Editing 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).<ref>{{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
}}</ref> 2B1Q is a four-level [[pulse-amplitude modulation]] (PAM-4) scheme without redundancy, mapping two [[binary numeral system|bits]] (2B) into one [[quaternary numeral system|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 ([[dibit]]s) 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]]. 

==References==
{{Reflist}}

{{Bit-encoding}}

[[Category:Line codes]]
[[Category:Integrated Services Digital Network]]


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