Editing 8b/10b encoding (section)

====Running disparity====
8b/10b coding is DC-free, meaning that the long-term ratio of ones and zeros transmitted is exactly 50%.  To achieve this, the difference between the number of ones transmitted and the number of zeros transmitted is always limited to ±2, and at the end of each symbol, it is either +1 or −1.  This difference is known as the ''running disparity'' (RD).

This scheme needs only two states for the running disparity of +1 and −1. It starts at −1.<ref>{{cite web |title=Thoughts on Gigabit Ethernet Physical |date=1996-04-01 |url=https://grouper.ieee.org/groups/802/3/z/public/presentations/mar1996/JTtgep.txt|author=Thatcher, Jonathan|work=IBM|access-date=2008-08-17}}</ref>

For each 5b/6b and 3b/4b code with an unequal number of ones and zeros, there are two bit patterns that can be used to transmit it: one with two more "1" bits, and one with all bits inverted and thus two more zeros.  Depending on the current running disparity of the signal, the encoding engine selects which of the two possible six- or four-bit sequences to send for the given data. Obviously, if the six-bit or four-bit code has equal numbers of ones and zeros, there is no choice to make, as the disparity would be unchanged, with the exceptions of sub-blocks D.07 (00111) and D.x.3 (011). In either case the disparity is still unchanged, but if RD is positive when D.07 is encountered 000111 is used, and if it is negative 111000 is used.  Likewise, if RD is positive when D.x.3 is encountered 0011 is used, and if it is negative 1100 is used.  This is accurately reflected in the charts below, but is worth making additional mention of as these are the only two sub-blocks with equal numbers of 1s and 0s that each have two possible encodings.

{| class="wikitable" style="text-align:center"
|+ Rules for running disparity
|-
! previous<br>RD !! Disparity of<br>code word !! Disparity<br>chosen !! next<br>RD
|-
| −1 ||  0 ||  0 || −1
|-
| −1 || ±2       || +2       || +1
|-
| +1 ||  0 ||  0 || +1
|-
| +1 || ±2       || −2       || −1
|}