外文翻譯---6數(shù)字數(shù)據(jù)傳輸：接口和調(diào)制解調(diào)器-wenkub.com
2025-01-15 00:00 本頁面
【正文】 而DCE設(shè)備之間必須協(xié)調(diào)工作，才能實現(xiàn)完整的數(shù)據(jù)翻譯過程。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。 數(shù)據(jù)電路終接設(shè)備任何能夠通過網(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）。通過這種方式，你和你朋友的大腦就像DTE一樣。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。 數(shù)據(jù)終端設(shè)備數(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。 數(shù)據(jù)終端設(shè)備和數(shù)據(jù)電路中接設(shè)備接口 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）。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。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)。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。在接受完n個比特后，接受方就等待停止位到達。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。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）。這些比特，通常是1，被稱為停止位。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。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。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）。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）。圖62顯示了n=8時并行傳輸?shù)墓ぷ鳡顩r。通過分組，我們可以一次發(fā)送n個比特而不是一個比特。在串行模式中，每個時鐘脈沖只發(fā)送一個比特。數(shù)據(jù)傳輸并行傳輸串行傳輸同步傳輸異步傳輸從一個設(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。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，或叫接口。數(shù)據(jù)通信與網(wǎng)絡(luò)187。 its electrical 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 several popular standards and are incorporated in the physical layer of the OSI model. DIGITAL DATA TRANSMISSIONOf 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 the 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 acplished 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, there are two subclasses of serial transmission: synchronous and asynchronous (see Figure 61).Parallel TransmissionBinary data, consisting of 1s and 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 data n bits at a time instead of one. This is called parallel transmission. Data transmissionParallelSerialSynchronousAsynchronousFigure 61 Data transmission The mechanism for parallel transmission is a conceptually simple one: use 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 62 shows how parallel transmission works for n= the eight wires are bundled in a cable with a connector at each end.Sender ReceiverWe need eight liness8 bit synchronously Figure 62 Parallel transmissionThe advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial
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