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Two-binary, one-quaternary (2B1Q) is a line code used in the U interface of the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI). 2B1Q is a four-level pulse amplitude modulation (PAM-4) scheme, mapping two bits (2B) into one quaternary symbol (1Q). A competing encoding technique in the ISDN basic rate U interface, mainly used in Europe, 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.