4B3T

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4B3T stands for 4 (four) Binary 3 (three) Ternary. It's a line encoding scheme used for ISDN BRI interface. It represents four binary bits using three pulses. It uses three states viz. + (positive pulse), no pulse - 0, - (negative pulse). This means we have 16 (2⁴) input combinations to represent using 27 output combinations. 000 is not used to avoid long periods without a transition. To avoid DC buildup on the physical medium inputs are either mapped to a single combination with no net DC (one + and one -) or to a pair of combinations with opposite net DC (e.g. ++- and --+). When transmitting the DC buildup is tracked and a combination chosen that has a DC component of the opposite sign to the running total.

This mapping from 4 bits to 3 ternary states is given in a table known as Modified Monitoring State 43 (MMS43). A competing encoding technique, used for the ISDN basic rate interface where 4B3T is not used, is 2B1Q.