100BASE-T4 runs on four pairs of Category 3 twisted-pair cable. An efficient encoding is needed to pack 100Mbps onto Category 3 cable.
As with 100BASE-TX, three different signal levels are used for 100BASE-T4. Instead of following fixed steps from low-to-middle, middle-to-high, and so on, however, the voltage levels are used as symbols that can appear in any order. These are called ternary symbols.
As was the case for MLT-3, the voltage levels correspond to these ternary symbol labels:
[-, 0, +] or [-1, 0, and 1].
Each 8−bit byte is mapped to a unique code−group consisting of six ternary symbols. There are 2<+z6.175>8<$z$> = 256 different 8−bit patterns, but there are 3<+z6.175>6<$z$> ...