4B3T

4B3T stands for 4 (four) Binary 3 (three) Ternary, a line encoding scheme used for ISDN BRI interface. 4B3T 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.

ee also

Other line codes that have 3 states:
* hybrid ternary code
* bipolar encoding
* MLT-3 encoding
* B3ZS