To minimize error propagation, bit pairs (dibits) are assigned to voltage levels according to a Gray code, as follows:
If the voltage is misread as an adjacent level, this causes only a 1-bit error in the decoded data. 2B1Q code is not DC-balanced. Symbol rate is half of data rate.
References
This article is based on material taken from the Free On-line Dictionary of Computingprior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.