(1) AMI code
AMI(Alternative Mark Inversion) code is the full name of alternate mark inversion code, its encoding rule is to alternately transform the message code “1″ (mark) to “+1″ and “-1″, while the “0″ (empty sign) remains unchanged. For example:
Message code: 0 1 1 0 0 0 0 0 0 0 0 1 1 0 0 1 1
AMI code: 0-1 +1 0 0 0 0 0 0 0 0 0 0 0 1 +1 0 0 0 0 1 +1
The waveform corresponding to AMI code is a pulse train with positive, negative and zero levels. It can be seen as a unipolar waveform deformation, that is, “0″ still corresponds to zero levels, and “1″ alternately corresponds to positive and negative levels.
The advantage of AMI code is that there is no DC component, and the high and low frequency components are small, and the energy is concentrated at the frequency of 1/2 yard speed
(Figure 6-4); The codec circuit is simple, and the error of code can be observed by using the rule of alternating polarity of signal. If it is an AMI-RZ waveform, after receiving it, as long as the full wave rectification, it can be changed into a unipolar RZ waveform, from which the bit timing component can be extracted. In view of the above advantages, AMI code has become one of the most commonly used transmission codes.
Disadvantages of AMI code: When the original code has a long “0″ string, the level of the signal does not jump for a long time, resulting in difficulty in extracting the timing signal. One of the effective ways to solve the problem of “0″ code is to use HDB3 code.
(2) HDB3 code
The full name of HDB3 code is third-order high-density bipolar code. It is an improved version of AMI code, the purpose of improvement is to maintain the advantages of AMI code and overcome its shortcomings, so that the number of “0″ does not exceed three. Its encoding rules are as follows:
Check the number of zeros connected to the message code. When the number of “0″ is less than or equal to 3, the coding rule is the same as that of the AMI code. When the number of consecutive zeros exceeds three, each of the four consecutive zeros is turned into a subsection and replaced by 000V. V(taking the value +1 or -1) should have the same polarity as the previous adjacent non-” 0 “pulse (because this breaks the rule of polarity alternation, V is called the destruction pulse). Adjacent V-code polarities must alternate. When the value of V code can meet the requirements in (2) but cannot meet this requirement, “0000″ is replaced by “B00V”. The value of B is the same as the following V pulse to solve this problem. Therefore, B is called the regulating pulse. The polarity of the number transmission after the V code should also alternate.
In addition to the advantages of AMI code, HDB3 code also limits the number of even “0″ code to 3, so that the timing information can be extracted when receiving. Therefore, HDB3 code is the most widely used code type in China and Europe and other countries, and the interface code type of law A PCM below four groups are HDB3 code.
In the above AMI code and HDB3 code, each binary signal code is converted into a one-bit three-level value (+1, 0,-1) code, so this type of code is also called 1B1T code. In addition, the HDBn code can be designed so that the number of “0″ does not exceed n.
(3) biphase code
Biphasic code is also known as Manchester code. It uses the positive and negative symmetric square waves of one period to represent “0″ and its inverting waveform to represent “1″. One of the coding rules is that the “0″ code is represented by the “01″ two-digit code, and the “1″ code is represented by the “10″ two-digit code, for example:
Message code: 1 1 0 0 0 1 0 1
Biphase code: 10 10 01 01 10 01 10
A bipolar code waveform is a bipolar NRZ waveform with only two levels of opposite polarity. It has a level jump in the center point of each symbol interval, so it contains rich bit timing information, and there is no DC component, and the coding process is simple. The disadvantage is that the occupied bandwidth is doubled, so that the frequency band utilization is reduced. Biphase code is suitable for short-range transmission of data terminal equipment, and it is often used as transmission code type in local area network.
(4) Differential biphase code
In order to solve the decoding errors caused by polarity reversal in biphasic codes, the concept of differential codes can be adopted. Biphasic codes are synchronized and represented by a level jump in the middle of each symbol’s duration (a jump from negative to positive represents a binary “0″ and a jump from positive to negative represents a binary “1″). In differential biphase coding, the level jump in the middle of each element is used for synchronization, and whether there is an additional jump at the beginning of each element is used to determine the signal code. If there is a jump, it indicates a binary “1″, and if there is no jump, it indicates a binary “0″. This code is often used in local area networks.
(5)CMI code
CMI code is short for mark reversal code, and similar to bipolar code, it is also a bipolar bipolar flat code. Its coding rules are: “1″ code is alternately represented by “11″ and “00″ two-digit codes; The 0 code is represented by 01, and its waveform is shown in Figure 6-5(c).
CMI code is easy to implement and contains rich timing information. In addition, since 10 is a disabled code group, more than three codes will not appear, and this rule can be used for macro error detection. This code has been recommended by ITU-T as the PCM quad-group interface code type, and is sometimes used in optical cable transmission systems with rates below 8.448Mb /s.
(6)Block coding
In order to improve the performance of line coding, some kind of redundancy is needed to ensure the synchronization and error detection ability of code patterns. The introduction of block coding can achieve both purposes to some extent. The form of block coding has nBmB code, nBmT code and so on.
nBmB code is a kind of block coding, which divides the n-bit binary code of the original information stream into a group, and replaces it into a new code group of M-bit binary code, where m>n. Because m>n, the new code set may have 2^m combinations, so there are more (2^m-2^n) combinations. In the 2 “combination, the favorable code group is selected as the allowed code group in some way, and the rest is used as the disabled code group to obtain good coding performance. For example, in a 4B5B encoding, replacing a 4-bit encoding with a 5-bit encoding, there are only 2^4=16 different combinations for a 4-bit grouping, and 2^5=32 different combinations for a 5-bit grouping. In order to achieve synchronization, we can select code groups in the manner of no more than one leading “0″ and two suffixes “0″, and the rest are disabled code groups. In this way, if there is a disabled code set at the receiving end, it indicates that there is a code error in the transmission process, thus improving the error detection ability of the system. The biphase codes and CMI codes described earlier can both be regarded as 1B2B codes.
In the optical fiber communication system, m=n+1 is often selected, and 1B2B code, 2B3B code, 3B4B code and 5B6B code are taken. Among them, the 5B6B code has been used in practice as a line transmission code for cubic groups and more than quadruple groups.
The nBmB code provides good synchronization and error detection, but it comes at a cost, that is, the required bandwidth increases.
The design idea of nBmT code is to convert n binary codes into m ternary codes, and m<n. For example, the 4B3T code, which converts four binary codes into three ternary codes. Obviously, at the same bit rate, the information capacity of 4B3T code is greater than that of 1B1T code, so the frequency band utilization can be improved. The 4B3T code and 8B6T code are suitable for higher rate data transmission systems, such as high-order group coaxial cable transmission systems.
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