"The open source network simulation tool ns-2 is an invaluable
tool for researchers working on wired or wireless networks. I came
across ns-2 while working on my thesis. I needed a network
simulator, and since my college lacked the brand-name heavyweights,
I had to look for an alternative that was free but could do the
job. While ns-2 is free, it's also pretty unfriendly.
"ns-2 is a variant of the REAL network simulator. Over the past
few years it has been evolving, and it is still far from complete.
Several organizations have been involved in its development,
including DARPA, Xerox, UCB, and Sun Microsystems. The objective
has been to make a network simulation tool to study and analyze new
ideas in detail before implementation..."