Two-binary, one-quaternary (2B1Q) is a

line code In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained c ...

used in the

U interface U or u, is the twenty-first and sixth-to-last letter and fifth vowel letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''u'' (pr ...

of the

Integrated Services Digital Network Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. W ...

(ISDN)

Basic Rate Interface Basic Rate Interface (BRI, 2B+D, 2B1D) or Basic Rate Access is an Integrated Services Digital Network (ISDN) configuration intended primarily for use in subscriber lines similar to those that have long been used for voice-grade telephone service ...

(BRI) and the high-bit-rate digital subscriber line (HDSL). 2B1Q is a four-level

pulse-amplitude modulation Pulse-amplitude modulation (PAM) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulse ...

(PAM-4) scheme without redundancy, mapping two bits (2B) into one

quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

symbol (1Q). A competing encoding technique in the ISDN basic rate U interface, mainly used in Europe, is

4B3T 4B3T, which stands for 4 (four) binary 3 (three) ternary, is a line encoding scheme used for ISDN PRI interface. 4B3T represents four binary bits using three pulses. Description It uses three states: * + (positive pulse), * 0 (no pulse), and * � ...

. To minimize

error propagation In statistics, propagation of uncertainty (or propagation of error) is the effect of variables' uncertainties (or errors, more specifically random errors) on the uncertainty of a function based on them. When the variables are the values of exp ...

, bit pairs ( dibits) are assigned to voltage levels according to a

Gray code The reflected binary code (RBC), also known as reflected binary (RB) or Gray code after Frank Gray, is an ordering of the binary numeral system such that two successive values differ in only one bit (binary digit). For example, the representati ...

, as follows: 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 In signal processing, when describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC ...

. Symbol rate is half of data rate.

References

Line codes Integrated Services Digital Network {{telecomm-stub