Hello Gurinder singh
I agree with the explanation given by Abhishek and Alisthu about the WDM OFDM.I just want to add that due to the long sequence length you will get the BER 0 i.e error free in optisystem samples (if you are using eye diagram analyzer) and falls to very high values of BER after that.If you are using the DUT signal analyzer i.e BER TEST SET then you will get the no. of error bite more clearly and you will get the noticeable results.Rest in general for the system BER is acceptable within 10^-9 to 10^-12.Visit the link given below as Ravil and Alessandro have given very good points in the discussion