Advantages and Disadvantages of Synchronous and Asynchronous Transmission
Synchronous transmission is the data transfer in units of data frames. Frame synchronization signals in the form of characters or bit combinations (the latter have high transmission efficiency and reliability) can be used. The transmitter or receiver provides a clock signal dedicated to synchronization. In short-distance high-speed large file transfer, the clock signal can be transmitted by a dedicated clock line. When the computer network adopts the synchronous transmission method, the clock synchronization signal is often embedded in the data signal frame to achieve clock synchronization between the receiver and the transmitter.
In digital communication, synchronization is very important. When the transmitter transmits data information to the receiver through the transmission medium, for example, each time a character (or data frame) data signal is sent, the receiver must recognize the start bit and end bit of the character (or frame) data signal. To correctly read each bit of the data signal of the character (or the frame) at the appropriate time, this is the basic synchronization problem between the receiver and the transmitter.
When sengding data signals in data frames, to ensure the integrity and accuracy of the transmitted signal, in addition to requiring the receiver to be able to recognize the start and end of the signal corresponding to each character (or data frame), to ensure that the reading starts and ends at the correct time Retrieving the signal means maintaining the integrity of the transmitted signal. It is also required to keep the clock and the transmitter at the same frequency to ensure that the number of signal units reads per unit time is the same, that is, to ensure the accuracy of the transmitted signal.
Therefore, when transferring data signals in data frames, the transmitter is required to take the following two measures for the transmitted signals: ①Add a start signal and a stop signal different from the data signal before and after the corresponding signal of each frame of data; ② Add a clock synchronization signal before the data signal of each frame to control the clock synchronization of the receiver.
Asynchronous transmission is the transmission of data in units of characters, using character synchronization signals in the form of bits. The transmitter and receiver have independent clocks (the frequency difference cannot be too much), and neither side provides clock synchronization signals to the other side.
The sender and receiver of the asynchronous transmission do not need to coordinate before the data can be transmitted: the sender can move data at any time, and the receiver must be ready to receive data at any time.
The asynchronous transmission mode is generally adopted between the computer host and the input and output devices. Such as a keyboard, a typical RS-232 serial port (used between a computer and a modem or an ASCII terminal device): the sender can send a character at any time (led by a start bit, and then send all the characters in succession, followed by A dumb bit longer than one bit).
The Advantages and Disadvantages of Asynchronous and Synchronous Data Transmission
Asynchronous transmission is the character-oriented transmission, while the synchronous transmission is bit-oriented.
The unit of asynchronous transmission is character and the unit of synchronous transmission is a frame.
Asynchronous transmission seizes the opportunity of re-synchronization through the start and stops codes of character start and end, while synchronous transmission extracts synchronization information from the data.
The asynchronous transmission has lower requirements on timing, and synchronous transmission often coordinates timing through a specific clock line.
Asynchronous transmission is less efficient than synchronous transmission.
To Sum Up
Both asynchronous transmission and synchronous transmission have the above basic synchronization problem: character synchronization or frame synchronization signals are generally used to identify the start and end of transmitting character signals or data frame signals. The main difference between the two is whether one of the transmitter or receiver sends a clock synchronization signal to the other party.