Fibre Channel 8B/10B encoding


Fibre Channel 8B/10B encoding

The Fibre Channel FC1 data link layer implements the 8b/10b encoding and decoding of signals.

The Fibre Channel 8B/10B coding scheme is also used in other telecommunications systems. Data is expanded using an algorithm that creates one of two possible 10-bit output values for each input 8-bit value. Each 8-bit input value can map either to a 10-bit output value with odd disparity, or to one with even disparity. This mapping is usually done at the time when parallel input data is converted into a serial output stream for transmission over a fibre channel link. The odd/even selection is done in such a way that a long-term zero disparity between ones and zeroes is maintained. This is often called "DC balancing".

The 8-bit to 10-bit conversion scheme uses only 512 of the possible 1024 output values. Of the remaining 512 unused output values, most contain either too many ones or too many zeroes so are not allowed. However this still leaves enough spare 10-bit odd+even coding pairs to allow for 12 special non-data characters.

The codes that represent the 256 data values are called the data (D) codes. The codes that represent the 12 special non-data characters are called the control (K) codes.

All of the codes can be described by stating 3 octal values. This is done with a naming convention of "Dxx.x" or "Kxx.x".

Example:

Input Data Bits: ABCDEFGH

Data is split: ABC DEFGH

Data is shuffled: DEFGH ABC

Now these bits are converted to decimal in the way they are paired.

Input data

C3 (HEX) = 11000011 = 110 00011 = 00011 110 = 3 6E 8B/10B = D03.6


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