Network, Networking Technology, Data Communication Terms, Glossary and Dictionary

4B/5B or 4B5B

4B/5B, also called 4-byte/5-byte or 4B5B, is a block encoding scheme used to send Fast Ethernet data and for MADI (Multichannel Audio Digital Interface). 4B5B works by mapping groups of four bits onto groups of 5 bits. Since there are 32 possible combinations of 5 bits and only 16 combinations of 4 bits the 16 groups of 5 bits with the most transitions in are used in order to provide as many transitions as possible. This is done as the transitions provide clocking information for the signal. Despite this 4B5B does not guarantee at least one transition for each bit period, however there are enough transistions to allow the clock signal to be recovered.

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4B/5B or 4B5B