[ns] Accuracy of wireless simulation with nodes sending data at high rate.

[Dominique Dhoutaut <dominique.dhoutaut@ens-lyon.fr>]

I'm a PhD student interested in wireless ad-hoc networks, and I have
done some simulation whith ns2.
But the results I got seems quite strange to me, and as I do not have
any real 802.11 card, I have no way to be sure these results are
accurate. (considering what I got, I hope they are not, or that I have
done some mistake )

Could someone give me some advice on the following :

I simulated 3 pair of nodes (a,b),(c,d) and (e,f).

- a is sending data to b, c to d and e to f.
- Each sender try to send as much data as possible (using CBR).
- I haven't done any modification to ns-allinone 2.1b7a which I use
- The following happens whatever wireless routing protocol I use
- I use the two-ray ground reflection radio propagation model
- The problem appears only if I use most of the bandwith

During the simulation, the two nodes of a pair stay very close to the
each other. (5 length units)
At the beginning of the simulation, the 3 pairs are very close.

One pair will stay at it's starting position, one will go away at a
speed s, and the third will go away in the same direction at a speed 2s.

we obtain something like this, the distance between each pair increasing
slowly.
(a)->(b)                              (c)->(d)
(e)->(f)

At the beginning, as all nodes are close, the three data streams are
fairly sharing the available bandwith.

We would have expected that at some point, each pair would have (more or
less suddently) gain access to the full bandwith, as the interferences
with the others pairs get lower and lower.
In the end, it effectively happens, but the problem is that there is an
intermediate state where the sender of the central pair cannot send data
to it's receiver anymore ! (c cannot send to d, which is only 5 length
units away !)

[as I wasn't sure it is permited to attach pictures to mails on this
mailing list, here is a link to a small graph :
http://www.ens-lyon.fr/~ddhoutau/images/ns_pb.gif ]

this graph show the data rate for each stream (in bit per second),
depending on time (in second).
Moreover, the *problem* is happening when the distance between the pairs
is between 1 and 2 times the maximum transmission range of a node !
(after 5 seconds, the central pair is out of the direct transmission
range of the others pairs (but of course is not yet out of their
interference range))

The red curve show what happens to the central pair (c)->(d) ... And
there I don't known what to do ....

I suspect a problem with some "not realy independant" random behavior of
my senders.
But I don't know where I should look ....