Home > Resources > Networking Glossary > Fast Ethernet >

print

Signal Encoding - 4B/5B

Examples of 4B/45 and 8B/6T Data Encoding

4B5B Example

In order to send information using 4B5B encoding, the data byte to be sent is first broken into two nibbles. If the byte is 0E, the first nibble is 0 and the second nibble is E. Next each nibble is remapped according to the 4B5B table. Hex 0 is remapped to the 4B5B code 11110. Hex E is remapped to the 4B5B code 11100.

In 100BASE-FX and 100BASE-TX, the 4B5B replacement happens at the PCS sublayer of the Physical layer. Information is then further encoded for transmission using NRZI in 100BASE-FX at the PMA sublayer, and MLT-3 in 100BASE-TX at the PMD sublayer.

8B/6T Example

In order to send information using 8B6T encoding, the value of the data byte is compared to the values in the 8B6T table. Every possible byte has a unique 6T code, a set of 6 tri-state symbols. Unlike 4B5B, 8B6T completely prepares the data for transmission; no further encoding is required.

100BASE-T4 is currently the only technology which uses 8B6T encoding. It performs 8B6T encoding at the PCS sublayer of the Physical layer. 100BASE-T4 then demultiplexes the 6T codes onto three wire pairs.