2B1Q

Two-binary, one-quaternary (2B1Q) is a physical layer encoding used for Integrated Services Digital Network (ISDN) basic rate interface. 2B1Q uses four signal levels, which are −450 mV, −150 mV, 150 mV and 450 mV, each (1Q) equivalent to two bits (2B). A competing encoding technique, also used for ISDN basic rate interfaces, is 4B3T.

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 will only cause a 1-bit error in the decoded data. 2B1Q is also used for some variants of HDSL.

Note that the 2B1Q code is not DC-balanced.

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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