8b/10b Encoding - How IT Works For The IBM Code

How It Works For The IBM Code

As the scheme name suggests, 8 bits of data are transmitted as a 10-bit entity called a symbol, or character. The low 5 bit of data are encoded into a 6-bit group (the 5b/6b portion) and the top 3 bits are encoded into a 4-bit group (the 3b/4b portion). These code groups are concatenated together to form the 10-bit symbol that is transmitted on the wire. The data symbols are often referred to as D.x.y where x ranges over 0–31 and y over 0–7. Standards using the 8b/10b encoding also define up to 12 special symbols (or control characters) that can be sent in place of a data symbol. They are often used to indicate start-of-frame, end-of-frame, link idle, skip and similar link-level conditions. At least one of them (i.e. a "comma" symbol) needs to be used to define the alignment of the 10 bit symbols. They are referred to as K.x.y and have different encodings from any of the D.x.y symbols.

Because 8b/10b encoding uses 10-bit symbols to encode 8-bit words, some of the possible 1024 (10 bit, 210) codes can be excluded to grant a run-length limit of 5 consecutive equal bits and grant that the difference of the count of 0s and 1s is no more than 2. Some of the 256 possible 8-bit words can be encoded in two different ways. Using these alternative encodings, the scheme is able to effect long-term DC-balance in the serial data stream. This permits the data stream to be transmitted through a channel with a high-pass characteristic, for example Ethernet's transformer-coupled unshielded twisted pair or optical receivers using automatic gain control.