2B1Q

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) and the high-bit-rate digital subscriber line (HDSL).[1] 2B1Q is a four-level pulse amplitude modulation (PAM-4) scheme without redundancy, 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 causes only a 1-bit error in the decoded data. 2B1Q code is not DC-balanced. Symbol rate is half of data rate.

References

1. Whitham D. Reeve (1995). Subscriber Loop Signaling and Transmission Handbook. IEEE Telecommunications Handbook Series. IEEE Press. ISBN 0780304403.

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.