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 low-speed communication. For example, the connection of a terminal to a computer 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 Transmission
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 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 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 dependent 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, synchronous transmission is faster than asynchronous 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. 6.2 DTE-DCE INTERFAC
At 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 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.
13
Data Terminal Equipment (DTE)
Data terminal equipment (DTE) includes any unit that functions either as a source of or 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 communicate 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’s say you have an idea that you want to communicate to a friend. Your brain creates the idea but cannot transmit that idea to your friend’s brain by itself. Unfortunately or fortunately, we are not a species of mind readers. Instead, your brain passes the idea to your vocal chords and mouth, which convert it to sound waves that can travel through the air or over a telephone line to your friend’s ear and from there to his or her brain, where it is converted back into information. In this model, your brain and your friend’s brain are DTEs. Your vocal chords and mouth are your DCE. His or her ear is also a DCE. The air or telephone wire is your transmission medium.
A DTE is any device that is a source of or destination for binary digital data. Data Circuit-Terminating Equipment (DCE)
Data circuit-terminating equipment (DCE) includes any functional 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 introduces 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 transmission medium and sends the converted signal to another DCE on the network. The second DCE takes the signal off the line, converts it to a form usable by its DTE, and delivers it. To make this communication possible, both the sending and receiving DCEs must use the same encoding method, much the way that if you want to communicate to someone who understands only Japanese, you must speak Japanese. The two DTEs do not need to be coordinated with each other, but each of them must be coordinated with its own DCE and the DCEs must be coordinated so that data translation occurs without loss of integrity.
A DCE is any device that transmits or receives data in the form of an analog or digital signal through a network.
14
6 数字数据传输：接口和调制解调器
（选自?数据通信与网络?， Behrouz Forouzan著）
我们将信息编码成可以传输的格式，下一步就是探讨传输过程了。信息处理设备如个人计算机能生成编码信号，通常还需要其它设备协助才能将这些信号在通信链路上传输。例如一台PC机产生数字信号，在将信号通过电话线发送之前，还需要一台附加设备来调制载波频率。在这过程中，我们怎样才能把数据从产生它的设备传送到下一个设备呢？解决办法是使用一捆导线，成为一种为通信链路，或叫接口。
因为接口连接的两个设备有可能不是一个厂家生产的，所以必须规定接口的特性并建立标准。接口特性包括机械规范（使用多少条导线来传输信号）、电气规范（预期信号的频率、振幅和相位）以及功能规范（如果使用多条导线，每条导线的功能是什么？）。这些特性在一些常用标准中都有描述并且被集成到了OSI7层模型的物理层中。
6.1数字数据传输
从一个设备向另一个设备发送数据主要考虑的是配线方式。对于配线问题主要考虑的因素是数据流。我们是否一次只发送一个比特，或是将比特成组发送以及如何成组？通过链路传输二进制数据可以采用并行模式或串行模式。在并行模式中，在每个时钟脉冲到来时多个比特被同时发送。在串行模式中，每个时钟脉冲只发送一个比特。尽管只有一种发送并行数据的方法，串行传输却有两个子类：同步方式和异步方式（参见图6-1）。 数据传输
6.1.1 并行传输
由0和1组成的二进制值可以组成n比特的位组。计算机使用和生成以比特为单位的数据，就像我们在英语会话时用词而不是一个个的字母来交流一样。通过分组，我们可以一次发送n个比特而不是一个比特。这称为并行传输。
从概念上说，并行传输的机制很简单：一次使用n条导线来传输n个比特。这种方式下，每个比特都使用专门的线路，而一组中的n个比特就可以在每个时钟脉冲从一个设备传输到另一个设备。图6-2显示了n=8时并行传输的工作状况。通常八根导
15
并行传输 串行传输 同步传输 异步传输 图6-1 数据传输
线被捆成一根电缆，两端都有连接头。
需要8条线图6-2 并行传输 并行传输的优势在于速度。当其它因素相同时，并行传输将比串行传输的速度快n倍，但同时也存在一个严重缺点：费用高。为进行数据传输，并行传输需要n条通信线路（本例中是导线）。因为如此昂贵，所以并行传输通常被限制在最长25英尺的距离内。 6.1.2 串行传输
在串行传输中，比特是一个一个一次发送的，因此在两个通信设备之间传输数据只要一条通信通道，而不是n条。
串行传输相对于并行传输的优点是：因为只需要一条通信信道，串行传输的的费用大约只是并行传输的n分之一。
因为在设备内部的传输是并行的，所以在发送端和线路之间以及接收端和线路之间的接口上，都需要有转换器（前者是并/串转换，后者是串/并转换）。
串行传输以两种方式进行：同步方式和异步方式。 (1) 异步传输
如果在传输中信号的时序并不重要，我们就将这种传输称为异步传输。它与同步方式不同的事，信息是以一种约定的模式来被接收和翻译的。只要遵照约定模式，接收设备就可以以不理会信息发送的节奏而能正确获取信息。约定模式是基于将比特组成字节。每一组比特（通常为八个）作为一个单位通过链路传输。发送端系统单独处理每个组，每处理完一个组就将其转发到链路上，并不理会时钟信号。
因为没有同步脉冲，接收方步可能通过及是方式来预测下一组比特何时到达。因而，为了通知接收方有新的比特组到达，在每字节的开头都要附加一个比特。这个比特，通常是0，被称为起始位。为了让接收方知道一个字节已经结束，在每字节尾部还要加上一个或多个比特。这些比特，通常是1，被称为停止位。利用以上的方法，每字节的大小至少增加到了10个比特，其中有8比特的信息在加上2个或更多的提示接收方的信号。另外，每发送完一个字节，可能还要跟上一段可变长的时间间隙。这段间隙或者通过信道控闲状态代表，或者通过附加的停止比特流代表。
在异步传输中，需要在每字节开始时发送一个起始位（0），然后在结束时发送一
16
8个比特一起发送 发送方 接收方