Hi Gurpreet
Alisth have explained well about your question.I just want to add that run your system with script mode and change data rate in global parameter window.For filter(electrical low pass) ) 0.75*Bit rate is a cut off frequency,you can change it to o.8* bit rate and see the results beacaus for my some of systems it works well.In optiwaves example of CSRZ,MDRZ and DPSK they have considered cut-off frequency 0.8* Bit rate.