Hi Sanjeev kumar,
I agree with the points of Dhiman kakati that the “problem occurs many a times when system bit rate and symbol rate do not match with that of your transmitter, so we have to set prbs generator bit rate in script form and write there Bit rate”.
I also think that when we design the networks as per our requirements , then we should know the different standard values of corresponding parameters as per ITU- standards.
We should not use random values of different parameters and always follow the standard one.
We could take help from the sample of different designs of optisystem.