中英文翻译.doc

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1、附录4：英文资料及中文翻译1.英文资料Communicating with DatalData signals are transmitted over various types of telephone circuits. They travel on wire from telephone pole to telephone pole, through underground cables, from mountain top to mountain top over microwave facilities, on the ocean floor in submarine cables

2、, and via communications satellites from continent to continent. Some type of data conversion equipment is required to change the digital machine signals to a form suitable for transmission over these facilities.The data machine which provides an input to the transmit section of the conversion equip

3、ment, or modulator ,can be a keyboard , printer, card reader, paper tape terminal computer or magnetic tape terminal. The output from the receive section of the converter, or demodulator, can be applied to a tape punch, printer, card punch, magnetic tape unit, computer, or visual display terminal. T

4、ypically, both the modulator and demodulator sections of the converter are combined into a two-way data transmitter-receiver, commonly called a data modem or data set.The typical full-duplex data transmission system including the originating data processing equipment and the interface assembly which

5、 consists of buffer and control units. The interface assembly at the transmitter accepts data at the rate determined by the operating speed of the data processor. stores the data temporarily, and regenerates it at a rate compatible with that of the data modem. At the receiving terminal the interface

6、 assembly accepts the received data, stores it, then feeds it to the data processor at the appropriate rate.Timing signals from the interface assembly at the transmitter are applied to the data modem to synchronize the computer and the data set .At the receiver, synchronization pulses are derived fr

7、om the data stream to synchronize the computer.When more than one data set feeds into a computer, the capacity of the interface equipment is of major concern since it must determine the time slot allocation for each line. Various types of interface assemblies are employed, such as magnetic core memo

8、ries, shift registers, and delay lines. Not all data communications terminals employ an interface between the data processor and the data modem. Without an interface, the input, data transmission, and output functions proceed simultaneously and at the same rate of speed. Since data signals are rarel

9、y in suitable form for transmission over the various types of transmission facilities, a signal coding process is normally performed. Ideally, the transmission medium should have linear attenuation and delay characteristics, but this is never so in practice, and transmission impairments are always p

10、resent to disturb the data signals. As a comparison, in voice communications a high degree of transmission irregularities can be tolerated. If a voice circuit has a heavy loss or is noisy, the speakers compensate automatically by increasing the intensity of their voices. If words are missed because

11、of transmission difficulties, they are often understood anyway because of the redundant nature of speech. In contrast, there is no inherent redundancy in data signals unless purposely inserted and, therefore, transmission variations car only be compensated for over a very small range. In addition, d

12、ata signals are sensitive to other transmission impairments which have little effect on speech.Coding is undertaken to alleviate transmission irregularities, to increase the information capacity of the system, to enable error detection, and to provide message security. The coding process in the data

13、 transmitter simply rearranges the applied data machine signals into some other format. At the receiving end the reverse processing is performed to recover the original machine signals.The diagrams show the two types of information signals that are applied in digital form to a data modem. Shown in A

14、 is a binary non-return to zero signal. In B the same signal is shown in the return to zero format. The difference between A and B is that in A successive marks or spaces follow one another, whereas in B there must be a return to the space level between successive marks. The voltage values of marks

15、and spaces are arbitrary and may be positive, negative, or both.Of primary concern when considering the transmission of data from one device to another is wiring. And of primary concern when considering the wiring is the data stream. Do we send one bit at a time, or do we group bits into larger grou

16、ps and. if so, how? The transmission of binary data across a link can be accomplished either in parallel mode or serial mode. In parallel mode, multiple bits are sent with each clock pulse. In serial mode, one bit is sent with each clock pulse. While there is only one -way to send parallel data, the

17、re are two subclasses of serial transmission: synchronous and asynchronous.Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed-upon patterns. As long as those patterns are followed, the receiving device can re

18、trieve the information without regard to the rhythm in which it is sent. Patterns are based on grouping the bit stream into bytes. Each group, usually eight bits, is sent along the link as a unit. The sending system handles each group independently, relaying it to the link whenever ready, without re

19、gard to a timer.Without a synchronizing pulse, the receiver cannot use timing to predict when the next group will arrive. To alert the receiver to the arrival of a new group, therefore, an extra bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit. To let the re

20、ceiver know that the byte is finished, one or more additional bits are appended to the end of the byte. These bits, usually 1s, are called stop bits. By this method, each byte is increased in size to at least 10 bits, of which 8 are information and 2 or more are signals to the receiver. In addition,

21、 the transmission of each byte may then be followed by a gap of varying duration. This gap can be represented either by an idle channel or by a stream of additional stop bits.The start and stop bits and the gap alert the receiver to the beginning and end of each byte and allow it to synchronize with

22、 the data stream. This mechanism is called asynchronous because, at the byte level, sender and receiver do not have to be synchronized. But within each byte, the receiver must still be synchronized with the incoming bit stream. That is, some synchronization is required, but only for the duration of

23、a single byte. The receiving device resynchronizes at the onset of each new byte. When the receiver detects a start bit, it sets a timer and begins counting bits as they come in. After n bits the receiver looks for a stop bit. As soon as it detects the stop bit, it ignores any received pulses until

24、it detects the next start bit.The addition of stop and start bits and the insertion of gaps into the bit stream make asynchronous transmission slower than forms of transmission that can operate without the addition of control information. But it is cheap and effective, two advantages that make it an

25、 attractive choice for situations like low-speed communication. For example, the connection of a terminal to a computer is a na1ural application for asynchronous transmission. A user types only one character at a time, types extremely slowly in data processing terms, and leaves unpredictable gaps of

26、 time between each character.In synchronous transmission, the bit stream is combined into longer frames, which may contain multiple bytes. Each byte, however, is introduced onto the transmission link without a gap between it and the next one. It is left to the receiver to separate the bit stream int

27、o bytes for decoding purposes. In other words, data are transmitted as an unbroken string of 1s and 0s, and the receiver separates that string into the bytes, or characters, it needs to reconstruct the information.It gives a schematic illustration of synchronous transmission. We have drawn in the di

28、visions between bytes. In reality, those divisions do not exist; the sender puts as data onto the line as one long string. If the sender wishes to send data in separate bursts, the gaps between bursts must be filled with a special sequence of 0s and 1s that means idle. The receiver counts the bits a

29、s they arrive and groups them in eight-bit units.Without gaps and start/stop bits, there is no built- in mechanism to help the receiving device adjust its bit synchronization in midstream. Timing becomes very important, therefore, because the accuracy of the received information is completely depend

30、ent on the ability of the receiving device to keep an accurate count of the bits as they come in.The advantage of synchronous transmission is speed. With no extra bits or gaps to introduce at the sending end and remove at the receiving end and, by extension, with fewer bits to move across the link,

31、synchronous transmission is faster than asynchronous transmission. For this reason, it is more useful for high-speed applications like the transmission of data from one computer to another. Byte synchronization is accomplished in the data link layer.By far the most popular serial interface between a

32、 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. I

33、n fact, the asynchronous serial data link is a very old form of data transmission system and has its origin in the era of teleprompter.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 on

34、ce used by native Americans. The fundamental problem encountered by all serial data transmission systems is how to split the incoming data stream 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 separat

35、ed by an intersymbol space, while the individual characters are separated by an inter character space, which is three times the duration of an intersymbol space.First we examine how the data stream is divided into individual bits and the bits grouped into characters in an asynchronous serial data li

36、nk. The key to the operation of this type of fink is both simple and ingenious. 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 informatio

37、n 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.When the transmitte

38、r 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 fine for a

39、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 as the idle state) for one or two bit periods. Now the transmitter may

40、send another character whenever it wishes.At the receiving end of an asynchronous serial data link, the receiver continually monitors the line 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 ce

41、nters, 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 parit

42、y error flag is set to indicate a transmission error. 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 fall

43、ing edge of the start bit and samples the incoming data every T seconds thereafter until the stop bit has been sampled. As the receivers clock is not synchronized with the transmitter clock, the sampling is not exact.The most obvious disadvantage of asynchronous data transmission is the need for a s

44、tart, 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

45、terminal is itself character oriented. However, if the data link is being used to, say, dump binary data to a magnetic tape, problems arise.2中文翻译数据通信数据信号在各种各样的话路上传输：它们通过导线从一根电杆传到另一根电杆；它们经过地下电缆传送；它们通过微波设备从一个山头传到另一个山头；它们通过海底电缆，通过通信卫星，从一个洲传到另一个洲。为了把数字化机器信号变换为适合在这些设备中传输的信号形式,需要使用某种类型的数据变换设备。向变换设备发送部分（即调

46、制器）提供输入的数据设备可以是键盘、打印机、卡片阅读器、纸带终端计算机或磁带终端机。变换器接收部分（即解调器）的输出可以适用于纸带凿孔机、打印机、卡片凿孔机、磁带机、计算机或视频显示终端。一般地说，变换器的调制部分和解调部分合并成为一个双向数据发送接收机，通常称之为数据调制解调器或数据传输机。典型的全双工数据传输系统，包括始发端数据处理设备和由缓冲器和控制单元组成的接口部件。发端的接口部件以数据处理机的处理速度所确定的速率接收数据，将它们暂时存储起来，并以与数据调制解调器兼容的速率予以转发。在接收端，接口部件接受所收到的数据，将它们存储起来，再以适当的速率送到数据处理机中去。来自发端接口部件的

47、定时信号被加到数据调制解调器上，以使计算机与数传机同步。在接收端，从数据流中取出同步脉冲使计算机同步。 当有一台以上数传机接至一台计算机时，接口设备的容量是主要问题，因为它必须确定分配给每条线路的时隙。有各种类型的接口部件可以使用，如磁芯存储器、移位寄存器和时延线。然而并不是所有的数据通信终端在数据处理机和数据调制解调器之间都使用接口。如果没有接口，那么输入、数据传输和输出这三个操作过程同时进行，而且速率相同。由于数据信号的形式一般不适宜在各种传输设备上传送，通常对信号要进行编码。在理想情况下，传输媒介应当具有线性衰减和线性时延的特性。但实际情况根本不是这样，传输损伤总是存在，干扰了数据信号。

48、相比之下，语声通信可以容忍极不规则的传输情况。如果电话电路的衰耗严重或噪声大，说话人就会提高嗓音，自动予以弥补。如果讲的某些单词因传输困难而没听见，双方往往仍可听懂，因为语言有冗余度。数据信号则与之相反，除非有意加入，它本身没有冗余度，所以传输质量的不稳定只能得到非常有限的补偿。另外，数据 信号对基本上不影响话音的其他传输质量下降很敏感。 为了减少不正常的传输情况，增加系统的信息容量，实现差错检测和消息保密，就要采用编码手段。数据发送端的编码仅仅是将所输入的数据信号重新排列成其他形式。在接收端则进行相反的过程（译码），恢复原来的数据信号。 所给的波形表示以数字形式输入到数据调制解调器的两类信息

49、信号。波形A是二进制不归零（NRZ）信号，波形B是同一信号的归零（RZ）形式。波形A与波形B的区别是：波形A中传号或空号连续不新地出现，而波形B中脉冲幅度必须在两个连续信号之间回到空号电平上来。传号和空号的电压值是任意的，可以是正值或负值，也可以是正负值兼而有之。当研究数据从一个设备向另一个设备传输时,我们关心的主要问题之一是连线。而考虑连线时，数据流又是我们所关心的问题。我们是一次发送一个比特呢,或者是成组发送它们呢？如果要成组发送，又如何做到这一点呢？通过链路来发送二进制数据的方法可以这样实现：要么采用并行方式，要么使用串行的模式。在并行模式中，在每一个时钟脉冲到来时,可同时发送多个比特。而在串行方式里，伴随每个时钟只发送一个比特。虽然只有一种并行发送数据的方法,但串行传输却有两类：同步传输和异步传输。 异步传输被如此称呼，是因为信号的定时并不重要。不同的是，信息是按事先约定的方式来接收和翻译的。只要遵照这些约定，接收器件就能够恢复信息，而不理会它们在发送时的节拍。约定的基础是将比特流组合成字节。每一个组合通常含有8个比特，它被作为一个单元在链路上发送。发送系统单独处理每个组合，当将组合准备停