 8b/10b encoding guarantees strict bounds on DC balance, transition density and run length, whereas 64b/66b provides statistical bounds on these properties.Most clock recovery circuits designed for SONET OC-192 and 64b/66b are specified to tolerate an 80-bit run length.Such a run cannot occur in 64b/66b because transitions are guaranteed at 66-bit intervals, and in fact long runs are very unlikely.