4B/5B Coding

MII

0

0

0

1

16 code

groups

 

 

 

 

 

 

 

 

4 x 25

Mbit/s PCS

4B/5B Encoder/Decoder

32 code

groups

 

 

 

0

1

0

0

1

 

 

 

 

 

125 MBaud

PMA

 

 

 

 

(C) Herbert Haas 2005/03/11

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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 %.

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