2B1Q
{{Short description|Line code}}
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).{{cite book
|last= Reeve
|first=Whitham D.
|title=Subscriber Loop Signaling and Transmission Handbook
|series=IEEE Telecommunications Handbook Series
|publisher=IEEE Press
|year=1995
|isbn=0780304403
|url-access=registration
|url=https://archive.org/details/subscriberloopsi00reev
}} 2B1Q is a four-level pulse-amplitude modulation (PAM-4) scheme without redundancy, mapping two bits (2B) into one quaternary symbol (1Q). 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 (dibits) are assigned to voltage levels according to a Gray code, as follows:
| class=wikitable
! 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.