The bit rate is set for all the system, including the fiber channel, and so is the symbol rate. So like I said, the symbol rate should be chosen according to the “bits per symbol” parameter in the coder or modulator. For example, if in a system with a bit rate of 7.5GHz this parameter is 3 (for a modulation scheme like 8-psk), then the symbol rate would be 2.5 GHz.