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. Symbol rate is half of data rate.
A competing encoding technique in the ISDN basic rate U interface, mainly used in Europe, is 4B3T.
Encoding
To minimize error propagation, bit pairs 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.