Multi-Level Transition (MLT-3) Encoding

MLT-3 encodes a bit as presence or lack of transition, exactly as in NRZI. What makes MLT-3 different is that the base waveform is a 3-state alternating wave. Rather than alternating between 0 and 1 as in Manchester encoding and NRZI, MLT-3 alternates from -1 to 0 to +1, back to 0, then back to -1, repeating indefinitely. A zero is encoded as a halt in the back-and-forth progression. It may be useful to think of MLT-3 as a stop-and-go sine wave, encoding 0 as stop and 1 as go. Using MLT-3, it is therefore possible to represent four or more bits with every complete waveform, at 0, +1, 0, and -1.

The data being transmitted in this example is the hexadecimal byte “0E”. First, the byte is broken down into four bit “nibbles”, giving a nibble of “0” and a nibble of “E”. Then, each nibble is looked up in a table to find the byte code associated with that hexadecimal number. The code for “0” hex is “11110” and the code for “E” hex is “11100”. Finally, the byte codes are transmitted onto the wire using MLT-3 encoding.

For a discussion of the reasoning behind the association of four byte “nibbles” with five byte codes, refer back to the DATA ENCODING SECTION of this Fast Ethernet section