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 ...

used in the U interface 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. Work ...

(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 servi ...

(BRI) and the

high-bit-rate digital subscriber line High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum over copper, twisted pair cables. HDSL was developed to ...

(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 year ...

symbol (1Q). A competing encoding technique in the ISDN basic rate U interface, mainly used in Europe, is 4B3T. 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 expe ...

, bit pairs (

dibit In computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels. In information theory, units of information a ...

s) 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 representa ...

, 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. Symbol rate is half of data rate.

References

Line codes Integrated Services Digital Network {{telecomm-stub