Data encoding technology
Source: Shangpin China |
Type: website encyclopedia |
Time: June 20, 2014
In data communication, according to different communication media, the signals transmitted on the channel can be divided into digital signals and analog signals. At the sending end, the digital data needs to be encoded by the encoder first. The encoder converts the digital data into digital signals that can be transmitted on the digital channel. If it is transmitted on the analog channel, the modulator modulates the digital signal into analog signals that can be transmitted on the analog channel. At the receiving end, the reverse operation is performed, that is, analog signal demodulation (demodulator) and digital signal decoding (decoder), and finally the original digital data is restored. In a word, the digital data generated by the source is not transmitted directly, and needs to be processed through coding and other processing processes. 1. Coding technology Coding technology, that is, to transform the number in some ways to get another group of numbers suitable for transmission, or to monitor the original number with other numbers to ensure that it will not be misjudged in the transmission process. In coding technology, coding types can be as follows: (1) Digital data to digital signal coding: such as Website construction For communication, the computer is directly connected with the communication medium coaxial cable or double ridge line through the network card. Its channel is a digital channel. The network card has the function of digital data encoding to generate digital signals. (2) Analog data to digital signal coding: if voice chat data is transmitted in network applications, the generated voice analog data needs to be sampled and processed by the encoder/decoder to form digital data, which is then converted into digital signals for transmission. (3) Digital data analog signal coding: For example, if the application of telephone dial-up networking is used, the digital data generated by the computer is transmitted through the telephone network of the analog transmission system, and both ends need a modulator/demodulator to modulate digital signals into analog signals or demodulate analog signals into digital signals. (4) Analog data to analog signal coding: This coding technology is generally used in telephone communication systems. No matter what kind of coding technology, digital transmission technology is the basis of data communication in today's digital network. Therefore, the content to be introduced in this section is mainly about the common digital coding technology of "digital data~digital signal" in data communication (for the digital signal coding technology of analog data, please refer to the description of Question 2-1 in Chapter 11). 2. Why does binary coding adopt "coding technology" In the computer system, although the storage form of data has been binary digital coding, in point-to-point data transmission, how to ensure the synchronization of transmission and reception at both ends of the communication, how to ensure that the binary coding on the read channel is correct, and how to directly convert the original digital data into digital signals cannot be solved, However, it is an effective way to redefine the original digital data with coding technology. There are three basic problems to be solved in digital coding technology: (1) In data communication, the problem of synchronous control between sending data at the sending end and receiving data at the receiving end. In order to achieve synchronous control of data transmission at both ends, synchronous control information needs to be carried in the transmitted binary digital data. (2) The channel communication capacity is limited. High quality coding technology is needed to improve the transmission rate of binary digital data. (3) Channel anti-interference capability. Which coding technology is used to reduce the channel error rate. The essence of adopting data coding technology is to process the original digital coding and add synchronous information, so that the transmission is faster and error free. 3. Common digital coding technology The commonly used digital coding technology is shown in Figure 2-3. 1) Non Return to Zero Coding (NRZ) Non return to zero coding is to use different level signals to represent the 0 or 1 of the binary code, and this level signal should cover the width of the whole symbol, with no return to zero in the middle. For example,+5V represents 1, OV represents 0, or ten 5V represents 1, and one 5V represents 0. Note: symbol is the basic unit of digital signal. In digital communication, it is often used to transmit a binary digit at an equal time interval. This equal time interval signal is called a binary symbol. The main disadvantage of non return to zero coding is that it occurs continuously in the data stream. Or 1, it is not easy for the receiving end to distinguish when each bit signal starts and ends, that is, it is impossible to read the correct bit string from the high and low level rectangular wave. For example, if the sending end sends 11011000 rectangular waves, if the transmission bit duration is shortened by half, the receiving end will read 111100111100000. The bit string of. In order to ensure correct data transmission, another channel must be used to simultaneously transmit clock synchronization signal while transmitting NRZ code, as shown in the upper end of Figure 2-3. When the number of 0 and 1 in the signal is uneven, this coding will produce the accumulation of DC components, resulting in signal distortion and distortion, reducing the reliability of transmission. Therefore, this coding is rarely used in LAN transmission. 2) Manchester Coding Manchester code has its own synchronization signal. In Manchester coding, the duration of each bit is divided into two halves. When bit 0 is transmitted, the signal bit middle level jumps from low to high; When bit 1 is transmitted, the signal bit middle level jumps from high to low. Since the voltage changes in the middle of each symbol, that is, there must be a level jump in the duration of each bit transmission, the receiver can easily use it as a synchronous clock. In addition, Manchester code does not contain DC component, but its coding efficiency is low. Therefore, this coding is only used for data transmission in 10Mbps LAN. 3) Differential Manchester Coding The differential Manchester coding is improved by the Chester coding, that is, the signal polarity (no jump) is not changed at the beginning of the signal bit to indicate 1; Changing the signal polarity (with jump) at the beginning of the signal bit means 0. The coding principle of the subject Chester and the differential Manchester is basically the same. Their common feature is that there is a bit synchronization clock in every bit of transmission, which has self synchronization capability and good anti-interference performance, and makes up for the defect of non return to zero. The difference between the two is that the differential is subject to Chester coding, and there is a jump in the middle of each clock bit, which is specially used for synchronization control. The transmission is 1 or 1. It is distinguished by whether there is jump at the beginning of each clock bit. Therefore, the differential Manchester code has relatively less change than the received Manchester code, and 16Mbps token ring network uses this differential Manchester code. However, because each bit of these two codes is converted into two levels, the efficiency of these two codes can only reach about 50%, which is not suitable for high-speed networks. 4) DNRZ code (Differential NRZ) DNRZ code is an improved form of NRZ code. It also uses signal polarity changes to represent 1 and 0, and a jump at the beginning of a bit indicates 1, while no jump indicates.. DNRZ code not only keeps the advantages of NRZ code, but also improves the anti-interference and easy synchronization of the signal. Moreover, the symbol rate in DNRZ coding is consistent with the coding clock rate, which has high coding efficiency and meets the requirements of high-speed network for signal coding. 5) 4B/5B code and 8B/10B code With the rapid development of network application technology, especially LAN technology, the application of optical fiber technology in Fast Ethernet and Gigabit Ethernet has become mature. Therefore, another encoding technology, 4B/5B encoding or 8B/10B encoding, is more used for data transmission in optical fiber media. 4B/5B coding: This coding technology is characterized by taking every 4 bits of the bit stream to be sent as a group, and then converting them into corresponding 5 bit codes according to the 4B/5B coding rules. The converted symbols can maintain the AC balance of the line and minimize the waveform spectrum in transmission. For example, FDDI, 10OBase TX and 10OBase FX LAN use this encoding technology. 8B/10B coding: It is to divide a group of consecutive 8-bit bit streams into two groups, one group of 3-bit and the other group of 5-bit. After coding, they become a group of 4-bit code and a group of 6-bit code respectively, thus forming a group of 10 bit data to send out. BB/10B coding mode is adopted in Gigabit Ethernet.
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