电子设计毕业设计-汽车尾灯控制电路设计论文资料-正文

Figure 6-1 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 6-2 shows how parallel transmission works for n=8.Typically the eight wires are bundled in a cable with a connector at each end.

8 bit synchronously

We need eight lines Figure 6-2 Parallel transmission

The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission. But there is a significant disadvantage:

cost. Parallel transmission requires n communication 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. Serial Transmission

In serial transmission one bit follows another, so we need only **munication channel rather than n to transmit data between **municating devices .

The advantage of serial over parallel transmission is that with only **munication channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of n.

**munication within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-parallel).

Serial transmission occurs in one of two ways: asynchronous or synchronous.

Sender Receiver

Asynchronous Transmission

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 retrieve 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 regard 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 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 least 10 bits, of which 8 are information and 2 or more are signals to the receiver. In addition, 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.

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.

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 and 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. 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 **e 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 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 low-**munication. 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 **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 high-speed applications like the transmission of data from **puter to another. Byte synchronization is accomplished in the data link layer. ** 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 **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 destination for binary digital data. At the physical layer, if can be a terminal, **puter, computer, printer, fax machine, or any other device that generates or consumes digital data. DTEs do not **municate 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 **munication 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 **munication 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.

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。

**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）。

** 并行传输 并行传输 串行传输 数据传输 图6-1 数据传输 同步传输 异步传输 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。通常八根导线被捆成一根电缆，两端都有连接头。

8个比特一起发送 接收方 发送方