According to the order of data transmission, it can be divided into parallel transmission and serial transmission; according to the synchronization method of data transmission, it can be divided into synchronous transmission and asynchronous transmission; according to the flow direction and time relationship of data transmission, it can be divided into Simplex, half-duplex and full-duplex data transmission.
Data transmission mode (data transmission mode) is the way data is transmitted on the channel.
Classification in order
Parallel transmission is the simultaneous transmission of data in groups on more than two parallel channels. For example, if 8-unit code characters are used, 8 channels can be used for parallel transmission, with one channel transmitting one character at a time. Therefore, character synchronization between the sending and receiving parties is achieved without additional measures. The disadvantages are that there are many transmission channels, the equipment is complex, and the cost is high, so it is rarely used.
Serial transmission is a data stream transmitted serially on a channel. This method is easy to implement. The disadvantage is that in order to solve the synchronization of code groups or characters between the receiving and transmitting parties, additional synchronization measures are required. Serial transmission is more commonly used.
Classification by method
In serial transmission, how does the receiving end correctly divide the sent data from the serial data stream? The actions taken by characters are called character synchronization. According to the different ways of realizing character synchronization, there are two ways of data transmission: asynchronous transmission and synchronous transmission.
Asynchronous transmission transmits one character code (5~8 bits) each time. A "start" signal is added in front of each character code sent. Its length is specified as 1 symbol and the polarity is " 0", followed by a stop signal. When the international telegraph number 2 is used, the stop signal length is 1.5 code elements. When the international telegraph number 5 (see data communication code) or other codes is used, the stop signal length is 1 or 2 code element, the polarity is "1". Characters can be sent continuously or individually; when no characters are sent, a stop signal is sent continuously. The starting moment of each character can be arbitrary (this is also the meaning of asynchronous transmission), but the length of each code element within the same character is equal. The receiving end detects and recognizes the "start" signal of a new character based on the transition from the stop signal to the start signal between characters ("1" → "0"), thereby correctly distinguishing each character. Therefore, this character synchronization method is also called start-stop synchronization. The advantage of this method is that it is relatively simple to achieve synchronization, and the clock signals of both sending and receiving parties do not need to be accurately synchronized. The disadvantage is that each character adds 2 to 3 bits, which reduces the transmission efficiency. It is often used for low-speed data transmission of 1200bit/s and below.
Synchronous transmission sends data signals with a fixed clock beat. In the serial data stream, the relative positions between each signal symbol are fixed. To correctly distinguish the sent characters from the received data stream, the receiving end must establish bit timing synchronization and frame synchronization. Bit timing synchronization is also called bit synchronization. Its function is 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 numbers from the received information stream. element to generate a receive data sequence. There are two methods for the DCE transmitter to generate timing: one is to generate bit timing in the data terminal equipment (DTE), and send the DTE data to the DCE at this timing beat. This method is called external synchronization. The other is to use the internal bit timing of the DCE to extract the DTE side data. This method is called internal synchronization. For the receiving end of the DCE, the received data is sent to the DTE based on the bit timing beat within the DCE. Frame synchronization is to correctly group or frame the received data sequence in order to correctly distinguish individual characters or other information. The advantage of the synchronous transmission method is that there is no need to add and stop code elements for each character separately, so the transmission efficiency is high. The disadvantage is that the implementation technology is more complex. Usually used for data transmission with a rate of 2400bit/s and above.
Classification by relationship
According to the flow direction and time relationship of data transmission, data transmission methods can be divided into simplex, half-duplex and full-duplex. Duplex data transmission.
Simplex data transmission means that data can only be transmitted in one specified direction between two data stations. 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 means that data can be transmitted in two directions between two data stations, but not at the same time. That is, the DTE at each end can be used as a data source or a data sink, but it cannot be used as a data source and data sink at the same time.
Full-duplex data transmission is between two data stations and can be transmitted in both directions simultaneously. That is, the DTE at each end can serve as a data source and data sink at the same time. Typically four-wire lines implement full-duplex data transmission. The second line realizes simplex or half-duplex data transmission. When using technologies such as frequency reuse, time division multiplexing or echo cancellation, the second-line line can also achieve full-duplex data transmission
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