Fibre Channel Protocol Architecture Fibre Channel Layered Model
8b/10b Encoding
• Adding two bits allows FC to:
– Differentiate control signals from data – Detect signaling errors – Reduce run length – Maintain clock synchronization – Provide DC balance – Simplify transmitter and receiver design
Adding two extra bits to each data byte consumes extra bandwidth, but it also has some advantages:
• A 10-bit encoding scheme provides 1024 unique characters—768 more than are required to represent every 8-bit character. Some of the extra 10-bit characters are used as control characters to differentiate control signals from actual data.
• The encoding scheme allows the receiver to easily detect many signalling errors. Only a subset of the 1024 possible 10-bit patterns is used, so an erroneous bit will often produce an invalid character. This is known as a Code Violation error.
• It ensures that there are a sufficient number of signal transitions in the signal to maintain clock synchronization—in other words, it avoids long run lengths (long strings of ones or zeros) that could cause the receiver clock to lose synchronization.