4b/5b Encoding

The bi-phase Manchester encoding can consume up to approximately twice the bandwidth of the original signal (20 MHz). While this was of little concern in coaxial cable transmission, the limited bandwidth of necessitated a more efficient encoding method for 100 Mbps transmission using a 4b/5b MLT code. Gigabit Ethernet utilises five levels and 8b/10b encoding, to provide even more efficient use of the limited cable bandwidth, sending 1 Gbps within 100 MHz of bandwidth.

The 4B/5B Encoding Method

4B/5B encoding is a type of 'Block coding'. This processes groups of bits rather than outputting a signal for each individual bit (as in Manchester encoding). A group of 4 bits is encoded so that an extra 5th bit is added. Since the input data is taken 4-bits at a time, there are 2^4, or 16 different bit patterns. The encoded bits use 5-bit, and hence have 2^5 or 32 different bit patterns. As a result, the 5-bit patterns can always have two '1's in them even if the data is all '0's a translation occurs to another of the bit patterns. This enables clock synchronisations required for reliable data transfer.

4-bit 5-bit

0 0000 11110
1 0001 01001
2 0010 10100
3 0011 10101
4 0100 01010
5 0101 01011
6 0110 01110
7 0111 01111
8 1000 10010
9 1001 10011
A 1010 10110
B 1011 10111
C 1100 11010
D 1101 11011
E 1110 11100
F 1111 11101

Multi-Level Transition (MLT) Signalling

A 100 MHz data stream that uses a 4B/5B encoding therefore results in a125 MHz signal. However in Fast Ethernet, this 125 MHz signal is itself encoded as a mult-level signal using three signal levels (instead of the two levels used in Manchester encoding). The three signal levels are -1, 0, +1. This reduces the bandwidth required o the physcial cable to only 31.25 MHz, which is within the specification of the CAT5e cable used in UTP.

MLT-3 Encoding


Gorry Fairhurst - Date: 01/02/2006 EG3557