2B1Q


Two-binary, one-quaternary is a line code used in the U interface of the Integrated Services Digital Network Basic Rate Interface and the high-bit-rate digital subscriber line. 2B1Q is a four-level pulse amplitude modulation scheme without redundancy, mapping two bits into one quaternary symbol. A competing encoding technique in the ISDN basic rate U interface, mainly used in Europe, is 4B3T.
To minimize error propagation, bit pairs are assigned to voltage levels according to a Gray code, as follows:
DibitSignal 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.