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In telecommunications, '''6b/8b''' is a line code that expands 6-[[bit]] codes to 8-bit symbols for the purposes of maintaining [[DC bias|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 <math>\tbinom 84</math> = 70.  Further excluding the patterns <code>11110000</code> and <code>00001111</code>, 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:

{|class=wikitable style="text-align:center"
! 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 <code>10</code><br/>''Example: 000111 → '''10'''000111''<br/>''Example: 101010 → '''10'''101010''
* The 14 6-bit codes with disparity +2, other than <code>001111</code>, are prefixed with <code>00</code><br/>''Example: 010111 → '''00'''010111''
* The 14 6-bit codes with disparity −2, other than <code>110000</code>, are prefixed with <code>11</code><br/>''Example: 101000 → '''11'''101000''
* The remaining 20 codes: 12 with disparity ±4, 2 with disparity ±6, <code>001111</code>, <code>110000</code>, and the 4 control codes, are assigned to codes beginning with <code>01</code> as follows:
{|class=wikitable style="text-align:center"
! Type !! Input !! Output
|rowspan=11|
! Type !! Input !! Output
|rowspan=11|
! Complement
|-
| −6 || <code>000000</code> || <code>010'''11'''00'''1'''</code>
| +6 || <code>111111</code> || <code>011'''00'''11'''0'''</code>
| 01_xx__x
|-
|rowspan=6| −4
| <code>000001</code> || <code>01'''11'''0001</code> 
|rowspan=6| +4
| <code>111110</code> || <code>01'''00'''1110</code>
|rowspan=2| 01xx____
|-
| <code>000010</code> || <code>01'''11'''0010</code> 
| <code>111101</code> || <code>01'''00'''1101</code>
|-
| <code>000100</code> || <code>01'''1'''0010'''1'''</code> 
| <code>111011</code> || <code>01'''0'''1101'''0'''</code>
|rowspan=2| 01x____x
|-
| <code>001000</code> || <code>01'''1'''0100'''1'''</code> 
| <code>110111</code> || <code>01'''0'''1011'''0'''</code>
|-
| <code>010000</code> || <code>010100'''11'''</code> 
| <code>101111</code> || <code>011011'''00'''</code>
|rowspan=2| 01_____xx
|-
| <code>100000</code> || <code>011000'''11'''</code> 
| <code>011111</code> || <code>010111'''00'''</code>
|-
| −2 || <code>110000</code> || <code>01110'''1'''00</code>
| +2 || <code>001111</code> || <code>01001'''0'''11</code>
| 01____x__
|-
|rowspan=2|Control
| <code>K 000111</code> || <code>01000111</code>  
|rowspan=2|Control
| <code>K 111000</code> || <code>01111000</code>
|rowspan=2|
|-
| <code>K 010101</code> || <code>01010101</code>
| <code>K 101010</code> || <code>01101010</code>
|}

Because we excluded the patterns <code>11110000</code> and <code>00001111</code>,

Note that no data symbol contains more than four consecutive matching bits, or begins or ends with more than three identical.
Thus, the longest run of identical bits that will be produced is 6.  (I.e. this is a (0,5) [[Run length limited|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.

== See also ==
* [[8b/10b encoding]], another fixed-table system with a higher [[code rate]] but less error detection.
* [[64b/66b encoding]], linear feedback register.

== External links ==
* Freepatents online, United States patent 6,876,315: [http://www.freepatentsonline.com/6876315.html DC-balanced 6B/8B transmission code with local parity].

{{Bit-encoding}}

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[[Category:Line codes]]


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