Synchronous Transmission/asynchronous transmission

Source: Internet
Author: User

In the process of network communication, both parties need to exchange data, which requires high collaboration. In order to correctly interpret the signal, the receiver must know exactly when the signal should be received and processed, so timing is crucial. In computer networks, timing is called bit synchronization. Synchronization requires the receiver to receive data according to the start time, end time, and speed of each bit sent by the sender. Otherwise, an error occurs. Usually synchronous or asynchronous transmission can be used for synchronization.

1.Asynchronous transmission(Asynchronous Transmission): Asynchronous Transmission divides bits into groups for Transmission. The group can be 8-bit 1 character or longer. The sender can send these bit groups at any time, and the receiver never knows when they will arrive. A common example is the communication between the computer keyboard and the host. Press the next primary key, numeric key, or special character key to send an 8-bit ASCII code. The keyboard can send code at any time, depending on the user input speed. The internal hardware must be able to receive a typed character at any time. Asynchronous transmission has a potential problem, that is, the receiver does not know when the data will arrive. Before it detects the data and responds, the first bit has passed. This is like someone coming up and talking to you unexpectedly, and you missed the first few words before you could respond. Therefore, the information transmitted asynchronously starts with a starting bit and notifies the recipient that the data has arrived. This gives the recipient the time to respond, receive, and cache data bits; at the end of the transfer, a stop bit indicates the termination of the transfer information. By convention, idle (no data is transmitted) lines actually carry a Signal Representing Binary 1, and the start bit of asynchronous transmission changes the signal to 0, other BITs change the signal with the transmitted data information. Finally, the stop position changes the signal back to 1, and the signal remains till the next start position. For example, the number "1" on the keyboard, according to the 8-bit extended ASCII encoding, will send "00110001", at the same time need to add a start bit before the 8-bit, followed by a stop bit. Asynchronous transmission is easy to implement. Because each information is added with "synchronous" information, the timing drift will not produce a large amount of accumulation, but will produce a large amount of overhead. In the preceding example, if two more bits are transferred for each eight bits, the total transmission load increases by 25%. It is not a problem for low-speed devices with small data transmission volumes, but for high-speed devices with large data transmission volumes, the value-added load of 25% is quite serious. Therefore, asynchronous transmission is often used on low-speed devices.


2.Synchronous Transmission(Synchronous Transmission): The bit groups for Synchronous Transmission are much larger. It does not send each character independently. Each character has its own Start and Stop bits, but sends them together. We call these combinations a data frame or frame for short. The first part of a data frame contains a set of synchronization characters. It is a unique bit combination, similar to the Start bit mentioned earlier, used to notify the recipient that a frame has arrived, however, it can also ensure that the sampling speed of the receiver is consistent with the bit arrival speed, so that the receiving and receiving sides can synchronize. The last part of a frame is a frame end mark. Like synchronization characters, it is also a unique Bit String, similar to the stop bit mentioned above, used to indicate that there is no data to be reached before the next frame starts. Synchronous transmission is usually much faster than asynchronous transmission. The receiver does not have to start or stop each character. Once the frame synchronization characters are detected, it receives them when the next data arrives. In addition, the cost of synchronous transmission is also relatively small. For example, a typical frame may have 500 bytes (4000 bits) of data, which may only contain 100 bits of overhead. At this time, the added bit increases the total number of transmitted BITs by 2.5%, which is equivalent to 25 in asynchronous transmission.
% Value-added is much smaller. As the actual data bit in the data frame increases, the percentage of overhead bit will be reduced accordingly. However, the longer the bit length of the data, the larger the buffer required for caching the data, which limits the size of a frame. In addition, the larger the frame, the longer it occupies the continuous time of the transmitted media. In extreme cases, this will cause other users to wait too long. In synchronous transmission mode, the sender and receiver clock are unified, and there is no interval between character transmission. Asynchronous transmission does not require the sender and receiver to have exactly the same clock. The transmission between characters is asynchronous. Synchronous and asynchronous transmissionDifferences:

1. asynchronous transmission is character-oriented transmission, while synchronous transmission is bit-oriented transmission.

2. The unit of asynchronous transmission is character while that of synchronous transmission is bytes.

3. asynchronous transmission captures the opportunity of re-synchronization through the start and end of characters and the stop code, while synchronous transmission extracts synchronization information from data.

4. asynchronous transmission has low requirements on time sequence. synchronous transmission usually coordinates the time sequence through specific clock lines.

5. asynchronous transmission is less efficient than synchronous transmission.

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