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If according to the order of data transmission can be divided into parallel transmission and serial transmission; if according to the synchronous mode of data transmission can be divided into synchronous transmission and asynchronous transmission; if according to the flow direction and time relationship of data transmission can be divided into simplex, half-duplex and full-duplex data transmission.

Data transmission mode is the way data is transmitted over a channel.

sort in order

Parallel transmission is the simultaneous transmission of data in groups on more than two parallel channels. For example, characters with 8 unit codes can be transmitted in parallel with 8 channels, one channel transmitting one character at a time. Therefore, the character synchronization of the transmitting and receiving sides is realized without additional measures. The disadvantage is that there are many transmission channels, complex equipment and high cost, so it is rarely used.

Serial transmission is the transmission of data streams in serial fashion over a single channel. This method is easy to implement. The disadvantage is that to solve the synchronization of receiving and sending code groups or characters, synchronization measures need to be added. Serial transmission is used more often.

Classification by mode

In serial transmission, how to divide the serial data stream from the serial data stream correctly is called character synchronization. According to the different ways to realize character synchronization, there are two ways of data transmission: asynchronous transmission and synchronous transmission.

Asynchronous transmission transmits a character code (5~ 8 bits) at a time, adding a "start" signal in front of each character code, the length of which is specified as 1 symbol, the polarity is "0," and a stop signal is added after it. When the international telegraph number is used, the stop signal length is 1.5 symbols. When the international five number (see data communication code) or other codes are used, the stop signal length is 1 or 2 symbols, and the polarity is "1." Characters can be sent continuously or separately; when no characters are sent, stop signals are sent continuously. The start of each character can be arbitrary (this is also the meaning of asynchronous transmission), but the symbol lengths within the same character are equal. The receiver detects the "start" signal for recognizing a new character according to the transition ("1"→"0") from the stop signal to the start signal between characters, so as to correctly distinguish one character from another. Therefore, such a character synchronization method is also called start-stop synchronization. The advantage of this method is that it is simple to realize synchronization, and the clock signals of both sides do not need accurate synchronization. The disadvantage is that each character increases by 2~ 3 bits, which reduces the transmission efficiency. It is often used for low-speed data transmission of 1200bit/s and below.

Synchronous transmission is the transmission of data signals at fixed clock beats. In serial data stream, the relative position of each signal symbol is fixed. If the receiver wants to distinguish the transmitted symbol from the received data stream correctly, it must establish bit timing synchronization and frame synchronization. Bit timing synchronization, also known as bit synchronization, is used to synchronize the bit timing clock signal at the receiving end of the data circuit terminating equipment (DCE) with the input signal received by the DCE, so that the DCE can correctly determine the signal symbols from the received information stream and generate the received data sequence. There are two ways to generate timing at the DCE transmitting end: one is to generate bit timing in the data terminal equipment (DTE) and send the DTE data to the DCE with this timing beat. This method is called external synchronization. Another method is to extract DTE data by using the bit timing inside DCE. This method is called internal synchronization. For the receiving end of the DCE, the received data is sent to the DTE at the bit timing beat in the DCE. Frame synchronization is the correct grouping or framing of received data sequences in order to correctly distinguish individual characters or other information. The advantage of synchronous transmission is that it does not need to add and stop symbols separately for each character, so the transmission efficiency is high. The disadvantage is that the implementation technology is more complex. Usually used for data transmission at rates of 2400bit/s and above.

Classification by relationship

According to the flow direction and time relationship of data transmission, the data transmission mode can be divided into simplex, half-duplex and full-duplex data transmission.

Simplex data transmission is the transmission of data between two data stations in only one specified direction. That is, the DTE at one end is fixed as the data source, and the DTE at the other end is fixed as the data sink.

Half-duplex data transmission is data transmission between two data stations in two directions, but not simultaneously. That is, the DTE at each end can be used as either a data source or a data sink, but not both.

Full-duplex data transmission is between two data stations and can be transmitted simultaneously in both directions. That is, the DTE at each end can act as both a data source and a data sink. Usually four-wire lines achieve full duplex data transmission. Two-wire line to achieve simplex or half-duplex data transmission. When frequency multiplexing, time division multiplexing or echo cancellation are used, two-wire lines can also realize full-duplex data transmission.

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