的不同与在中继器和网桥之间的不同一样：交换器依据MAC地址做出决策，而集线器根本不做任何决策

Because of the decisions that switches make, they make a LAN much more efficient. They do this by ―switching‖ data only out the port to which the proper host is connected. In contrast, a hub sends the data out all its ports so that all the hosts have to see and process (accept or reject) all the data.

由于交换器做出决策，因而它们使LAN高效的多。它们只将数据交换到与合适的主机连接的端口上。相反的，集线器却是将数据发到它全部端口上，以至于所有的主机不得不看到并处理（接收或拒绝）所有的数据。

Switches, at first glance, often look like hubs. Both hubs and switches have many connection ports because part of their function is connectivity concentration(allowing many devices to be connected to one point in the network). The difference between a hub and a switch is what happens inside the device. Figure 6-2 shows the symbol for a switch. The arrows on top represent the separate paths data can take in a switch, unlike the hub, where all data flows on all paths.

粗看起来，交换器和集线器往往很像。无论是集线器和交换器都有许多连接端口，因为它们的部分功能是集中连接（让许多部件被连到网络的一个点上）。集线器与交换器之间的差别发生在部件内部。图6-2显示了交换器的表示符号。顶部的箭头代表了在交换器中数据可取的路由。它与集线器不同，在那里所有数据流向所有的路径之中。

The purpose of a switch is to concentrate connectivity while making data transmission more efficient. For now, think of the switch as something that combines the connectivity of a hub with traffic regulation of a bridge on each port. It switches frames from incoming ports (interface) to outgoing ports while providing each port with full bandwidth (the transmission speed of data on the network backbone).

交换器的目的是集中连接且使数据传输更为高效。眼下，可将交换器看成这样一个部件，它能将集线器的连通性和网桥在每个端口上的流量规则结合起来。它将输入端口（接口）的帧交换到输出端口上，而且提供每个端口整个宽带（即在网络主干上数据的传输速率）。

Routers

The router is the first device you work with that is at the OSI network layer, otherwise known as layer 3. working at layer 3 allows the router to make decisions based on network addresses as opposed to individual layer 2 MAC addresses. Routers also can connect different layer 2 technologies, such as Ethernet, token ring, and FDDI. However, because of their capability to route packets based on layer 3 information, routers have become the backbone of the Internet, running the IP protocol.

路由器

路由器是与OSI网络层，或称为第三层，打交道的第一个器件。工作在第三层使路由器依据网络地址做出决策，而不是依据第二层的MAC地址。路由器也能与不同的第二层技术相接，如以太网、令牌环和FDDI.。然而，由于它们有根据第三层信息为分组选择路由的能力，路由器已经成了运行IP协议的互联网骨干。

The purpose of a router is to examine incoming packets (layer 3 data), choose the best patch for them through the network, and then switch them to the proper outgoing port. Routers are the most important traffic-regulating devices on large networks. They enable virtually an type of computer to communicate with any other computer anywhere in the world! While performing

Each network is only capable of transporting one specific service for which it was intentionally designed. Only in a limited number of cases and by using additional equipment (e.g. a modem) and with an inefficient use of its resources can it be adapted to other services.

? 业务独立性

每个网络只能传送一种特定的业务（信息类型），该网络是为该业务精心设计的。仅在有限的几种情况下利用附加设备（例如调制解调器），以及低效利用资源时，该网络才适用与其它业务。

? Inflexibility

Advances in audio, video and speech coding and compression algorithms and progress in Very Large Systems Integration (VLSI) technology influence the bit rate generated by a certain service and thus change the service requirements for the network. In the future, new services with unknown requirements will appear. For the time being it is yet unclear, e.g. what the requirements in terms of bit rate for HDTV will be. A specialized network has great difficulties in adapting to changing or new service requirements.

? 不灵活性

在声音、图像、语音编码和压缩算法以及在特大系统综合（VLSI）技术方面的进步影响着一定业务的比特率，因而改变着对网络的业务要求。在将来，未知需求的新业务将会出现。例如，HDTV(高清晰度TV)将在比特率上有何要求当前仍不清楚。一个专用化网络在适应变化和新业务要求方面具有极大困难。

? Inefficiency

The internal available resources are used inefficiently. Resources which are available in one network cannot be made available to other networks.

Taking into account all these considerations on flexibility, service dependence and resource usage, it is consequently very important in the future that only a single network exists and that this network of the future (B-ISDN) is service-independent. This implies a single network capable of transporting all services, sharing all its available resources between the different services.

? 低效

内在可用资源低效适用。在一个网络中可用的资源不能对其它网络同样有效。

考虑到所有这些因素，即灵活性、业务独立性和资源利用程度，未来极其重要的一点即仅有一个网络存在，且这个未来网络（B-ISDN）是与业务无关的。这意味着单个网络可传送所有业务，在多种不同的业务之间共享其所有可用资源。

A single service-independent network will not suffer from the disadvantages described above, but it will have the following main advantages:

单一的与业务无关的网络将没有上述缺点，但它将有下述特点： ? Flexible and future-safe

Advances in the state of the art of coding algorithms and VLSI technology may reduce the bandwidth of existing tele-services. A network capable of transporting all types of services will be able to adapt itself to changing or new needs.

? 灵活和面向未来

在编码算法和VLSI技术方面的进步可以减少现有电信业务的带宽。可传送所有业务类型的网络将能对变化或新的需求自我适应。 ? Efficient in the use of its available resources

All available resources can be shared between all services, such that an optimal statistical sharing of the resources can be obtained.

? 有效利用可用资源

所有的业务可共享所有可用资源，以达到资源在统计意义上的最优分配。 ? Less expensive

Since only one network needs to be designed, manufactured and maintained, the overall costs of the design, manufacturing, operations and maintenance will be smaller.

? 费用较低

由于仅需设计、制造、和维护一个网络，设计、制造、运行和维护的整个开销将变得更小。

these basic functions, routers also can execute many other tasks, which may be mentioned later.

路由器是的作用是检查流入的分组（第三层的数据），为它们选择通过网络的最佳路径，然后把它们交换到适当的输出端口。在大型网络中，路由器是最为重要的流量控制器件。它们事实上能使任何类型的计算机与世界上任何地方的任何其它的计算机进行通信！在实现这些基本功能时，路由器还能执行许多其它任务，这些也许今后会提及。

Routers differ from bridges in several respects. First, bridging occurs at layer2, the data link layer, whereas routing occurs at layer 3 ,the network layer of the OSI reference model. Second, bridges use physical or MAC addresses to make data forwarding decisions. Routers use different addressing scheme that occurs at layer 3 to make forwarding decisions. They use network-layer addresses, which are referred to as Internet protocol (IP), or logical, addresses, rather than MAC addresses. Routers match information in the routing table with the data’s destination IP address and send incoming data toward the correct sub-network and host. Because IP addresses are implemented in software and refer to the network on which a device is located, sometimes these layer 3 addresses are referred to as protocol addresses or network addresses. Physical, or MAC addresses, usually are assigned by the NIC manufacturer and hard-coded into the NIC. Network-layer addresses, or IP addresses, on the other hand, usually are assigned by the network administrator.

路由器与网桥在许多方面不同。首先，网桥发生在第二层，即数据链路层，而路由器发生在第三层，即OSI参考模型的网络层。第二，网桥利用物理的或MAC地址对数据送出做出决定。路由器利用不同的寻址体系对数据前进做出决定，该寻址体系发生在第三层。它们利用网络层地址，此地址称为互联协议地址。或逻辑地址，而不是MAC地址。路由器将路由表中的信息与数据的目的IP地址选配并将流入的数据发往正确的子网和主机。由于IP地址是由软件实现并是指目的设备所在的网络，有时这些第三层地址就被称为协议地址或网络地址。物理地址，或MAC地址通常由网络接口卡的生产厂家所指定，并且被硬件编码放在网络接口卡之中。而另一方面，网络层地址，或IP地址，通常由网络管理者指定。

The symbol for a router, shown in figure 6-3, is suggestive of its tow primary purposes: path selection, and switching of packets to the best route. A router can have many different types of interface ports.

示于图6-3的路由器的符号说明了它的两个作用：路由选择和将分组交换到最佳路由。一个路由器可有许多不同类型的接口。

Clouds

The cloud symbol in figure 6-4 suggests another network, or perhaps the entire Internet. It reminds you that there is a way to connect to that other network (for example, the Internet), but does not supply all the details of either the connection or the network.

云

在图6-4中表示了云符号说明了另一个网络，或可以是整个互联网。它提示你，这里有连到其它网络（例如互联网）的途径，但并不是提供连接或网络的所有细节。

The physical features of the cloud are many. To help you understand , you might think of all the devices that connect your computer to some very distant computer, perhaps on other continent. There is no single picture that could display all the processes and equipment that world be involved in making that connection.

云的物理特征是很多的。为了弄懂这一点，你可以想想把你的计算机连接到某个很远的，也许是别的大陆上的计算机所需的所有器件。不可能有一幅图画能显示在做出这种连接时所包括的全部过程和设备。

It is important to remember that, at this point in the book, you are interested only in how LANs connect to large WANs and to the Internet (the ultimate WAN), so that any computer can talk to any other computer, any place and any time. Because the cloud is not really a single device, but represents a collection of devices that operate at all levels of the OSI mode., it could be classified as a layer 1 though 7 device.

重要的是应记住，我们只对LAN是怎样连接到大的WAN和连接到Internet(最大的或极限的WAN)感兴趣，以至于任何计算机可以与任何其它计算机在任何地点、任何时间进行通信。由于云不是一个实际的器件，但表示了在OSI参考模型所有层运行的部件的综合，因而它可能被分类为第一层到第七层。

Network segments

The term segment has many meanings in networking. The correct definition depends on the situation in which its is used. Historically, a segment identifies the layer 1 media that is the common patch for data transmission in a LAN. As previously mentioned, there is a maximum length for data transmission on each type of media. Each time an electronic device is used to extend the length or to manage data on the media, a new physical segment is created. Some people refer to segments by the term wires, although the ―wire‖ might by optical fiber, wireless medium, or copper wire.

网络段 在联网中，“段”这个词有多种意义。正确的定义取决于它所采用的场合。从历史上看，一个段表明了第一层的介质，而且是在一个LAN中数据传输的通常长度。正如以前所提及的，在每种介质上的数据通信有一个最大长度。每次采用一个电子设备延伸长度或管理介质上的数据，就产生了一个新的物理段。一些人用词“线路”来表示段，然而“线路”可为光纤、无线介质或铜线。

UNIT 7

Internet 互联网

The Internet is a giant network of computers located all over the world that communicate with each other.

Internet是由位于世界各地相互通信的计算机连接而成的巨大的计算机网络。

The Internet is a international collection of computer networks that all understand a standard system of addresses and commands, connected together through backbone systems. It was started in 1969, when the U.S. Department of Defense established a nationwide network to connect a handful of universities and contractors. The original idea was to increase computing capacity that could be shared by users in many locations and to find out what it would take for computer networks to survive a nuclear war or other disaster by providing multiple path between users. People on the ARPNET quickly discovered that the could exchange messages and conduct electronic ―conferences‖ with distant colleagues for purposes that had nothing to duo with the military industrial complex. If somebody else had something interesting stored on his or her computer, it was a simple matter to obtain a copy.

Internet是计算机网络的国际性的集合，这些网络都符合具有地址和命令的标准体系，并经骨干网连在一起。Internet始建于1969年，当时美国国防部为连接一些大学和协议企业而建立了一个全国性网络。最初的想法是要增加计算机能力并可由许多地点的用户共享，并且通过提供用户间多条路径来找到哪一种计算机网络能够在核战或其他灾难中幸存。

ARPNET（这种全国网络最初的名称）上的用户很快就发现它们可以与远距离的同事交换消息，并且进行某种目的的电子“会议”，而这些目的与军事工业企业没有任何关系。如果另外一些人在其计算机中存有有趣的东西，得到其拷贝是很容易的事（假定拥有者没有进行保护）。

Over the years, additional networks joined which added access to more and more computers. The first international connection, to Norway and England, were added in 1973. Today thousands of networks and millions of computers are connected to the Internet. It is growing so quickly that nobody can say exactly how many users ―On the Net‖.

几年间，新的网络接入使越来越多的计算机加入近来。在1973年进行了第一次与挪威和英国的国际连接。今天，有成千上万的计算机网络和数百万台计算机与Internet相连。Internet发展如此之快以至于没有人能准确的说出网上有多少用户。

The Internet is the largest repository of information which can provide very very large network resources. The network resources can be divided into network facilities resources and network information resources. The network facilities resources provide us the ability of remote computation and communication. The network information resources provide us all kinds of information services, such as science, education, business, history, law, art, and entertainment, etc.

Internet是最大的信息宝库，它可以提供非常巨大的网络资源。这种网络资源可分为网络设备和网络信息资源。网络设备资源使我们能够进行远程计算和通信。网络信息资源向我们提供各种各样的信息服务，如科学、教育、商务、历史、法律、艺术和娱乐等等。

The goal of your use of the Internet is exchanging messages or obtaining information. What you need to know is that you can exchange message with other computers on the Internet and use you computer as a remote terminal on distant computers. But the internal details of the link are less important, as long as it works. If you connect computers together on a network, each computer must have a unique address, which could be either a work or a number. For example, the address of Sam’s computer could be Sam, or a number.

使用Internet的目的是交换消息或获得信息。你只需知道你可以与Internet上的其他计算机交换消息并将你的计算机用作远端计算机的远程终端，而链路的内部细节并不太重要，只要网络能工作就行。若将多台计算机连接到网络上，每台计算机须有惟一的地址，地址可以是一个字或一个数字。例如Sam的计算机地址可以是Sam或一个数字。

The Internet is a huge interconnected system, but is uses just a handful of method to move data around. Until the recent explosion of public interest in the Internet, the vast majority of the computers on the Net use the Unix operating system. As a result, the standard Unix commands for certain Internet services have entered the online community’s languages as both nouns and verbs to describe the services themselves. Some of the services that the Internet can provide are: Mail, Remote use of another computer (Telnet), File Transfer (FTP), News, and Live conversation.

虽然Internet是一个巨大的互联系统，但它仅使用一点简单办法就将数据传来传去。近来公众对Internet产生了极大的兴趣，在这之前，绝大多数网上计算机都使用Unix操作系统。结果，用于某些Internet业务的标准Unix命令已经进入联机团体语言当作名词和动词来描述业务本身。Internet可以提供的一些服务为：电子邮件、远程使用其他计算机、文件传送、电子新闻和实况对话。

The most commonly used network service is electronic mail (E-mail).or simply as mail. Mail permits network users to send textual messages to each other. Computer and networks handle delivering the mail, so the communication mail user do not have to handle details of delivery, and do not have to be present at the same time or place.

最常使用的网络服务是电子邮件，或简称邮件。电子邮件允许网络用户彼此传送文本消息。邮件的传递有计算机和网络处理，邮件用户不必关心传递的细节，也不必同时在场。

The simplest way to access a file on another host is to copy it across the network to your local host. FTP can do this.

从其他主机中获得的文件的最简单的方式是通过网络将其拷贝到你的计算机上。文件传送（FTP）可完成这项工作。

Presently, a user with an account on any Internet machine can establish a live connection to any other machine on the Net from the terminal in his own office or laboratory. It is only necessary to use the Unix command that sets up a remoter terminal connection (Telnet), followed by the address of the distant machine.

目前，用户在Internet建立账户后，就可以从其办公室和实验室的终端上与网上其他计算机建立实时连接。只需使用Unix命令Telnet来建立远程终端连接，命令后跟上远端计算机的地址即可。

Before you can use the Internet, you must choose a way to move data between the Internet an you PC. This link may be a high-speed data communication circuits, a local-area network( LAN), a telephone line or a radio channel. Most likely, you will use a Modem attached to your telephone line to talk to the Internet. Naturally, the quality of your Internet connection and service, like many other things in life, is dictated by the amount of money you are willing to spend.

可使用Internet之前，必须使用一种方法在你的PC机和Internet之间传送数据。这种里那几的链路可以是高速数据通信电路、局域网（LAN）、电话线路或无线信道。最有可能的是，你使用Modem连到电话线上与Internet对话。当然，像生活中许多其他的事物一样，与Internet连接和服务的质量是有你所花钱的数量决定的。

Although all these services can well satisfy the needs of the users for information exchange, a definite requirement is needed for the users. Not only should the user know there the resources locate, but also he should know some operating commands concerned. To ease the searching burden of the users, recently some convenient searching tools appear, such as Gopher, WWW and Netscape.

虽然所有这项服务可以很好的满足用户对信息交换的需要，但用户仍旧还需要具有一些特定的先决条件。用户不仅要知道信息资源所处的位置，而且要知道一些有关的操作命令。为了减轻用户寻找信息的负担，近来出现了一些方便的搜索工具，如gopher，www和Netscape

World wide web (WWW) is a networked hypertext protocol and user interface. It provides access to multiple services and documents like Gopher does but is more ambitious in its method. A jump to other Internet service can be triggered by a mouse click on a ―hot-linked‖ word, image, or icon on the Web page.

全球网(WWW)是一种网络的超文本协议和用户界面。像Gopher一样，它提供多种服务和文件接入方法，但其方法更加有前途。向Internet其他服务的跳转可在网页上由鼠标器点击“热链接”的字、图像或小相框来启动。

As more and more systems join the Internet, and as more and more forms of information can be converted to digital form, the amount of stuff available to Internet users continues to grow. At some points very soon after the nationwide (and later worldwide) Internet started to grow, people began to treat the net as a community, with its own tradition and customs. For example, some body would ask a question in a conference, and a complete stranger would send back an answer: after the same questions were repeated several times by people who hadn’t seen the original answers, somebody else gathered a list of ―frequently asked questions‖ and placed it where

newcomers could find it .

随着越来越多的系统加入Internet，同时随着越来越多的信息可以转变成数字形式，Internet用户所能得到的东西也在继续增加。随着国家（后来是国际）Internet的发展，人们很快在某些方面开始将互联网看作是一个社区，有自己的传统和习惯。例如某些人会在会议上提出一个问题，一个完全陌生的人会传送一个答案；由于一些人没有看到最初的答案而多次重复这一问题，这时另外一些人会搜集一系列“经常提到的问题”并将其放置在新来者能够找到的地方。

So we can say that the Internet is your PC’s window to the rest of the world. 所以我们可以说，Internet是你的PC机通向世界其他地方的窗口。

UNIT 8

Introduction to Optical Fiber Communication

One of the most important technological developments during the 1980s has been the emergence of optical fiber communication as a major international industry. One indication of the extent of this development is the total length of installed fiber, which was estimated to be 3.2million kilometers in the U.S. alone by the end of 1987. Over 90% of this fiber was placed in service during the time period of 1982-1987. Long-haul trunk installations have been dominated, accounting for about 95% of the fiber in the U.S.

光纤通信介绍

80年代一项最重要的技术发展是光纤通信成为一个主要的国际性产业。用光纤敷设总长度可以表明其发展程度。据估计，到1987年底仅美国的关切敷设总长将达320万公里，其中90％以上是在1982-1987年间敷设并开通的，而长途干线占主导地位，数量约为光纤总长的95％。

Although telecommunication is the rationale for most of the current interest in fiber optics, this was not the case during the early days of the technology. The researchers who produced the first clad glass optical fibers in the early 1950s were not thinking of using them for communications; they wanted to make imaging bundles for endoscopy. Fiber optics was already a well-established commercial technology when the famous paper by Kao and Hockham, suggesting the use of low-loss optical fibers for communication, appeared in 1966.

虽然现在人民对纤维光学的兴趣主要在于通信，但早期发展纤维光学的目的并不在此。50年代初研究人员制造出第一根具有包层玻璃光纤时，并不想用于通信而是想用它们传送内窥镜需要的成像光束。1966年Kao和Hockham发表了那篇著名的论文，建议将低损耗光纤用于通信，此时纤维光学已发展为一项很实用的技术了。

The first low-loss (20dB/km) silica fiber was described in a publication which appeared in October of 1970. The date of this publication is sometimes cited as the beginning of the era of fiber communication. Although this development did receive considerable attention in the research community at the time, it was far from inevitable that a major industry would evolve. The 20dB/km loss figure was still too high for long-hall telecommunication systems. The fibers were fragile, and a way to protect them would have to be found. There were no suitable light sources. Researchers did not know whether field termination and splicing of optical cables would ever be practical. Finally, there were serious doubts as to whether these components could ever be produced economically enough for the technology to play a major role in the marketplace.

1970年10月，第一根低损耗（20 dB/km）石英光纤问世了。有时将这以日期作为光纤通信时代的开端。虽然这一成果当时在研究领域确实引起了极大的关注，但因此就认为一项产业会发展起来尚为时太早: 每千米20dB的损耗对于长途通信系统仍然是太大了；光纤易断裂，必须寻找保护方法；没有合适的光源。研究人员不知道光缆终结和拼接是否会发展到实用阶段，最后，这些部件是否能够足够经济的生产出来，从而使之在市场上占用重要的地位，它们更是存有严重的疑虑。

Although the technological barriers appeared formidable, the economic potential was very significant. As a consequence, research and development activity expanded rapidly, and a number of important issues were resolved during the early 1970s.

虽然技术障碍好像不可逾越，但经济潜力却非常明显。正因为如此，在70年代早期研究和开发工作开展迅速，一些重要问题得以解决。

During the middle and late 1970s, the rate of progress toward marketable products accelerated as the emphasis shifted from research to engineering. Fibers with losses approaching the Rayleigh limit of 2 dB/km at a wavelength of 0.8um,0.3 dB/km at 1.3um, and 0.15 dB/km at 1.55um,were produced in the laboratory. Microbend loss problems were overcome through the use of improved fiber coatings and cabling techniques. Rugged cables and multifibre connectors were produced for field installation. Room temperature threshold currents for commercial gallium aluminum arsenide lasers operating in the 0.8 to 0.85um spectral region were reduced to the 20 to 30 mA range, and projected lifetimes in the 100 000 to 1000 000 hour range were claimed for both lasers and LEDs. Light sources and improved photodetectors which operated near 1.3um were developed to take advantage of the low fiber loss and dispersion in this ―longer wavelength region‖. Several major field trials were undertaken during this period, including AT&T’s Atlanta experiment and Chicago installation, and Japan’s subscriber access project.

70年代中后期，由于发展重点研究领域转入工程，因而加速发展了适应市场的产品。在实验室研制的光纤衰减值接近瑞利极限值：0.8um波长处为2 dB/km，1.3um波长处为0.3 dB/km和1.55um波长处为0.15 dB/km。通过改进光纤外涂层方法和成缆技术，克服了微弯损耗。生产了加强型光纤和多纤和多纤连接器用于室外作业。工作在0.8-0.85um波长区的商用镓铝砷激光器的室温阈值电流减少到20-30mA范围。据称，激光器和发光管的设计寿命达10万-100万小时。开发了工作于1.3um波长附近的光源和改进的光检测器，从而可以利用光纤在长波长区的低损耗和低色散特性。这一时期进行的室外试验教重要的有AT&T于1976年在亚特兰大的实验，1977年在芝加哥的实验和1977年日本的用户通路项目。

Improvements in component performance, cost, and reliability by 1980 led to major commitments on the part of telephone companies. Fiber soon became the preferred transmission medium for long-haul trunks. Some early installations used 0.8um light sources and graded-index multimode fiber, but by 1983, designers of intercity links were thinking in terms of 1.3um, single-mode systems. The single-mode fiber, used in conjunction with a 1.3um laser, provides a bandwidth advantage which translates into increased repeater spacings for high data rate systems.

到了80年代，光纤器件在性能、价格和可靠性方面的改善使众多电话公司受益匪浅。光纤很快成为长途干线的首选传输媒质。一些早期敷设的光纤线路采用0.8um光源和渐变折射率多模光纤，但到1983年，城市间线路的设计者们就考虑使用1.3um单模光纤系统了。单模光纤与1.3um激光器连接，可以提供宽带特性，增加了高数据速率系统的中继距离。

Data rates for installed fiber optic system have recently move into gigabit per second range. Such system use the spectrally pure distributed-feedback lasers to minimize fiber dispersion effects. Fibers designed for low dispersion at 1.55UM wavelength, which corresponds to

minimum fiber loss, are now commonly used in long distance transmission. The use of wavelength multiplexing to further increase the fiber information capacity is becoming more widespread.

最近敷设的光纤系统的数据速率已移至每秒吉比特范围。这种系统采用光谱纯的分布反馈激光器，将光纤色散效应减至最小。在1.55um波长上设计的低色散光纤，相应地具有低损耗特性，目前广泛用于长途通信。为进一步增加光纤的信息容量，逐渐广泛采用波分复用的方法。

The potential of fiber optics in other areas is only beginning to be realized. Fiber optic networks for computer systems and offices are becoming more prominent. In the telephone system, the use of fiber optics for interconnecting central offices within a metropolitan area and for lower levels in the switching hierarchy is still increasing rapidly. Fiber links to the home have been used in demonstration projects. Many observers believe that national telephone systems will eventually be upgraded to handle video bandwidths by using fiber optics. These wideband subscriber loop systems would provide access to services such as picturephone, video entertainment. Widespread installation of these broadband services will become economically feasible.

人们对于光纤在其他领域的潜力刚刚开始认识。用于计算机系统和办公室的光纤网络逐渐变的更加重要。在电话系统中，光纤在主要城市地区中心交换局间互联和低级交换中的使用继续迅速增加。入户光缆已经有了示范工程。许多观察家相信，全国电话系统将使用光纤传输视频宽带信号而逐渐升级。这些宽带用户环路系统将可视电话、视频娱乐节目等业务提供通路。宽带业务广泛使用光纤将会变的经济可行。

UNIT 9

Cellular Mobil Telephone System

One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems: limited service capability, poor service performance, and inefficient frequency spectrum utilization. 蜂窝式移动电话系统

开发蜂窝式移动电话系统并将其在许多城市中推广应用的原因之一是传统的移动电话系统存在容量有限、性能差、频普利用率低的缺点。

A major problem facing the radio communication industry is the limitation of the available radio frequency spectrum. In setting allocation policy, the Federal Communications Commission (FCC) seeks systems which need minimal bandwidth but provide high usage and consumer satisfaction.

无线通信领域面临的一个主要问题是可使用的无线电频普有限。在确定分配政策时，美国联邦通信委员会寻求的是只需要最小的带宽却能提供高使用率并使用户满意的系统。

The ideal mobile telephone system would operate within a limited assigned frequency band and would serve an almost unlimited number of users in unlimited areas. Three major approaches to achieve the ideal are:

理想的移动电话系统将在有限的给定频段上工作，但却可以向任意多的地区中几乎是任意多的用户提供服务。实现这种理想系统的方法主要有三种：

1. Single-sideband (SSB) , which divides the allocated frequency band into maximum

numbers of channels;

2. Cellular, which reuses the allocated frequency band in different geographic

locations;

3. Spread spectrum, frequency-hopped, which generates many codes over a wide

frequency band.

1. 单边带，可将给定的频段分为最多的信道。

2. 蜂窝式，可使给定的频段在不同的地理位置上重复使用。 3. 扩展频普与跳频，能在宽频带上产生许多代码。

In 1971, the computer industry entered a new era. Microprocessors and minicomputers are now used for controlling many complicated features and functions with less power and size than was previously possible. Large-Scale Integrated (LSI) circuit technology reduced the size of mobile transceivers so that they easily fit the standard automobile. These achievements were a few of the requirements for developing advanced mobile phone systems and encouraging engineers to pursue this direction.

1971年，计算机工业进入了一个新纪元。现在许多复杂的特性和功能都使用微处理机和小型计算机进行控制，这比以前的控制方法减少了功率，缩小了体积。大规模集成电路技术减少了移动收发两用机的体积，而容易安装在标准汽车中。这些成果满足了发展先进移动电话系统的一些要求，促使工程人员进一步的向这个方向努力。

Another factor was the price reduction of the mobile telephone unit. LSI technology and mass production contribute to reduced cost so that in the near future an average-income family should be able to afford a mobile telephone unit.

另一个因素是移动电话机的价格下降。大规模集成电路技术和批量生产带来的价格下降，使在不远的将来普通收入水平的家庭也用的起移动电话。

On Jan.4, 1979, the FCC authorized lllinois Bell Telephone Co. to conduct a developmental cellular system in the Chicago area and make a limited commercial offering of its cellular service to the public. In addition, American Radio Telephone Service Inc. (ARTS) was authorized to operate a cellular system in the Washington, D.C.---Baltimore, Md., area. These first systems showed the technological feasibility of cellular service.

1979年1月４日，美国联邦通信委员会授权伊利诺斯贝尔电话公司在芝加哥地区开发蜂窝式系统，并向公众提供一定的商业性服务。另外，美国无线电话服务公司被授权在首都华盛顿到马里兰州的巴尔的摩地区建立蜂窝式系统。这些早期系统显示了蜂窝式移动通信技术可行性。

Why 800 MHZ? The FCC’s decision to choose 800 MHZ was made because of severe spectrum limitations at lower frequency bands. FM broadcasting services operate in the vicinity of 100 MHZ. The television broadcasting service starts at 41 MHZ and extends up to 960 MHZ. Air-to-ground systems use 118 to 136MHZ;military aircraft use 255 to 400 MHZ. The maritime mobile service is located in the vicinity of 160 MHZ. Also fixedstation services are allocated portions of the 30 to 100 MHZ band. Therefore, it was hard for the FCC to allocate a spectrum in the lower portions of the 30 to 400 MHZ band since the services of this band had become so crowded. On the other hand, mobile radio transmission cannot be applied at 10 GHZ or above because severe propagation path loss, multipath fading, and rain activity make the medium improper for mobile communications.

为什么选择800MHZ作为蜂窝式电话通信的频率？美国联邦通信委员会选择800MHZ是因为低频段的频普非常有限。跳频广播工作在100MHZ附近，电视广播工作在41-960MHZ.空对地系统使用118-136MHZ,军用飞机使用225-400MHZ,海事移动服务在160MHZ附近，地面固定无线电台使用30-100MHZ.30-400MHZ低频段这样拥挤

使得联邦通信委员会很难将移动电话频率安排在此波段。另一方面，移动无线传输不能使用10GHZ以上的波段，因为严重的传输路径损耗、多径衰落和降雨，使该波段的空间媒质不适宜移动通信。

Fortunately, 800 MHZ originally assigned to educational TV channels. Cable TV service became a big factor in the mid-70s and shared the load of providing TV channels. This situation opened up the 800 MHZ band to some extent, and the FCC allocated a 40 MHZ system at 800 MHZ to mobile radio cellular systems.

幸运的是，800MHZ原来安排的教育电视。70年代中期，有线电视的发展分担了电视频道的业务，在某种程度上缓解了800MHZ波段的压力，使联邦通信委员会在800MHZ处为移动无线蜂窝系统分配了40MHZ的频宽。

A basic cellular system consists of three parts: a mobile unit, a cell site, and a mobile telephone switching office (MTSO) , as Fig.9-1 shows, with connections to link the three subsystems.

一个蜂窝式系统基本上由三部分组成：移动机、无线基站和移动电话交换局，如图9-1所示，并且这三部分互相联通。

1.Mobile units. A mobile telephone unit consists a control unit, a transceiver, and an antenna system.

2.Cell site. The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets,antennas,a power plant, and data terminals.

3.MTSO. The switching office, the central coordinating element for all cell sites, consists of the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing and handles billing activities.

4.Connections. The radio and high-speed data links connect the three subsystem. Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units.

１．移动机。移动电话机包括控制单元、收发信号机和天线系统。

２．无线基地站。无线基地站在移动电话交换局和移动机之间提供接口，它有控制单元、无线机柜、天线、电源装置、和数据终端。

３．移动电话交换局。它是所有无线基地站的协调中心，包括蜂窝处理机和蜂窝交换机。它与电话公司的地区交换局接口，控制呼叫处理并进行计费业务。

４．连线。无线线路和高速数据线连接以上三个部分。每个移动机每次只占用通信线路的一个频道。频道不是固定的，可以是服务区分配的全部频道中的任何一个。每一个无线基站具有多频道能力，可同时联通许多移动机。

The MTSO is the heart of the cellular mobile system. Its processor provides central coordination and cellular administration.

移动电话交换局是蜂窝移动系统的中心，它的处理机起到了中心协调和蜂窝管理的作用。

The cellular switch, which can be either analog or digital, switches calls to connect mobile subscriber to other mobile subscribers and to the nationwide telephone network. It also contains data links providing supervision links between the processor and the switch and between the cell sites and the processor . The radio links carries the voice and signaling between the mobile unit and the cell site. Microwave radio links or wire lines carry both voice and data between the cell site and the MTSO.

蜂窝交换机既可以是模拟的也可以是数字的，它对移动用户之间和移动用户与全国电话网之间的呼叫进行交换连接。它还有在处理机与交换机间和在无线基地站与处理机间起监控作用的数据线。无线线路在移动机和无线基地站间传输话音信号和信令。微波无线线路或有线线路在无线基地站和移动电话交换局之间传送音频信号和数据信号。 UNIT 10

GSM (Global System for Mobile Communication) 全球移动通信系统

The success of mobile systems across the world is a sign that communication is moving towards a more personalized, convenient system. People who have to use a mobile phone on business soon begin to realize that the ability to phone any time, any place in one’s personal life rapidly becomes a necessity, not a convenience.

世界范围移动通信的成功标志着通信正在向着更加个人化，更加方便的通信系统迈进。在公务中需使用移动电话的人们很快就认识到在个人生活中随时随地打电话不仅仅带来了方便，而且是一种必须。

The speed and rapidity with which the personal communications revolution takes place is, unlike fixed transmission systems, highly dependent on technology and communication standards.

不像固定通信系统，个人通信的快速发展在很大程度上依赖于技术和通信标准的发展。

For mobile the three key elements to achieving service take-up are the cost, the size and the weight of the phone, and the cost and quality of the link. If any of these are wrong, especially the first two, then market growth is liable to be severely restricted.

移动通信取得业务腾飞的三个关键因素是费用、手机的大小和重量以及链路的收费和质量。如果其中任何一个不能令人满意，尤其是前两项，移动通信的市场增长就有可能受到严重的限制。

The fixed telephone service is global and the interconnection varies from coaxial cable to optical fiber and satellite. The national standards are different, but with common interfaces and interface conversion, interconnection can take place. For mobile the problem is far more complex, with the need to roam creating a need for complex networks and systems. Thus in mobile the question of standards is far more crucial to success than fixed systems. In addition, there is also the vexed question of spectrum allocation in the mobile area.

固定电话业务是全球性的，系统的相互采用不同的方式由同轴电缆到光纤和卫星。虽然各国的标准不同，但却采用共同的接口和接口转换装置，使得相互连接可以进行。对移动通信来说，问题要更加的复杂，这是由于需要漫游业务就要有更复杂的网络系统。这样比起固定通信系统，移动通信的标准问题是取得成功更为关键的因素。此外，在移动通信领域，频普的分配也是令人苦恼的问题。

Mobile systems originally operated in analog mode in the 450MHZ band, moving later to 900MHZ with digital GSM and then to 1 800 MHZ with personal communication systems. The history of mobility can split into generations. The first generation systems were the advanced mobile phone systems (AMPS) in the US, total access communication system (TACS) in most of Europe and Nordic mobile telephone system (NMT), which were all

analogue systems. The second generation is much dominated by the standard first set out in Europe by the group special mobile (GSM) committee, which was designed as a global mobile communication system.

移动通信系统最初以模拟方式工作，其频带在450MHZ，后来逐步发展到900MHZ的数字GSM，系统，下一步是向工作在1800MHZ的个人通信系统发展。移动通信的发展可分为几代。第一代系统如美国的高级移动电话系统（AMPS）、欧洲多数国家的全接入通信系统（TACS） 以及北欧的移动电话系统（NMT）,这些都是模拟系统。第二代移动通信系统在很大程度上是由欧洲特别移动小组（GSM）委员会所制定的标准决定的，设计这一系统作为一种全球移动通信系统。

The GSM system is based on a cellular communications principle which was first proposed as a concept in the 1940s by Bell System engineers in the US. The idea came out of the need to increase network capacity and got round the fact that broadcast mobile networks, operating in densely populated areas, could be jammed by a very small number of simultaneous calls. The power of the cellular system was that it allowed frequency reuse.

GSM系统基于蜂窝通信系统原则。这一概念是由美国贝尔系统工程师在40年代首先提出来的。这一思想出自于增加网络容量的需要以及解决网络堵塞的问题。在人口稠密地区运行的广播式移动网络系统会由于很少的几个用户同时呼叫而引起堵塞。蜂窝系统的威力在于允许频率的再利用。

The cellular concept is defined by two features, frequency reuse and cell splitting. Frequency reuse comes into play by using radio channels on the same frequency in coverage areas that are far enough apart not to cause co-channel interference. This allows handling of simultaneous calls that exceed the theoretical spectral capacity. Cell splitting is necessary when the traffic demand on a cell has reached the maximum and the cell is then divided into a microcellular system. The shape of cell in a cellular system is always depicted as a hexagon and the cluster size can be seven, nine or twelve.

蜂窝的概念有两个特点确定，即频率重复利用和蜂窝裂变。在覆盖区域相隔足够远而不至于引起公共信道干扰时，通过使用同一频率的无线信道，频率再利用才起作用。这样可以处理出现并超过了理论频普容量的同时呼叫。当小区的业务需求增到最大时，就要进行蜂窝裂变，蜂窝再被分成微蜂窝系统。蜂窝系统的小区形状常被描绘成六边形，一群六边形小区的数量可以是７个、９个、或12个。

The GSM system requires a number of functions to be created for a fully operational mobile system.

对于一个充分运营的移动系统来说，GSM系统需要建立一些功能。

The cell coverage area is controlled by a base station which is itself made up of two elements. The first element is the transmission system which communicates out to the mobile and also receives information from it to set up and maintain call when actually in operation. The base station transceiver (BST) is controlled by the base station controller (BSC), which communicates with the mobile switching center (MSC)---the essential link to the local public switched telephone network (PSTN), and to the subscriber data which is stored in registers within the system. The subscriber registers allow the GSM system to check a subscriber who requests the use of the network , allow access and then set up the charging function, etc.

小区的覆盖面由一个基站控制，基站本身由两个单元组成。第一个单元是传输系统，它在实际运行中与移动台进行通信以建立并保持通话。第二是基站收发机（BTS）,由基站控制器（BSC）控制,而BSC与移动交换中心（MSC）进行通信。这条通信链路，

对于本地公共交换电话网（PSTN）的链接以及连接存储在系统寄存器中的用户数据来说，都十分的重要。用户寄存器使GSM系统核查需要使用网络的用户，允许接入并建立收费功能等。

The GSM system was allocated part of the 900 MHZ band at the 1978 World Administration Conference (WAC) , the actual bands being 890 to 915 MHZ for the uplink transmission and 935 to 960 MHZ for the downlink. The access method is time division multiple access (TDMA).

1978年由世界管理会议（WAC）分配给GSM 900 MHZ 的部分频带，实际的频带是890-915MHZ用于上行传输，935-960MHZ用于下行传输。接入方式是时分多址（TDMA）方式。

The GSM system operates in a burst transmission mode with 124 radio channels in the 900MHZ band, and these bursts can carry different types of information. The first type of information is speech, which is coded at 6.5 kbit/s or 13 kbit/s. The second type is data, which can be sent at 3.6 kbit/s, 6 kbit/s or 12.6 kbit/s. These two forms of transmission are the useful parts of the transmission, but have to be supported by overhead information which is sent in control channels (CCH).

GSM系统以突发脉冲序列传输方式工作。在900MHZ频带有124个无线信道，这些突发脉冲序列可以传送不同的信息。第一类信息是话音，编码速率为6.5 kbit/s 或 13 kbit/s。第二类信息是数据，可以3.6 kbit/s, 6 kbit/s或12.6 kbit/s的速率传输。这两类信息是传输信息的有用部分，但是传输还必须得到额外开销信息的支持，开销信息通过控制信道传输。

The use of digital radio transmission and the advanced handover algorithms between radio cells in GSM network allows for significantly better frequency usage than in analogue cellular systems, thus increasing the number of subscribers that can be served. Since GSM provides common standard, cellular subscribers will also be able to use their telephones over the entire GSM service area. Roaming is fully automatic between and within all countries covered by GSM system. In addition to international roaming , GSM provides new services, such as high-speed data communication, facsimile and short message service .The GSM technical specifications are designed to work in concert with other standards, e.g. ISDN. Interworking between the standard is in this way assured . In the long term perspective cellular systems, using a digital technology, will become the universal method of telecommunication.

GSM网络使用数字无线传输和先进的无线越区切换算法，可以得到比模拟蜂窝系

统好的多的频率利用，因而增加了服务的用户数。由于GSM可提供共同的标准，蜂窝用户就可以在整个GSM服务区使用其电话。在GSM覆盖的国内或国际区域内，其漫游是完全自动的。除了国际漫游外，GSM还可提供新型用户服务，如高速数据通信、传真和短电文业务。GSM技术规范可与其它的通信标准保持一致，如ISDN.这样可保证标准间的互通。展望未来，利用数字技术的蜂窝系统将成为通信的通用方式。

The third generation mobile communication system currently being developed in Europe is intended to integrate all the different services of second generation systems and cover a much wider range of broadband services ( voice, data, video and multimedia) consistent and compatible with technology developments taking place within the fixed telecommunication networks.

目前欧洲正在开发第三代移动通信系统，其目的是要综合第二代系统的所有不同

业务并覆盖更广泛的业务（话音、数据、视频和多媒体）范围，而且还要与固定电信网络的技术发展保持一致和兼容。 UNIT 11

Circuit Switching and Packet Switching

There are two basic types of switching techniques: circuit switching and message switching. In circuit switching, a total path of connected lines is set up from the origin to the destination at the time the call is made, and the path remains allocated to the source-destination pair (whether used or not) until it is released by the communicating parties. The switches, called circuit switches (or office exchange in telephone jargon), have no capability of storing or manipulating user’s data on their way to the destination. The circuit is set up by a special signaling message that finds its way through the network, seizing channels in the path as it proceeds. Once the path is established, a return signal informs the source to begin transmission. Direct transmission of data from source to destination can then take place without any intervention on the part of the subnet.

交换技术有两种基本类型：电路交换和报文交换。在电路交换中，当呼叫发生时，由呼叫源点到终点之间要建立整个通路的连线，而且在通信双方释放该电路之前，此通路一直保持分配给这对源点－终点（不管通路是否使用）。被称为电路交换机（或以电话行业用语称为局交换）的交换设备没有存储或控制用户送往终点路由数据的能力。电路由特殊的信令建立，该信令通过网络选择路由，并在其进程中确定信道。一旦路由建立，一个返回信号就通知呼叫源开始传输（数据）。接着，由源点到终点间进行直接的数据传输，此期间对该通信子网不会做任何干预。

In message switching, the transmission unit is a well-defined block of data called a message. In addition to the text to be transmitted, a message comprises a header and a checksum. The header contains information regarding the source and destination addresses as well as other control information; the checksum is used for error control purposes. The switching element is a computer referred to as a message processor, with processing and storage capabilities. Messages travel independently and asynchronously, finding their own way from source to destination. First the message is transmitted from the host to the message processor to which it is attached. Once the message is entirely received, the message processor examines its header, and accordingly decides on the next outgoing channel on which to transmit it. If this selected channel is busy, the message waits in a queue until the channel becomes free, at which time transmission begins. At the next message processor, the message is again received, stored, examined, and transmitted on some outgoing channel, and the same process continues until the message is delivered to its destination. This transmission technique is also referred to as the store-and-forward transmission technique.

在报文交换中，传输单元是一个被精心定义的数据块，该数据块被称为报文。除了要发送的内容外，报文还包括有报头和校验项。报头含有源地址和目的地址的信息，以及其它的控制信息，而校验项用于误码控制。交换单元是一台被称为报文处理器的计算机，它具有处理和存储的能力。报文独立并异步的传输，在源点与终点间选择自己的传送路由。首先报文由主机送往与之相连的报文处理机。一旦报文被完全收到，报文处理机就检查它其报头，并相应的决定该报文传送的下一个输出信道。如果这个所选信道忙，则该报文就排队等待，直到此信道空闲时开始发送。在下一个报文处理

器中，报文被再次收到、存储、检查，并在某个输出信道上再发送出去。相同的过程继续进行着，直到报文交到目的地为止。这种传送技术亦被称为存储转发传输技术。

A variation of message is broken up into several pieces of a given maximum length, called packets. As with message switching, each packet contains a header and a checksum. Packets are transmitted independently in a store-and-forware manner.

分组交换是报文交换的一种变形。在分组交换中，报文被以指定的最大长度分成若干个被称为分组的段节。与报文交换一样，每个分组都含有一个报头和校验项。分组以存储转发方式独立传送。

With circuit switching, there is always an initial connection cost incurred in setting up the circuit .It is cost-effective only in those situations where once the circuit is set up there s a guaranteed steady flow of information transfer to amortize the initial cost. This is certainly the case with voice communication in the traditional way, and indeed circuit switching is the technique used in the telephone system. Communication among computers, however, is characterized as bursty. Burstiness is a result of the high degree of randomness encountered in the message-generation process and the message size, and of the low delay constraint required by the user. The users and devices require the communication resources relatively infrequently; but when they do, they require a relatively rapid response. If a fixed dedicated end-to-end circuit were to be set up connecting the end users, then one must assign enough transmission bandwidth to the circuit in order to meet the delay constraint with the consequence that the resulting channel utilization is low. If the circuit of high bandwidth were set up and released at each message transmission request, then the set-up time would be large compared to the transmission time of the message, resulting again in low channel utilization. Therefore, for bursty users (which can also be characterized by high peak-to-average date rate requirements), store-and-forward transmission techniques offer a more cost-effective solution, since a message occupies a particular communications link only for the duration of its transmission on that link; the rest of the time it is stored at some intermediate message switch and the link is available for other transmissions. Thus the main advantage of store-and-forward transmission over circuit switching is that the communication bandwidth is dynamically allocated, and the allocation is done on the fine basis of a particular link in the network and a particular message(for a particular source-destination pair).

在电路交换的情况下，建立电路总要对开始的接续付出代价。只有在这种情况下，即一旦电路建立后，信息的传送确保持续、源源不断，以便摊分初始花费，才能提高价格的效率比。传统方式的语音通信就属于这种情况，因而电路交换确实是电话系统中使用的技术。但是，计算机通信具有突发特性。突发性是报文产生过程和报文长度高度的随机性所造成的，也是用户对时延要求很短造成的结果。用户和设备不怎么经常的用到通信资源，但是当它们用到时，它们就要求相当迅速的反应。如果要建立一个固定的专用的端到端电路以连接两个用户，则必须对该电路分配足够的传输带宽以合乎对时延的要求，其结果是电路的利用率低。如果对每个报文传输要求都要建立和释放大带宽的电路，则与报文的传输时间相比，电路建立的时值将很大，造成很低的电路利用率。因此，对突发性用户（峰值速率于平均速率很高为该用户的特征），存储转发传输技术提供了一个更低价高效的解决办法，因为只有在报文传送的时间里，报文才占据一条特定的链路。在其它时间，报文是被存储在某个中间交换机中，因而此时的链路可用于其他传输。这样，与电路交换相比，存储转发方式的主要优点是通信

带宽的动态分配，而且这种分配是以网络中的特定链路和特定报文（对一个特定的源点－终点对来说）为基础的。

Packet switching achieves the benefits discussed so far and offers added features. It provides the full advantage of the dynamic allocation of the bandwidth, even when messages are long. Indeed, with packet switching, many packets of the same message may be in transmission simultaneously over consecutive link of a path from source to destination, thus achieving a ―pipelining‖ effect and reducing considerably the overall transmission delay of the message as compared to message switching .It tends to require smaller storage allocation at the intermediate switches. It also has better error characteristics and leads to more efficient error recovery procedures, as it deals with smaller entities. Needless to say, packet switching presents design problems of its own, such as the need to reorder packets of a given message that may arrive at the destination node out of sequence.

分组交换除具有以上讨论的优点外，还具有一些特点。它提供动态分配的全部优势，甚至当报文很长时依然如此。由于有分组交换，一个报文的多个分组确定可以通过源点到终点的通路中的多条链路同时传送，因而达到“管道传送”的效应。与报文交换相比，它大大的减少了报文整体的传送时延。在中间交换设备中，这种方式只需要较小的存储分配区域。分组交换的误码特性较好，由于它只涉及很短的长度，因而导致了高效的纠错方式。当然，分组交换也有它设计上的麻烦，例如当报文无序的到达目的节点时，需要重新对核对报文进行分组排序。

UNIT 12

ATM

异步转移模式

ATM (Asynchronous Transfer Mode) is both a multiplexing and switching technique. It was initially intended to handle high bit rates , but it has in fact proved to be a universal technique for transporting and switching any type of digitized information at a wide variety of bit rates.

ATM（异步转移模式）既是复用技术又是交换技术。最初，人们是想用ATM来处理高比特率的数字信号，事实确证明它是一种通用的技术，可以用来传输和交换任何类型并具有各种比特率的数字化信息。

ATM transfers information in short packets call ―cells‖ with a fixed length of 48 bytes plus five header bytes, irrespective of the underlying type of transmission. Cell routing is based on the principle of logical channels with dual identification: the cell header contains the identifier of the basic connection to which the cell belongs－called a virtual circuit (VC) and the identifier of the group of VCs to which the connection belongs－called a virtual path (VP).

无论传输的信息是什么，ATM都以称作“信元”的短的分组采传送信息。信元是由固定的48字节加上5个字节的信头组成。信元寻找路由是基于带有双重识别的逻辑信道原理：信元头包含了信元所属的基本连接识别符，这种基本连接称作虚电路（VC）, 另一种是连接所属的VC组识别符，称作虚路径（VP）.

ATM is related to both circuit and packet modes. Because of the simplicity of the protocol used, the transfer of cells to the network nodes can be handled entirely by hardware, which leads to very short transit time and high usage of transmission paths, even at bit rates

of several hundred megabits a second. On the other hand, ATM retains all the flexibility of the packet mode, enabling only required information to be conveyed, offering a simple, unique multiplexing method irrespective of the bit rates of the different information flows, and allowing these bit rates to be varied.

ATM既与电路方式有关又与分组方式有关。由于使用简单的协议，信元至网络节点的转移可完全有硬件处理完成，这就缩短了传送的时间，提高了传输路径的速率，使比特率甚至可以达到每秒几百兆比特。另一方面，ATM保留了分组方式的所有灵活性：只传送所需要的信息，提供简单、独特的复用方法而不管不同信息流的比特率，并且允许比特率变化。

ATM dated from the beginning of the 1980s: at the time, people were trying to find the most suitable technique for switching high bit rate channel at more than 100 Mbit/s. In1988, the ITU approved recommendation I. 121 which ratified the choice of ATM as the target transfer mode for broadband networks for all types of information, including low bit rate information such as voice. In 1991 several operators and manufacturers founded the ATM Forum to expedite standardization. The ATM Forum now has more than 600 members and has a significant influence on ATM standards and specifications. The first ATM produces appeared on the market in 1992: they were for local area networks and were designed to solve the problems of sharing the same bearer circuit between computer terminals as they continue to increase in numbers and power.

ATM开始于80年代初，那时人们试图找一种更合适的技术用于交换超过100Mbit/S的高比特率信道。1988年，ITU批准了I.121建议，该建议选择ATM作为用于各种类型信息宽带网络的目标传送模式，其中包括如话音的低比特率信息。1991年一些运营公司和厂商建立了ATM论坛以加速ATM标准化工作。现在ATM论坛的成员已经超过600个，它对ATM的标准化和规范化有着重大影响。第一批ATM产品1992年面世，主要用于局域网，其设计主要是用来解决计算机终端间随计算机数量和功能不断增加所带来的共享同一承载电路的问题。

An ATM network can be considered, in a first approximation, as being three overlaid functional levels: a services and applications level, an ATM network level and a transmission level. The applications provide an end-to-end service. They use the logical connections of ATM network level which in turn multiplexes and logically routes the information flow as ATM cells go through the transmission links shared by logical connections called virtual connections. The transmission level provides these physical links and handles the actual physical transport of the cells.

ATM网络可以近似的看做是有三个覆盖功能层组成：业务和应用层，ATM网络层和传输层。应用层提供端到端的业务。应用层使用ATM网络层的逻辑连接，当信元通过由逻辑连接（称作虚连接）共享的传输链路时，ATM网络层依次对信息流复用并寻找信息流的逻辑路由。传输层提供物理链路并处理信元的实际物理传输。

An ATM network can transport and switch voice, data and video which, seen from the access, use traditional digital interfaces with the same quality of service. This means that a physical connection between any two terminals can be replaced with an equivalent logical connection which is multiplexed with other in a common transmission link. The resource is shared dynamically between all the connections.

ATM网络能够传输和交换话音、数据和视频业务，从接入的角度看这些业务使用传统的数字接口并具有同样的服务质量。这就意味着任何两个终端间的物理连接都可

由等效的逻辑连接代替，逻辑连接可在公用的传输链路中与其他的逻辑连接复用。资源可在所有的连接中动态共享。

Compared with the synchronous time division multiplexing techniques which rigidly link service to resource, the asynchronous technique has the advantage of occupying the transmission link only in proportion to the exact requirement.

与同步时分复用技术相比，同步复用技术僵硬的将业务与传输资源相连，而异步技术的优势是根据其确切的需要来占用传输链路。

The ATM technique completely separates the applications and services transported over a network from the transmission resources used. The ability to construct virtual networks means that the physical network can be shared by many users dynamically and in real time, thereby achieving cost-effective use of infrastructure, for high bit rate services too. Investments at all levels are also future-proofed, because of the different applications can be reallocated in time over the same network infrastructure as requirements arise. ATM offers a unique way of coordinating different networks carrying different services into a single physical network.

ATM技术将网络传输的应用和业务与所使用的传输资源完全分开。构成虚网络的能力意味着物理网络可以由许多用户动态时实的共享，因而使网络结构得到高价高效的使用，对高比特率业务也一样。对所有网络层的投资都是适应未来需要的，因为不同的应用在出现新的需求时可及时在同一网络结构中进行重新分配。ATM提供一种独特的方式将传输不同业务的网络协调成单一的物理网络。

The advantages described above have explained the enthusiasm for ATM . 以上所描述的ATM优点说明了为什么人们对ATM充满热情。

As digitization and image encoding progress, interactive video services, and more generally multimedia services, are starting to emerge. Their impact on the network will be considerable. Today, ATM is the only transfer technique to offer the high bit rates and flexibility required by these services.

随着数字化和图像编码技术的进步，交互视频业务和更通常的多媒体业务开始出现。这些业务将会对网络产生很大影响。今天，ATM是唯一能够提供这些业务所需的高比特率和灵活性的传输技术。

ATM, much more than any other telecommunications technique , is able to meet the current and the future requirements of both operators and users. Compared with other techniques that may compete in certain applications, ATM is special ( mainly due to its universal nature, both in terms of bit rate and type of information transferred）. ATM offers a switching function for all bit rates and this is particularly suitable for high and variable bit rates.

ATM，远比任何其它电信技术更能满足运营公司和用户对当前和未来业务的需求。与其它有可能在某些应用领域于ATM竞争的技术相比，ATM（主要由于其通用性，无论是比特率还是传输信息的类型）都具有特殊的优点。ATM对所有比特率的信号都可提供交换功能，这一点特别适合于高比特率和可变化比特率信号。

ATM’s specific features will make it the preeminent nature vehicle for multimedia services and especially for varying bit rate video, and will make it one of the essential components of future information superhighways offering new services such as video on demand. In the short term, ATM is also proving of great interests to the operators , because of the flexibility and virtuality that it can introduce into networks, by separating the concept of

connection form that of physical resources. This simplifies network management functions and makes optimum use of resources, particularly through statistical multiplexing and the creation of virtual private networks.

ATM的独特性将使它成为卓越的多媒体业务的自然载体，特别是对于可变比特率的视频，并且使它成为能够提供如视频点播新业务的未来信息高速公路必不可少的一部分。在很短的时间内，运营这对ATM产生了很大的兴趣，主要是由于将连接的概念与实际资源分开所引入网络的灵活性和虚拟性。这就简化了网络的管理功能并能最佳地使用网络资源，特别是通过统计复用和建立虚拟专用网络。

Of course, there is still a long way to go before the ATM techniques is in general use ,but a revolution is underway which will deeply affect the worlds of telecommunications , data processing and video. The impact of this upheaval will without any doubt be greater than the advent of digital techniques in analogue networks.

当然，在ATM技术普遍使用之前仍会有很长的路要走，但是这场正在进行的技术革命将会深刻的影响数据处理和视频处理，影响电信世界。这一革命所产生的影响无疑会比在模拟网络中出现数字技术的影响要大的多。 UNIT 13

The Public Telecommunications Network

公共电信网

By far the largest circuit-switched network is the public telecommunications network－the telephone network. This is actually a collection of national networks interconnected to form an international service. Although originally designed and implemented to service analog telephone subscribers, it handles substantial data traffic via modem, and is gradually being converted to a digital network.

迄今最大的电路交换王是公共电信网－电话网，实际上是由一些国家网相互连接而形成的国际业务。虽然原来的设计是为模拟电话用户服务的，但是它通过利用调制解调器处理大量的数据业务，而正在逐渐变为数字网。

The public telecommunications network, as with any communications network, can be described using four generic architectural components.

如同任何通信网一样，公用电信王可以用四个一般的结构组件来描述。

? Stations: generally denoted as subscribers, these are the devices that attach to the

network.

? Interfaces: the interface between the stations and the network, referred to in the

phone system as the local loop.

? Nodes: the switching centers in the network.

? Links: the branches between nodes, referred to as trunks. ? 站：一般只用户，这些是接入网络的装置。

? 接口：站和网络间的接口，在电话系统中称为本地环路。 ? 节点：网络交换的中心

? 链路：节点之间的支路，称为中继线。

Most of the subscribers on the network are telephones. The telephone contains a transmitter and receive for converting back and forth between analog voice and analog electrical signals. With the introduction of the digital data system, some subscribers that transmit digital signals have been incorporated into the network.

网络中的大多数用户是电话机。电话机包括一个送话器和一个受话器，用来在模拟话音和模拟电信号之间进行变换。随着数字数据系统的引入，有些传输数字信号的用户已经并入该网。

The local loop is a pair of wires, generally twisted pair, that connects a subscriber to one of the nodes in the network. The local loop generally covers a distance of a few kilometers to a few tens of kilometers at most.

本地环路是将用户连接到网络中的一个节点的一对导线，通常是双绞线。一般本地环路的覆盖范围从几公里到几十公里。

A two-wire connection is inherently half-duplex; that is, it can carry voice in one direction at a time only. Similarly, it can carry digital signals in on direction at a time only. For full-duplex DDS connections, two twisted pair links are generally used.

两线连接本质上是半双工的，即它一次只可传送一个方向的话音。同样，它一次仅能传送一个方向上的数字信号。对于全双工DDS连接，一般采用两对双绞线链路。

Each subscriber connects via local loop to a switching center, known as an end office. Typically, an end office will support many thousands of subscribers in a localized area. There are over 19 000 end offices in the United States, so it is clearly impractical for each end office to have a direct link to each other end office; this would require on the order of 2*108 links. Rather, intermediate switching nodes are used. Designers have found it convenient to organize these nodes into a hierarchy or tree topology (Fig. 13-1), consisting of five classes of switching centers of nodes:

每个用户通过本地环路接到一个称为端局的交换中心。通常，一个端局支持一个局部地区的数千个用户。在美国有19000多个端局，所以要想每个端局都有到其它每一个端局的直接链路显然是不实际的，这将需要2*108数量级的链路。因此，应采用中间交换节点，设计人员发现可以很方便的把这些节点组织成有五级交换中心或节点构成的层次结构或树状拓扑（见第152页图13-1）

Class 1: regional center. Class 2: sectional center Class 3: primary center. Class 4: toll center. Class 5: end office 第1级：大区中心 第2级：地区中心 第3级：初级中心 第4级：长途中心 第5级：端局

Subscribers connect directly to an end office, which must perform the same functions as a PBX. The remaining centers simply serve the function of concentrating traffic so as to reduce transmission facility requirements.

用户直接连接到端局，端局必须执行PBX的相同功能。其余的中心仅提供集中通信业务量的功能，以减少对传输设备的需求。

The switching centers are linked together by trunks. These trunks are designed to carry multiple voice-frequency circuits using either FDM or synchronous TDM.

交换中心由中继线连接在一起。这些中继线的设计采用FDM或同步TDM来传送多条音频线路。

Two additional elements are needed to complete the picture. In addition to the five classes of switching centers listed above, the network is augmented with additional switching nodes called tandem switches. Theses are used to interconnect adjacent end office. Finally, PBX facilities connect to the network not via local loop but via trunk. Since the PBX services multiple subscribers, a multiplexed link is needed to the end office. Generally, this link has lower capacity than the total number of PBX subscribers, reflecting the fact that, at any given time, only a fraction of the subscribers will be engaged in external calls.

还需要两个附加的组成部分来完成这个结构图。除了上面列出的五级交换中心外，网络用另外的称之为汇接局的交换节点加以扩展。这是用于连接邻近的端局的。最后，PBX设备不是通过本地环路而是通过中继线接到网中。由于PBX服务多个用户，所以需要一条复用链路连到端局。一般说来这条链路上的容量较PBX用户总数为低，这反映了在任一给定的时间只有部分用户进行外线呼叫这一事实。

The structure depicted in fig.13-1 is referred to as having a tree topology. Actually, it is a set of 10 trees, each rooted in a regional center. The 10 regional centers are meshed together to provide full connectivity. If this were the extent of architecture, routing would be quite simple. Consider a request from one subscriber to establish a connection with another. The following rules would apply:

图13-1所画的结构称为树状拓扑。实际上，它一共是10棵树，每一棵扎根于大区中心。这10个大区中心以网状形式连接在一起来提供全连通。如果结构的规模仅到此为止，则路由选择就相当简单了。设有一用户要与另一用户建立连接，则可以采用以下规则：

1. If both subscribers attach to the same end office, that end office makes the

connection.

2. If (1) fails, the subscribers attach to different end offices .If those end offices

attach to the same toll center, a connection is established between end office via the toll center.

１．如果这两个用户接到同一端局，则由该端局建立连接。

２．如果不是1的情况而是用户接到不同的端局，若那些端局接到同一个长途中

心，则端局间的连接由该长途中心建立。

And so on. The search continues up the hierarchy until a common node is reached. If

the two subscribers are under the aegis of different regional centers, the circuit will involve a trunk between regional centers, for a total of nine trunks in the path between the subscribers.

如此继续下去。搜索沿层次而上直至到达公共节点。如果两个用户是在不同的大

区中心的覆盖之下，电路将包括两个大区中心间的一条中继线，用户间的路径总共可有9条中继线。

This architecture has several drawbacks. First, a tremendous amount of traffic must be

carried at the upper levels of the hierarchy. Second, the loss or saturation of a single switching center decouples the network into isolated subnetworks. Finally, signal quality degrades as the number of switches and trunks increases.

这种结构存在几个缺点。首先，在较高的层次间要传送大的通信量。其次，单个

交换中心的故障或饱和会该网络拆成孤立的子网。最后，当交换的次数和中继的段数量增加时，信号的质量要降低。

To compensate for these problems, a large number of high-usage trunks augment the

basic architecture. High-usage trunks are used for direct connection between switching centers

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