外文翻譯----數(shù)字數(shù)據(jù)傳輸接口和調(diào)制解調(diào)器

1、<p><b>　　英文資料及中文翻譯</b></p><p>　　6 TRANSMISSIONS OF DIGITAL DATA: </p><p>　　INTERFACES AND MODEMS</p><p>　　(From Introduction to Data Communications and Net Working,

2、 </p><p>　　Behrouz Forouzan)</p><p>　　Once we have encoder our information into a format that can be transmitted, the next step is to investigate the transmission process itself. Information-pro

3、cessing equipment such as PCs generate encoded signals but ordinarily require assistance to transmit those signals over a communication link. For example, a PC generates a digital signal but needs an additional device to

4、 modulate a carrier frequency before it is sent over a telephone line. How do we relay encoded data from the generating devi</p><p>　　Because an interface links two devices not necessarily made by the same m

5、anufacturer, its characteristics must be defined and standards must be established. Characteristics of an interface include its mechanical specifications (how many wires are used to transport the signal); its electrical

6、specifications (the frequency, amplitude, and phase of the expected signal); and its functional specifications (if multiple wires are used, what does each one do?). These characteristics are all described by</p>&

7、lt;p>　　6.1 DIGITAL DATA TRANSMISSION</p><p>　　Of primary concern when considering the transmission of data from one device to another is the wiring. And of primary concern when considering the wiring is t

8、he data stream. Do we send one bit at a time, or do we group bits into larger groups and, if so, how? The transmission of binary data across a link can be accomplished either in parallel mode or serial mode. In parallel

9、mode, multiple bits are sent with each clock pulse. In serial mode, one bit is sent with each clock pulse. While there is </p><p>　　Parallel Transmission</p><p>　　Binary data, consisting of 1s a

10、nd 0s, may be organized into groups of n bits each. Computers produce and consume data in groups of bits much as we conceive of and use spoken language in the form of words rather than letters. By grouping, we can send d

11、ata n bits at a time instead of one. This is called parallel transmission.</p><p>　　Figure 6-1　Data transmission</p><p>　　The mechanism for parallel transmission is a conceptually simple one: u

12、se n wires to send n bits at one time. That way each bit has its own wire, and all n bits of one group can be transmitted with each clock pulse from one device to another. Figure 6-2 shows how parallel transmission works

13、 for n=8.Typically the eight wires are bundled in a cable with a connector at each end.</p><p>　　Figure 6-2　Parallel transmission</p><p>　　The advantage of parallel transmission is speed. All el

14、se being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission. But there is a significant disadvantage: </p><p>　　cost. Parallel transmission requires n comm

15、unication lines (wires in the example) just to transmit the data stream. Because this is expensive, parallel transmission is usually limited to short distances, up to a maximum of say 25 feet.</p><p>　　Seria

16、l Transmission</p><p>　　In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices .</p><p>　　The

17、advantage of serial over parallel transmission is that with only one communication channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of n.</p><p>　　Since communi

18、cation within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-parallel).</p><p>　　Serial transmission occurs in one of two ways: asynchronou

19、s or synchronous.</p><p>　　Asynchronous Transmission</p><p>　　Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated

20、 by agreed-upon patterns. As long as those patterns are followed, the receiving device can retrieve the information without regard to the rhythm in which it is sent. Patterns are based on grouping the bit stream into byt

21、es. 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 w</p><p>　　Without a synchronizing pulse, the receiver canno

22、t 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

23、. To let the receiver 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 le

24、ast 10 bits</p><p>　　In asynchronous transmission we send one start bit (0) at the beginning and one or more stop bits (1s) at the end of each byte. There may be a gap between each byte.</p><p>

25、　　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 the data stream. This mechanism is called asynchronous because, at the byte level, sender an

26、d receiver do not have to be synchronized. But within each byte, the receiver must still be synchronized with the incoming bit stream. This is, some synchronization is required, but only for the duration of a single byte

27、. The receiving device resynchronizes at the onset of each new</p><p>　　Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.</p>

28、<p>　　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.

29、 But it is cheap and effective, two advantages that make it an attractive choice for situations like low-speed communication. For example, the connection of a terminal to a computer is a natural application for asynchron

30、ous transmission. A user types only one character at a time, types extre</p><p>　　Synchronous Transmission</p><p>　　In synchronous transmission, the bit stream is combined into longer “frames,”

31、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 into bytes for decoding purp

32、oses. 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.</p><p>　　In synch

33、ronous transmission we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits.</p><p>　　Without gaps and start/stop bits, there is no built-in

34、 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 dependent on the ability of th

35、e receiving device to keep an accurate count of the bits as they come in. </p><p>　　The advantage of synchronous transmission is speed. With no extra bits or gaps to introduce at the sending end and remove a

36、t the receiving end and, by extension, with fewer bits to move across the link, synchronous transmission is faster than asynchronous transmission is faster than asynchronous transmission. For this reason, it is more usef

37、ul for high-speed applications like the transmission of data from one computer to another. Byte synchronization is accomplished in the data link layer.</p><p>　　6.2 DTE-DCE INTERFAC</p><p>　　At

38、this point we must clarify two terms important to computer networking: data terminal equipment (DTE). There are usually four basic functional units involved in the communication of data: a DTE and DCE on one end and a DC

39、E and DTE on the other end. The DTE generates the data and passes them, along with any necessary control characters, to a DCE. The DCE does the job of converting the signal to a format appropriate to the transmission med

40、ium and introducing it onto the network link. When the si</p><p>　　Data Terminal Equipment (DTE)</p><p>　　Data terminal equipment (DTE) includes any unit that functions either as a source of or

41、as a destination for binary digital data. At the physical layer, if can be a terminal, microcomputer, computer, printer, fax machine, or any other device that generates or consumes digital data. DTEs do not often communi

42、cate directly with one another, they generate and consume information but need an intermediary to be able to communicate. Think of a DTE as operating the way your brain does when you talk. Let</p><p>　　A DTE

43、 is any device that is a source of or destination for binary digital data.</p><p>　　Data Circuit-Terminating Equipment (DCE)</p><p>　　Data circuit-terminating equipment (DCE) includes any functi

44、onal unit that transmits or receives data in the form of an analog or digital signal through a network. At the physical layer, a DCE takes data generated by a DTE, converts them to an appropriate signal, and then introdu

45、ces the signal onto the telecommunication link. Commonly used DCEs at this layer include modems . In any network, a DTE generates digital data and passes it to a DCE; the DCE converts the data to a form acceptable to the

46、</p><p>　　A DCE is any device that transmits or receives data in the form of an analog or digital signal through a network.</p><p>　　6 數(shù)字數(shù)據(jù)傳輸：接口和調(diào)制解調(diào)器</p><p>　?。ㄟx自«數(shù)據(jù)通信與網(wǎng)絡(luò)»

47、;， Behrouz Forouzan著）</p><p>　　我們將信息編碼成可以傳輸?shù)母袷?，下一步就是探討傳輸過程了。信息處理設(shè)備如個人計算機能生成編碼信號，通常還需要其它設(shè)備協(xié)助才能將這些信號在通信鏈路上傳輸。例如一臺PC機產(chǎn)生數(shù)字信號，在將信號通過電話線發(fā)送之前，還需要一臺附加設(shè)備來調(diào)制載波頻率。在這過程中，我們怎樣才能把數(shù)據(jù)從產(chǎn)生它的設(shè)備傳送到下一個設(shè)備呢？解決辦法是使用一捆導(dǎo)線，成為一種為通信鏈路，或

48、叫接口。</p><p>　　因為接口連接的兩個設(shè)備有可能不是一個廠家生產(chǎn)的，所以必須規(guī)定接口的特性并建立標準。接口特性包括機械規(guī)范（使用多少條導(dǎo)線來傳輸信號）、電氣規(guī)范（預(yù)期信號的頻率、振幅和相位）以及功能規(guī)范（如果使用多條導(dǎo)線，每條導(dǎo)線的功能是什么？）。這些特性在一些常用標準中都有描述并且被集成到了OSI7層模型的物理層中。</p><p><b>　　6.1數(shù)字數(shù)據(jù)傳輸&l

49、t;/b></p><p>　　從一個設(shè)備向另一個設(shè)備發(fā)送數(shù)據(jù)主要考慮的是配線方式。對于配線問題主要考慮的因素是數(shù)據(jù)流。我們是否一次只發(fā)送一個比特，或是將比特成組發(fā)送以及如何成組？通過鏈路傳輸二進制數(shù)據(jù)可以采用并行模式或串行模式。在并行模式中，在每個時鐘脈沖到來時多個比特被同時發(fā)送。在串行模式中，每個時鐘脈沖只發(fā)送一個比特。盡管只有一種發(fā)送并行數(shù)據(jù)的方法，串行傳輸卻有兩個子類：同步方式和異步方式（參見圖6-

50、1）。</p><p>　　圖6-1 數(shù)據(jù)傳輸</p><p>　　6.1.1 并行傳輸</p><p>　　由0和1組成的二進制值可以組成n比特的位組。計算機使用和生成以比特為單位的數(shù)據(jù)，就像我們在英語會話時用詞而不是一個個的字母來交流一樣。通過分組，我們可以一次發(fā)送n個比特而不是一個比特。這稱為并行傳輸。</p><p>　　從概念上說

51、，并行傳輸?shù)臋C制很簡單：一次使用n條導(dǎo)線來傳輸n個比特。這種方式下，每個比特都使用專門的線路，而一組中的n個比特就可以在每個時鐘脈沖從一個設(shè)備傳輸?shù)搅硪粋€設(shè)備。圖6-2顯示了n=8時并行傳輸?shù)墓ぷ鳡顩r。通常八根導(dǎo)線被捆成一根電纜，兩端都有連接頭。</p><p>　　圖6-2 并行傳輸</p><p>　　并行傳輸?shù)膬?yōu)勢在于速度。當其它因素相同時，并行傳輸將比串行傳輸?shù)乃俣瓤靚倍，但同時

52、也存在一個嚴重缺點：費用高。為進行數(shù)據(jù)傳輸，并行傳輸需要n條通信線路（本例中是導(dǎo)線）。因為如此昂貴，所以并行傳輸通常被限制在最長25英尺的距離內(nèi)。</p><p>　　6.1.2 串行傳輸</p><p>　　在串行傳輸中，比特是一個一個一次發(fā)送的，因此在兩個通信設(shè)備之間傳輸數(shù)據(jù)只要一條通信通道，而不是n條。</p><p>　　串行傳輸相對于并行傳輸?shù)膬?yōu)點是：因為

53、只需要一條通信信道，串行傳輸?shù)牡馁M用大約只是并行傳輸?shù)膎分之一。</p><p>　　因為在設(shè)備內(nèi)部的傳輸是并行的，所以在發(fā)送端和線路之間以及接收端和線路之間的接口上，都需要有轉(zhuǎn)換器（前者是并/串轉(zhuǎn)換，后者是串/并轉(zhuǎn)換）。</p><p>　　串行傳輸以兩種方式進行：同步方式和異步方式。</p><p><b>　　(1) 異步傳輸</b>&l

54、t;/p><p>　　如果在傳輸中信號的時序并不重要，我們就將這種傳輸稱為異步傳輸。它與同步方式不同的事，信息是以一種約定的模式來被接收和翻譯的。只要遵照約定模式，接收設(shè)備就可以以不理會信息發(fā)送的節(jié)奏而能正確獲取信息。約定模式是基于將比特組成字節(jié)。每一組比特（通常為八個）作為一個單位通過鏈路傳輸。發(fā)送端系統(tǒng)單獨處理每個組，每處理完一個組就將其轉(zhuǎn)發(fā)到鏈路上，并不理會時鐘信號。</p><p>　

55、　因為沒有同步脈沖，接收方步可能通過及是方式來預(yù)測下一組比特何時到達。因而，為了通知接收方有新的比特組到達，在每字節(jié)的開頭都要附加一個比特。這個比特，通常是0，被稱為起始位。為了讓接收方知道一個字節(jié)已經(jīng)結(jié)束，在每字節(jié)尾部還要加上一個或多個比特。這些比特，通常是1，被稱為停止位。利用以上的方法，每字節(jié)的大小至少增加到了10個比特，其中有8比特的信息在加上2個或更多的提示接收方的信號。另外，每發(fā)送完一個字節(jié)，可能還要跟上一段可變長的時間間隙

56、。這段間隙或者通過信道控閑狀態(tài)代表，或者通過附加的停止比特流代表。</p><p>　　在異步傳輸中，需要在每字節(jié)開始時發(fā)送一個起始位（0），然后在結(jié)束時發(fā)送一個或多個停止位（1）。在字節(jié)之間可以插入間隙。</p><p>　　起始位、停止位和間隙將一個字節(jié)的起始和終止提示給接收放，使得接收方可以根據(jù)數(shù)據(jù)流進行同步。因為在字節(jié)這一級別，發(fā)送方和接收方不需要進行同步，所以這種傳輸方式稱為異步

57、傳輸。但是在每一字節(jié)內(nèi)，接受方仍要根據(jù)比特流來進行同步。也就是說，一定程度上的同步還是存在的，但僅僅局限在一個字節(jié)的時間內(nèi)。在每一個字節(jié)的開始，接收端設(shè)備就進行重同步。當接收方檢測到一個起始位后，就啟動一個時鐘，并隨著到來的比特開始記數(shù)。在接受完n個比特后，接受方就等待停止位到達。當檢測到停止位到達時，接受方在下一個起始位到達前忽略接收的所有信號。</p><p>　　異步傳輸意味著在字節(jié)級別以異步方式進行，但是

58、每比特仍需要同步，他們的時延是一致的。</p><p>　　相對于不需要控制信息的傳輸方式，異步傳輸由于加入了起始位、停止位以及比特流間插入了間隙而顯得慢一些。但是這種方式既便宜又有效，這兩大優(yōu)點使得在低速通信這一類情形下異步傳輸方式顯得很有吸引力。例如，一臺終端到計算機的連接很自然就是一種異步傳輸?shù)膽?yīng)用實例。用戶一次只敲一個字符，這在數(shù)據(jù)通信領(lǐng)域內(nèi)是十分低速的，同時還在字符之間引入了不可預(yù)計長短的時間間隙。&l

59、t;/p><p><b>　　(2) 同步傳輸</b></p><p>　　在同步傳輸中，比特流被組裝成更長的“幀”，一幀包含有許多個字節(jié)。與異步方式不同的是，引入幀內(nèi)的字節(jié)與字節(jié)之間沒有間隙，需要接收方在解碼時將比特流分解成字節(jié)。也就是說，數(shù)據(jù)被當作不簡短的0、1比特流傳輸，而接收方來將比特流分割成重建信息所需的一個個字節(jié)。</p><p>　　

60、在同步傳輸中，不插入起始/停止比特或間隙就將比特依次發(fā)送出去，完全有接收方負責重組比特。</p><p>　　因為沒有間隙和起始/停止位，就沒有勒比特流內(nèi)部的同步機制可以幫助接收端設(shè)備在處理比特流時調(diào)整比特同步。因為所接收數(shù)據(jù)的準備性完全依賴于接收端設(shè)備根據(jù)比特到達進行精確的比特計數(shù)的能力，所以時序變得十分重要。</p><p>　　同步傳輸?shù)膬?yōu)點是速度快。因為在發(fā)送端不需要插入附加的比特

61、和間隙，再接收端也不需要去掉這些比特和間隙，因而在傳輸方式在類似計算幾件數(shù)據(jù)串是這樣的高速應(yīng)用中更有效。字節(jié)同步在數(shù)據(jù)鏈路層實現(xiàn)。</p><p>　　6.2 數(shù)據(jù)終端設(shè)備和數(shù)據(jù)電路中接設(shè)備接口</p><p>　　在這里必須首先分清計算機網(wǎng)絡(luò)中的兩個重要概念：數(shù)據(jù)終端設(shè)備（DTE）和數(shù)據(jù)電路終結(jié)設(shè)備（DCE）。在數(shù)據(jù)通信中經(jīng)常涉及到四個基本功能單元：兩端各有一個DTE和一個DCE。數(shù)據(jù)終

62、端設(shè)備（DCE）將信號轉(zhuǎn)換成適用于傳輸介質(zhì)的形式并將它發(fā)送到網(wǎng)絡(luò)鏈路中。當信號到達另一端時，相反的過程將發(fā)生。</p><p>　　6.2.1 數(shù)據(jù)終端設(shè)備</p><p>　　數(shù)據(jù)終端設(shè)備（DTE）包括所有具有作為二進制數(shù)字數(shù)據(jù)源點或終點能力的單元。在物理層，這可以是一臺終端、一臺小型計算機、計算機、打印機、傳真機或是任何產(chǎn)生和處理數(shù)據(jù)的設(shè)備。數(shù)據(jù)終端設(shè)備之間并不經(jīng)常直接通信，他們產(chǎn)生或

63、處理數(shù)據(jù)，然而通信需要一種能夠用于傳輸?shù)闹虚g形式?？梢园袲TE的工作方式想象成當你說話時大腦的工作。比如說你有一個想法要與朋友交流，你的大腦產(chǎn)生了這個想法但并不能將它直接傳送到你的朋友的大腦中。由幸或者說不幸的是，我們都不是頭腦閱讀者。相反，你的大腦將想法傳送給你的聲帶和嘴，讓他們將想法轉(zhuǎn)換成聲波通過空氣或是電話線傳輸?shù)侥闩笥训亩?，并由此進入他的大腦。在對方的大腦中，聲波信號被還原成信息。通過這種方式，你和你朋友的大腦就像DTE一樣。

64、你的聲帶和嘴是DCE，你朋友的耳朵也是DCE?？諝饣螂娫捑€就是你們之間的傳輸媒介。</p><p>　　一個數(shù)據(jù)終端設(shè)備（DTE）就是可作為二進制數(shù)字數(shù)據(jù)來源和終點的任何設(shè)備。</p><p>　　6.2.2 數(shù)據(jù)電路終接設(shè)備</p><p>　　任何能夠通過網(wǎng)絡(luò)發(fā)送和接收模擬或數(shù)字形式數(shù)據(jù)的功能單元都是數(shù)據(jù)電路終接設(shè)備（DCE）。在物理層，一個數(shù)據(jù)電路終接設(shè)備（D

65、CE）接收從數(shù)據(jù)終端設(shè)備（DTE）中產(chǎn)生的數(shù)據(jù)，將她們轉(zhuǎn)換為相應(yīng)信號，然后將這些信號發(fā)送到傳輸鏈路上。在這一層中常用的DCE設(shè)備由調(diào)制解調(diào)器。在任何一個網(wǎng)絡(luò)中，一個DTE設(shè)備產(chǎn)生數(shù)字數(shù)據(jù)并將它傳送到DCE設(shè)備，DCE設(shè)備將這些數(shù)據(jù)轉(zhuǎn)化可以在傳輸媒體上傳送的格式并將轉(zhuǎn)化后的信號發(fā)送給網(wǎng)絡(luò)上另一個DCE設(shè)備。第二個DCE設(shè)備從線路上接收信號，將信號轉(zhuǎn)化為與它相連的DTE設(shè)備可用的格式，然后轉(zhuǎn)發(fā)信息。為實現(xiàn)這一通信，發(fā)送和接收的DCE設(shè)備必須