The 4B5B code is a term used in telecommunications, refers to a line code that maps clearly reversible four to five data bits of code bits. Areas of application of this code are for Fast Ethernet 100BASE -TX in combination with MLT -3 line coding for copper cables and NRZI line coding at the Fiber Distributed Data Interface ( FDDI). Other applications include the Multichannel Audio Digital Interface ( MADI ) according to the standard AES 10th

Due to the insertion of additional bits, the coded bit rate is increased by a factor of 5/4 with respect to the Nutzdatenbitrate.

4B5B table

Data symbols are denoted by 0 to 9 and A.. F, subsequently control codes.

In the figure that follows this table long "0" - or "1 " episodes avoided, which could complicate the clock recovery. It may for this purpose in the conversion table in the data words ( 0 to 9 and A.. F) be no more than a leading "0" and no more than 2 final "0" with the tax code, this rule is by a combination secured by two control words. Thus, the 4B5B code is equal to a (0,3) RLL code.

The control codes are grouped in pairs according to the following table. For Fast Ethernet, 100BASE -TX, a data frame starts for example with a JK - pair. The data frame ends at FDDI with a TT pair, with 100BASE- TX with a TR pair.