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'' (pro ...

of the Integrated Services Digital Network (ISDN) Basic Rate Interface (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 (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, 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 ar ...

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 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 D ...

. Symbol rate is half of data rate.

References

Line codes Integrated Services Digital Network {{telecomm-stub