4B-5B Coding principle

4b/5b Coding Principle

What is 4b/5b encoding?

4b/5b Encoding is one of the PHY layer encoding types in Gigabit Ethernet and Industrial Ethernet , which is the 5bit binary number to represent 4bit Binary number

Why do I need to encode 4b/5b ?

In the communication network, the receiving end needs to recover the clock information from the received data to ensure synchronization, which requires the transmission of the line in the binary stream has enough to jump , that is, there can not be too many consecutive high or low level, otherwise unable to extract clock information .

Manchester(Manchester) encoding ensures that the code flow in the line has a full jump because it is electrically flat from "-1" to "+1" of the jump to indicate "1" , Electric leveling from" +1" to "1" jumps to indicate " 0" , but the efficiency of this coding method is too low, only 50% , equivalent to the effective bandwidth of the circuit in exchange for signal jumps, 10 Gigabit Ethernet is usedManchesterEncoding, although the effective bandwidth of the line is only10Mbps, but the actual bandwidth is20Mbps。

The combination of 4b/5b encoding and MLT-3 encoding for Gigabit Ethernet, the transmission code stream is 4b/5b encoded, then MLT-3 encoded, and finally on the line transfer; Gigabit Ethernet is 8b/10b encoding and NRZ encoding combination, Gigabit Ethernet is 64b/66b encoded, andPCIE 3.0 is 128b/130b encoded.

What are the 4b/5b coding rules?

4b/5b encoding is actually using 5bit binary code to represent 4bit binary code. The efficiency of this encoding is 80% , than Manchester code is high. 4b/5b The purpose of the encoding has been mentioned earlier, which is to let the bitstream generate enough jumps. 4 bits binary total 16 combination , 5 bits binary total 32 combinations, how to choose >32 combinations >16 What about the species to use? There are two rules that need to be met:

1). each 5 -bit code group does not contain more than 3 "0";

2). or a 5 bit code group contains not less than 2 "1";

How did this rule come about? This is going to be explained in terms of the characteristics of MLT-3 yards. MLT -3 yards of the characteristics of the simple saying is: every "1" Jump change, every "0" does not jump change. In order to make the 4b/5b encoded in the code stream there are enough jumps in the code stream need to have as many as possible "1" and as little as possible "0".

The feature of this encoding is to send the data stream every 4bit as a group, and then follow the 4B/5B encoding rules to convert it to the corresponding 5bit code. There are 32 combinations of 5bit codes, but only 16 of them correspond to 16 of 4bit codes, the other 16 are not used or used as control codes to indicate the start and end of frames, the status of fiber optic lines (stationary, idle, pause) and so on. Three applications are similar to the concept of FDDI, 100base-tx, and 100BASE-fx8b/10b encoding and 4b/5b, for example, in the case of Gigabit Ethernet, the 8B/10B encoding method is used.

The 4B/5B encoding is an information encoding scheme used in the optical Distributed Data Interface (Fddi,fiber distributed-Interface) of the Gigabit Fast Ethernet. This encoding is characterized by the data stream to be sent every 4bit as a group, each four-bit binary code is represented by 5-bit encoding, this 5-bit encoding is called the code group, and is transmitted by NRZI mode.

The 4B/5B encoding is an information encoding scheme used in the optical Distributed Data Interface (Fddi,fiber distributed-Interface) of the Gigabit Fast Ethernet. This encoding is characterized by the data stream to be sent every 4bit as a group, each four-bit binary code is represented by 5-bit encoding, this 5-bit encoding is called the code group, and is transmitted by NRZI mode.

In the communication system, the communication speed and the modulation rate in the transmission line, the so-called modulation rate refers to the number of line state changes in unit time, in baud (baud) units. The most important advantage of practical 4b/5b code is that it can reduce the modulation rate in the transmission line and reduce the requirement of the circuit.

For example, if Manchester code is used, the data transfer rate is one-second of the baud rate if it beats two times within each modulation interval. In Fast Ethernet, the data transfer rate is 100Mbps, and if you use Manchester code, the baud rate will

Up to 200Mbps, the technical requirements for the transmission media and equipment will be increased, increasing the transmission costs. If 4B/5B encoding is used, at a transfer rate of 100Mbps, the modulation rate is: 100m÷ (4/5) =125m (baud). That is, the baud rate is 125M baud, significantly lower than the Manchester code 200M baud, which makes it possible to use unshielded twisted pair in Fast Ethernet .

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4B-5B Coding principle