High Quality Content by WIKIPEDIA articles! 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: • + (positive pulse), • 0 (no pulse), and • − (negative pulse). This means we have 24 = 16 input combinations to represent, using 33 = 27 output combinations. 000 is not used to avoid long periods without a transition. To achieve an overall zero DC bias, 6 triplets are used which have no DC component (+0−, +−0, 0+−, 0−+, −+0, −0+), and the remaining 20 are grouped into 10 pairs with differing disparity (e.g. ++− and −−+). When transmitting, the DC bias is tracked and a combination chosen that has a DC component of the opposite sign to the running total.