通信英语The_Principle_of_PCM译文

1、图解:译文“蓝色”UNIT 1 The Principle of PCMPCM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

2、 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 oc

3、cupying 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 codin

4、g 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300HZ3.4KHZ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 ex

5、ample 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 o

6、f 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

7、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 equipmen

8、t 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 al

9、ways 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

10、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 particula

11、r 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 attenuatio

12、n 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 th

13、is 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 assoc

14、iated 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-d

15、igit 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

16、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

17、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 im

18、portant! 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 synchronizat

19、ion 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

20、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 an

21、d 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 telepr

22、inter.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 sig

23、nals 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 se

24、parated 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

25、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

26、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 unit

27、s 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

28、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

29、 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 transm

30、itter 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

31、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 d

32、etected, 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 calcu

33、lated 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

34、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 receivers local clock, which samples each incoming bit at its nominal center. Suppose the receiver clock waits T/2 seconds from the falling e

35、dge 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 receivers 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

36、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 les

37、s 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 ,probl

38、ems 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 StandardsISO联网标准The early development of LANs, MANs, and WANs was chaotic in many ways.

39、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 intr

40、oduced.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 implementa

41、tions 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

42、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 addres

43、s 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

44、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 descri

45、ptive 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

46、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

47、 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

48、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

49、 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

50、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. Dividin

51、g 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：l It breaks network communication into smaller, simpler parts that are easier to develop.l It facilitates standardization of network components to allow multiplevendor de

52、velopment and support.l It allows different types of network hardware and software to communicate with each other.l It prevents changes in one layer from affecting the other layers, so that they can develop more quickly.l It breaks network communication into smaller parts to make learning it easier

53、to understand.l 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。l 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。l 它容许不同类型的网络硬、软件互相通信。l 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。l 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 se

54、ven 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：l Layer7: the application layerl Layer6: the presentation layerl Layer5: the session layerl Layer4: the transport

55、 layerl Layer3: the network layerl Layer2: the data link layer l Layer1: the physical layerl 第7层：应用层l 第6层：表示层l 第5层：会话层l 第4层：传输层l 第3层：网络层l 第2层：数据链路层l 第1层：物理层Now lets 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 layersThe three

56、 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 layerThe application layer is the O

57、SI layer that is closest to the user. It provides network services, such as file access and printing, to the users 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

58、 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

59、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 layerThe 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, t