HI AASIF BASHIR DAR…
WELL AS FAR AS I GOT YOUR QUESTION YOU ARE TALKING ABOUT GLOBAL DATE RATE OF YOUR SYSTEM AND THE BIT RATE OF DATA GENERATED BY THE INFORMATION SOURCE.These two bit rates are different due to implementation of different multiplexing and modulating schemes as well as the signal coding.So the data rate of the system should be always graeter than data rate of the bit sequence.
You are right in saying that it is not possible to sweep or iterate the layout data rate as this option is unavailable in optisystem.hence we go for the sweep or iteration in the of data rate in PBRS.
with regards