But NetSim is not for free Ashish Bagwari Sir. What could be the alternative solution for this, As I am working with no pay it is very hard for me to purchase at this moment.
There are Matlab based Vienna simulators, and they have two options: link level and system level. It is free but you need to get in touch with the builders.
There is also prominent ns3 simulator, and it is Linux and C++ based. It is open source.