同步传输和异步传输

Serial Data transmission can occur in two methods,

串行数据传输可以通过两种方法进行：

1. Asynchronous Transmission

   异步传输

2. Synchronous Transmission

   同步传输

1)异步传输 (1) Asynchronous Transmission)

It is named Asynchronous transmission because the timing of a signal is unimportant. In this transmission process transmitted information is encoded with start and stop bits, specifying the beginning and end of each character. As long as some pattern is followed, the receiving device can retrieve the information without regard to which it is sent.

之所以称为异步传输 ，是因为信号的时序无关紧要。 在此传输过程中，已传输的信息使用开始和停止位进行编码，指定每个字符的开始和结束。 只要遵循某种模式，接收设备就可以检索信息，而与发送信息的方式无关。

Patterns are based on grouping the bitstreams into a byte usually eight bits is sent along with the link as a unit. The sender sends each group of data independently, relaying it to the link whenever ready, without regard to a timer.

模式基于将位流分组为一个字节，通常将八个位与链接一起作为一个单元发送。 发送方独立发送每组数据，并在不考虑计时器的情况下随时将其中继到链接。

• The receiver cannot use timing to predict the arrival time of the next group so that synchronizing pulse is required. To notify the receiving system to the arrival of a new group, therefore, an extra bit is added to the beginning of each byte.

  接收机无法使用时序来预测下一组的到达时间，因此需要同步脉冲。 因此，为了通知接收系统有新的组到达，需要在每个字节的开头添加一个额外的位。

• This 0s bit is referred to as the start bit. Telling the receiver that the byte is finished, one or more additional bits are appended to the end of the byte. This is bit is called a stop bit.

  此0s位称为起始位。 告诉接收者该字节已完成，在该字节的末尾附加一个或多个其他位。 该位称为停止位。

• By this method, each byte is increased in size to at least 10 bits, of which 8 are information and two or more are signals to the receiver.

  通过这种方法，每个字节的大小增加到至少10位，其中8位是信息，两个或更多是给接收器的信号。

• Also, the transmission of each byte may gap of varying duration. This gap between information can be represented either by an idle channel or by a stream of additional stop bits.

  同样，每个字节的传输可能会间隔不同的持续时间。 信息之间的间隙可以通过空闲信道或附加停止位流来表示。

• The bits of a byte that is 8 bits are transmitted simultaneously on separate wires. If two devices are close together computer or printer so the communication within the computer.

  字节的8位是在单独的导线上同时传输的。 如果两个设备靠近计算机或打印机，那么计算机内部的通信。

Examples of Asynchronous Transmission: emails, forums, letters, radios and televisions.

异步传输的示例 ：电子邮件，论坛，信件，广播和电视。

Asynchronous Transmission

异步传输

This figure shows the schematic illustration of asynchronous transmission. In this example, the start bits are 0s and the stop bits are 1s and the gap is represented by an idle line rather than by additional stop bits.

该图显示了异步传输的示意图 。 在此示例中，起始位为0，停止位为1，并且间隙由空闲线而不是其他停止位表示。

In this diagram, the ASCII character would be transmitted using 10 bits. In this transmission scheme "0100 0001" changes into "1 0100 0001 0". The extra bits, depending on the parity bit, at the start and end of the data transmission.

在此图中，ASCII字符将使用10位进行传输。 在该传输方案中，“ 0100 0001”变为“ 1 0100 0001 0”。 在数据传输的开始和结束时，取决于奇偶校验位的额外位。

This starts and stops bits tells the receiver that character is coming and also the character has ended.

这将开始和停止位，以告知接收者角色即将到来且角色已经结束。

This scheme of transmission is used when data are transmitted irregularly as opposed to in a solid stream.

当与固态流相反地不规则地传输数据时，使用这种传输方案。

Characteristics of asynchronous communication are as follows:

异步通信的特征如下：

• Extra bits are added to the start and end of the character stream.

  多余的位将添加到字符流的开头和结尾。

• Between two characters there may exist gaps or spaces.

  两个字符之间可能存在间隙或空格。

• The idle time is not constant between bytes as only one byte is sent at a time.

  字节之间的空闲时间不是恒定的，因为一次仅发送一个字节。

• The reception of data is done at different clock frequencies.

  数据的接收是在不同的时钟频率下进行的。

Advantages:

优点：

• Synchronization between devices is not necessary.

  设备之间不需要同步。

• It is a low-cost scheme.

  这是一种低成本的方案。

Disadvantages:

缺点：

• Low transmission due to the use of 'start' and 'stop' bits and gaps between data chunks.

  由于使用了“开始”和“停止”位以及数据块之间的间隙，因此传输速度很低。

• Timing errors take place.

  发生计时错误。

2)同步传输 (2) Synchronous Transmission)

The bitstreams are combined into longer "frames" in synchronous transmission, which may contain multiple bytes. Each byte, however, is introduced onto the transmission link without a gap between it and the next one.

比特流在同步传输中组合成更长的“帧”，其中可能包含多个字节。 但是，每个字节都被引入到传输链路中，并且与下一个字节之间没有间隙。

It is left to the receiver to separate the bitstreams into bytes for decoding purposes.

留给接收机将比特流分成字节以进行解码。

• In other words, data or information is transmitted in the form of an unbroken string of 1s and 0s, and the receiver separates that string into the bytes or characters, it needs to reconstruct the information.

  换句话说，数据或信息以不间断的1s和0s字符串的形式发送，并且接收器将该字符串分成字节或字符，它需要重建信息。

• In this transmission, we send bits one after another without start/stop bits or gaps. Grouping of bits is the receiver's responsibility.

  在此传输中，我们一个接一个地发送比特，没有开始/停止比特或间隙。 比特分组是接收者的责任。

Examples of Synchronous Transmission are chat rooms, video conferencing, telephonic conversations, face-to-face interactions, etc.

同步传输的示例包括聊天室，视频会议，电话对话，面对面的交互等。

Synchronous Transmission

同步传输

Characteristics of Synchronous Transmission:

同步传输的特点：

• Between transmitted characters, there are no spaces.

  传输的字符之间没有空格。

• Timing is very important as 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.

  定时非常重要，因为接收到的信息的准确性完全取决于接收设备在比特进来时保持准确计数的能力。

• Special ‘syn’ characters are sent before the data being sent.

  特殊的“ syn”字符在发送数据之前发送。

• These syn characters are placed between chunks of data for timing functions.

  这些同步字符放置在数据块之间，用于计时功能。

Advantages:

优点：

• Data speed is much higher because of no extra bits at the sending end and at the receiving end.

  由于在发送端和接收端没有多余的位，因此数据速度要高得多。

• Timing errors are reduced due to syn.

  时序误差因syn而减少。

• More useful for high-speed applications.

  对于高速应用程序更有用。

Disadvantages:

缺点：

• Timing is responsible for the accuracy of data

  时序负责数据的准确性

• It is required that transmitter and receiver be properly synchronized

  要求发送器和接收器正确同步

翻译自: https://www.includehelp.com/computer-networks/synchronous-and-asynchronous-transmission-data-communication.aspx

同步传输和异步传输