On Jan 12, 2007, at 8:00 PM, [EMAIL PROTECTED] wrote: > for the sake of completion. >> Luqman wanted us to know: >> >> Though errors are inserted in the received packets these errors >> occur at >> the same place during every run of simulation. >> >> I think this is not intended so. Random variable should contain >> different numbers every time random function is called. > > After reading up in the ns-manual while searching for "random" and > "seed", I found that "ns-ransom 0" choses the seed at run time whereas > otherwise a static seed was being used. > > Hope this helps others with same questions.
Right, the way that the RNG in ns-2 is setup, the default seed is the same every time unless you change it. If you want to make sure that the different runs of your experiment are independent, you should use the substream feature available in ns-2. A further description of this can be found in Section 4 of this paper: M.C. Weigle, Improving Confidence in Network Simulations, Proceedings of the Winter Simulation Conference, Monterey, CA, December 2006, pp. 2188-2194. http://www.cs.odu.edu/~mweigle/papers/wintersim06.pdf This paper also contains the same example found in the ns Manual at http://www.isi.edu/nsnam/ns/doc/node265.html Essentially, you want to call next-substream enough times to advance the random number stream past where you used it in the previous experiment. For example, if you want to run the experiment 3 times: 1st experiment - call next-substream 0 times 2nd experiment - call next-substream 1 time 3rd experiment - call next-substream 2 times Be VERY careful with setting the seed according to the time of day. Unless you print out the seed and record it, you won't be able to reproduce an experiment exactly. More importantly, if you set the seed by the time-of-day, there is no way to guarantee that the multiple runs of your simulation are independent. -Michele -- Michele Weigle Assistant Professor Department of Computer Science Old Dominion University Norfolk, VA 23539 mweigle at cs.odu.edu http://www.cs.odu.edu/~mweigle/
