Urgent : Zigbee Simulation Software

I am planning to build a Zigbee network,which will consist of sending and receiving devices built by ATmega 168 and AT86RF230. It will take some time for me to get the necessary hardware.So during this time,it will be good if I can simulate my proposed network in a software.

Would anyone please tell,is there any good software for such a simulation-where I can build,simulate and test the functionality of a Zigbee network?

Thanks in advance.