Differences between synchronous transmission and asynchronous transmission
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The difference between synchronous and asynchronous synchronization-that is, when you ask me to go to dinner, I will go to dinner with you when I hear it; if I don't hear it, you will not stop calling until I tell you to hear it, to eat together.
Asynchronous-you call me and eat it yourself. After receiving the message, I may leave immediately or wait until I get off work. Also, the call is synchronous, and the message sending is asynchronous.
Differences between synchronous and asynchronous transmission
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. 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.
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. Generally, synchronous or asynchronous transmission can be used.
Synchronous processing of the input method.
1. 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, but the receiver never knows
When will they 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 be
Code is sent 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 be
Arrived. 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 transmission, a stop bit table
Indicates the termination of the transfer information. According to the Convention, idle (no data is transmitted) lines actually carry a Signal Representing Binary 1, the start bit of asynchronous transmission changes the signal to 0, and other BITs make the signal pass along
The input data information changes. Finally, the stop position changes the signal back to 1, and the signal remains till the next start position. For example, if the number "1" on the keyboard is based on an 8-bit extended ASCII code
Send "00110001" and add a start bit before the 8-bit, followed by a stop bit.
The implementation of asynchronous transmission is relatively easy, because each information is added with "synchronous" information, because
The shift of timing will not produce a large amount of accumulation, but it will produce a lot of overhead. In the preceding example, if two more bits are transferred for each eight bits, the total transmission load increases by 25%. Low data transmission speeds
Equipment is not a problem, 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. For example
If it is written to the disk, the asynchronous mode is written to the buffer first and then to the target disk, for example, the original write 1 2 3 4
5, but only 1 is written on the target end. Synchronization is written to the local disk without receiving it, which may cause buffer blocking. Synchronous transmission is usually much faster than asynchronous transmission. The receiver does not have to perform each character
Start and Stop operations. 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 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, synchronization
The transmission overhead 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. In this case, the added BITs make the total number of BITs transmitted
The number is increased by 2.5%, which is 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.
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. Http://cxjr.21ic.org/user1/4641/archives/2008/45279.html
