[ns] side effects of c++ random variables on tcl random variables?

[Martina Umlauft]

Hi all!

I have a weird question regarding the random generator.

Ok, I tried to check this in the manual and I got more confused than 
before -

1. the manual at http://www.isi.edu/nsnam/ns/doc/index.html dated 
2005-09-23 is for which version of ns? I assume 2.28 - is this correct?

2. if yes, then lots have changed from ns2.1b9a random generation, 
right? We have people here with experience in the old version but it 
seems I can't rely on their input.

3. the actual PROBLEM:
   -) we use random numbers from C++ and from Tcl - we have physical 
layer calculations in C++ and traffic generators written in pure Tcl.

   -) I set up the random numbers "like they used to be" in Tcl, eg.

      set rng1 [new RNG];
      $rng1 seed $seed;
      set rtime [RandomVariable/Exponential];
      $rtime use-rng $rng1;
      $rtime set avg_ 14.1;
      ...

      Apparently this should not be done like this anymore in ns2.28?

   -) The objective is to run the same traffic with and without relay 
stations - which of course means that the phy calculations differ. For 
some reason the generated traffic (yes, I checked at the sender side on 
the application layer level) is not the same with and without relay 
stations - it seems like someone (I suspect the C++ code) is "eating 
away" random numbers from the stream...

So the QUESTION is: can there be side effects from C++ towards the Tcl 
random generators? If the C++ code uses more random number values will 
this affect the random number values my pure tcl traffic generators get?

If yes (which I strongly suspect now) - how do I avoid it? I have to 
admit I don't really understand what the ns manual is trying to tell in 
Section 23.1...

Has anybody encountered such a problem before and could provide some 
guidance? Thank you very much.


lg,
     Martina Umlauft        martina.umlauft at vienna.at
-----------------------------------------------------
                            http://frauenweb.at/~tina