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4B/5B Coding
4 x 25
Mbit/s
125 MBaud
(C) Herbert Haas
2005/03/11
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
MII
0
0
0
PCS
4B/5B Encoder/Decoder
0
1
0
PMA
12.5 %.
16 code
1
groups
32 code
groups
0
1
13
13
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