4b/5b Clock 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 and a MLT-3 line 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.