6b/8b encoding

In telecommunications, 6b/8b is a line code that expands 6-bit codes to 8-bit symbols for the purposes of maintaining DC-balance in a communications system.

Each 8-bit output symbol contains 4 zero bits and 4 one bits, so the code can, like a parity bit, detect all single-bit errors.

The number of binomial coefficient 8-bit patterns with 4 bits set is ${\displaystyle {\tbinom {8}{4}}}$ = 70. Further excluding the patterns `11110000` and `00001111`, this allows 68 coded patterns: 64 data codes, plus 4 additional control codes.

Coding rules

The 64 possible 6-bit input codes can be classified according to their disparity, the number of 1 bits minus the number of 0 bits:

Ones Zeros Disparity Number
0 6 −6 1
1 5 −4 6
2 4 −2 15
3 3 0 20
4 2 +2 15
5 1 +4 6
6 0 +6 1

The 6-bit input codes are mapped to 8-bit output symbols as follows:

• The 20 6-bit codes with disparity 0 are prefixed with `10`
Example: 000111 → 10000111
Example: 101010 → 10101010
• The 14 6-bit codes with disparity +2, other than `001111`, are prefixed with `00`
Example: 010111 → 00010111
• The 14 6-bit codes with disparity −2, other than `110000`, are prefixed with `11`
Example: 101000 → 11101000
• The remaining 20 codes: 12 with disparity ±4, 2 with disparity ±6, `001111`, `110000`, and the 4 control codes, are assigned to codes beginning with `01` as follows:
Type Input Output Type Input Output Complement −6 `000000` `01011001` +6 `111111` `01100110` 01_xx__x −4 `000001` `01110001` +4 `111110` `01001110` 01xx____ `000010` `01110010` `111101` `01001101` `000100` `01100101` `111011` `01011010` 01x____x `001000` `01101001` `110111` `01010110` `010000` `01010011` `101111` `01101100` 01_____xx `100000` `01100011` `011111` `01011100` −2 `110000` `01110100` +2 `001111` `01001011` 01____x__ Control `K 000111` `01000111` Control `K 111000` `01111000` `K 010101` `01010101` `K 101010` `01101010`

Obviously, no data symbol contains more than four consecutive matching bits, and because the patterns `11110000` and `00001111` are excluded, no data symbol begins or ends with more than three identical bits. Thus, the longest run of identical bits that will be produced is 6. (I.e. this is a (0,5) RLL code, with a worst-case running disparity of +3 to −3.)

Any occurrence of 6 consecutive identical bits constitutes a comma sequence or sync mark or syncword; it identifies the symbol boundaries precisely. Those 6 bits straddle the inter-symbol boundary with exactly 3 of those identical bits at the end of one symbol, and 3 of those identical bits at the start of the following next symbol.