[Ns-developers] [ns-2.33] bug in WirelessPhyExt::sendUp()

[antoine.trux@nokia.com]

Mathieu,

This whole discussion started from the fact that, when in state
SEARCHING, the code in WirelessPhyExt::sendUp(), in order to determine
whether the reception succeeds, checks not only that the incoming signal
is strong enough (which is correct), but also that the incoming signal
is above the carrier-sense (which we think is clearly incorrect).

For example, suppose we are using 5 GHz, 20 MHz channel spacing, and
BPSK with 1/2 coding rate. According to the Standard, the minimum
receiver sensitivity is then -82 dBm, and the carrier-sense threshold is
also -82 dBm. Suppose the receiver's real sensitivity is -89 dBm. Now,
suppose the PHY layer is in state SEARCHING, there is no signal, and a
signal of -85 dBm arrives. According to WirelessPhyExt::sendUp(), this
signal is dropped, because its level is below the carrier-sense
threshold (i.e., below -82 dBm), whereas in reality the signal can be
received (because it is above -89 dBm).

> the questions are:
>   a) whether or not a real application will see a difference in
> simulations (and in the real world)

This depends on the values of the receiver's real sensitivity and the
carrier-sense threshold. The larger the difference between these two
values, the larger the discrepancy between simulation results and the
real world.

>   b) which values of the two thresholds would better modelize a real
> device, do these values change a lot from one device to the other ?

I don't know how much the receiver's real sensitivity varies from device
to device, but the real sensitivity can significantly depart from the
minimum mandated by the Standard. Suffice it to say that I didn't invent
the figure of -89 dBm in the above example.

>   c) whether or not the new model will be really "better" 
> (that is, its
> predictions will more accurately reflect real-world experimental
> measurements)

Depends on which kind of simulations you are running. See also my answer
to a) above. Anyway, I see no point in using a false criterion for
signal reception, even if the simulation results happened to be correct
for certain parameter values.

Antoine