2B1Q

Two-binary, one-quaternary (2B1Q) is a physical layer encoding used in the U interface of the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI). 2B1Q uses four signal levels, mapping two bits (2B) into one quaternary symbol (1Q). A competing encoding technique, also used for ISDN basic rate U interface, is 4B3T.

To minimize error propagation, bit pairs (dibits) are assigned to voltage levels according to a Gray code, as follows:

Dibit	Signal level
10	+450 mV
11	+150 mV
01	−150 mV
00	−450 mV

If the voltage is misread as an adjacent level, this will only cause a 1-bit error in the decoded data. 2B1Q is also used for some variants of HDSL. 2B1Q code is not DC-balanced. Symbol rate is half of data rate.

This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.