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

[antoine.trux@nokia.com]

Mathieu,

We understand that simulations are only simulations, with necessarily
many simplifications involved (as you point out), and their results must
be taken with a (big) grain of salt.

That being said, we see no point in using a reception criterion that has
nothing to do with the operation of the devices. Thus, instead of:

	(reception succeeds)
	IF
	(SINR >= a certain threshold) (1)
	AND
	(signal power >= carrier-sense threshold) (2)

we prefer to drop criterion (2), because carrier-sense threshold is not
related to signal reception.

<devil's advocate>

By the line of reasoning of your message, we could just as well replace
criterion (2) by another completely arbitrary one. For example:
	(signal power >= carrier-sense threshold + 1 dBm) (2')
or:
	(signal power >= carrier-sense threshold - 2.42 dBm) (2'')
or even (why not?):
	(signal power >= carrier-sense threshold + PI - square root of
my age in years) (2''')

None of (2'), (2'') and (2''') has anything to do with the real
functioning of WLAN devices. But then, "do you know of a rigorous
analysis that shows that the results of simulations using these criteria
significantly depart from real world experiments"? If such results do
depart from real world experiments, "why would the discrepancies come
from using these criteria"? "How would we verify that they come from
using these criteria"?

(2'), (2'') and (2''') are completely arbitrary, but "there are so many
other things in this model which are also completely arbitrary that
pointing out this specific item in isolation without further context is
just meaningless."

</devil's advocate>

As far as we are concerned, we prefer not to use criteria such as
(2)-(2'''), although we have no "proof" that not using them results in
better modelization of the real world.

Antoine

P.S.:

> > Suffice it to say that I didn't invent
> > the figure of -89 dBm in the above example.
> 
> So, where is it coming from ?

The figure of -89 dBm for receiver's sensitivity is the requirement of a
mobile device manufacturer (no name...) for the parameters that I gave.
As you can see, this requirement is tighter than the minimum one given
in the Standard (-82 dBm).

> -----Original Message-----
> From: ext Mathieu Lacage [mailto:mathieu.lacage at sophia.inria.fr] 
> Sent: 25 June, 2008 18:13
> To: Trux Antoine (Nokia-NRC/Helsinki)
> Cc: ns-developers at ISI.EDU
> Subject: RE: [Ns-developers] [ns-2.33] bug in WirelessPhyExt::sendUp()
> 
> 
> On Wed, 2008-06-25 at 16:11 +0300, antoine.trux at nokia.com wrote:
> 
> > > 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.
> 
> I have never seen a rigorous analysis of a real-world experiment which
> shows proof of that. i.e., the difference between the real-world
> experiment and the simulation could come from any number of 
> things. Why
> would it come from _that_ ? How do you verify it comes from _that_ ?
> 
> > >   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.
> 
> So, where is it coming from ? Part of the reason why 802.11 PHY
> simulation models suck so much is that there is little to no 
> litterature
> on experiment-backed model design for these PHY layers. So, 
> what we have
> is drawn from intuitions from our understanding of the way 
> these devices
> are designed, which is quite unlikely to match anything close 
> to a real
> world device. Or, if it does, no one wrote about a paper I am aware of
> about the success of their intuitions backed by experimental data.
> 
> > >   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
> 
> I take issue with this "false" adjective. There is nothing 
> "false" about
> the current behavior: it is merely an arbitrary model and there are so
> many other things in this model which are also completely 
> arbitrary that
> pointing out this specific item in isolation without further 
> context is
> just meaningless. 
> 
> For example, the whole idea that there is something called 
> the "power of
> a signal" which is constant over the signal reception is 
> meaningless and
> there are many other underlying assumptions about the current model
> which are similarly totally meaningless from a physical 
> perspective. So,
> is it a "false" model ? Of course it is. But does this model show good
> correlation with real-world data ? I don't know and showing proof of
> that would go a long way towards really improving the state 
> of the ns-2
> (or ns-3) 802.11 PHY models.
> 
> > signal reception, even if the simulation results happened 
> to be correct
> > for certain parameter values.
> 
> Of, course, it depends on which kind of simulation you are running,
> there is no doubt about it: it is trivial to devise a simulation which
> behaves wildly differently with different parameter values. The
> question, though, is whether what you are proposing is really better
> from the perpective of modelizing a real world device. 
> Intellectually, I
> agree with you that what you suggest is probably a good idea. I have
> never seen proof of it though and _that_ was my point. There 
> has been a
> recent flurry of activity and papers around that topic, so it 
> might the
> case that I have missed something in which case, I would be more than
> happy to be pointed out to relevant material :)
> 
> Mathieu
> 
>