On Sep 25, 2008, at 1:46 PM, funofnet Funofnet wrote:
>
>
> >> Hi all,
> >>
> >> I noticed that sometimes when I generate two link gain model files
> >> ("linkgain.out")
> >> by the "LinkLayerModel" java class for the same configuration, then
> >> I ran a simulation to compute for example the packet delivery rate
> >> of my application, I obtained incomparable results.
> >>
> >> Let me give you an example for best understanding:
> >> A simulation with the first linkgain file lets me get an
> PDR=0,6.....
> >> in contrast, a simulation with a second linkgain file (I repeat:
> for
> >> the same configuration) lets me get an PDR=0,05....
> >>
> >>
> >>
> >> It is a strange thing which lets Tossim simulation results to be
> not
> >> RELIABLE!!!!!
> >>
>
> >Do you mean "deterministic?"
>
> >Of course the link model generator is not deterministic; it considers
> >hardware variations. The point is that two sets of nodes, placed in
> >identical physical layouts, might have different packet reception
> >ratios due to sensitivity variation across the nodes. You're testing
> >two different networks, and so the results are, well, different. If
> >you want to test the same network, use the same link gain file.
>
> >Phil
>
> Thanks professor,
> But the amazing thing is the huge difference between the two results
> (0.6 and 0.05)
> Normally, results should be comparable (0.6 , 0.5, 0.7 in the worst
> case 0.3).
Why? For a CC2420, a 1.5dB chance in the SNR is the difference between
a PRR of 90% and 10%. Noise floor variations are much larger than
1.5dB. Not to mention the fact that the signal itself varies by at
least that much over time. Kannan has a paper appearing in SenSys on
this, you can take a look at the tech report version here:
http://sing.stanford.edu/pubs/sing-07-01.pdf
Phil
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