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(C) Herbert Haas
2005/03/11
4B/5B Coding
4B/5B Encoder/Decoder
PMA
PCS
MII
1000
010 10
16 code
groups
32 code
groups
4 x 25
Mbit/s
125 MBaud
The diagram above shows the basic principle of the 4B5B block coding principle,
which is used by 802.3u and also by FDDI. The basic idea is to transform any
arbitrary 4 bit word into a (relatively) balanced 5 bit word. This is done by a fast
table lookup.
Balancing the code has many advantages: better bandwidth utilization, better
laser efficiency (constant temperature), better bit-synchronization (PLL), etc.
Note that the signaling overhead is 5/4 12.5 %.