ns2 simulator is one among best tool for wireless sensor network..

I used NS3 for WSN simulation

I recommend to use Castalia which is an OMNET++ based WSN simulator.

• I do agree with Eren Demir.. Castalia is the easier and better tool for WSN.

 There is also tossim.

The NS2 is best solution for WSN and also check NS3, but if you are new in simulation so i prefer first learn NS2. 

you can also use mixim for omnett++ or opnet 

Thanks all!

Cooja is a good cross-level simulator, and you can simulate your software (OS, protocol stack) and also emulate your embedded hardware system. So Cooja is better for you to test a complete system. But if you are dedicated to WSN's protocols simulation, omnet++ and Castalia are good choices, I totally agree with the previous answers.

@Yacine, I want to simulate both th SW and the underlying HW, so I will have a look at Cooja as suggested by Yibo Chen.

If you want to test code then I would recommend NetSim

www.tetcos.com

for network layer, I find Network Simulator  (NS-2) and OMneT++ very effective tools. You may choose any of them and get good results.

Thanks!

There no exclusive simulator for WSN all adhoc networks can be simulated using NS2, OMNET, OPNET, QUALNET, You can use emulators like exata to do testing in hardware there performance. Else you can configure several CC3000 or aurdino boards to form a network and then program using embedded c 

@Poornima: thanks, I will have a look at it.

Use can use ns2

I will suggest you NS2 and 3, first install Ubuntu on your system and run NS2 if you run on virtual machine so it will be very slow.