外文翻譯---6數字數據傳輸：接口和調制解調器(編輯修改稿)

　

【文章內容簡介】 fter n bits the receiver looks for a stop bit. As soon as it detects the stop bit, it ignores any received pulses until it detects the next start bit.Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized。 their durations are the same.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 attractive choice for situations like lowspeed munication. For example, the connection of a terminal to a puter is a natural 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 time between each character.Synchronous TransmissionIn synchronous transmission, the bit stream is bined 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 into 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.In synchronous transmission we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits.Without gaps and start/stop bits, there is no builtin mechanism to help the receiving device adjust its bit synchronization in midstream. Timing bees very important, therefore, because the accuracy of the received information is pletely dependent on the ability of the receiving device to keep an accurate count of the bits as they e 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, synchronous transmission is faster than asynchronous transmission is faster than asynchronous transmission. For this reason, it is more useful for highspeed applications like the transmission of data from one puter to another. Byte synchronization is acplished in the data link layer. DTEDCE INTERFACAt this point we must clarify two terms important to puter networking: data terminal equipment (DTE). There are usually four basic functional units involved in the munication of data: a DTE and DCE on one end and a DCE 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 medium and introducing it onto the network link. When the signal arrives at the receiving end, this process is reversed.Data Terminal Equipment (DTE)Data terminal equipment (DTE) includes any unit that functions either as a source of or as a