[Ns-developers] Channel scanning in 802.11

[Gustavo Carneiro]

A while back I had written some wifi scanning code.  I have re-synced with
ns-3-dev and uploaded the code here, in case it is useful to someone:

http://code.nsnam.org/gjc/ns-3-wifi-scanning/

Caveats:

  1- The code is experimental;

  2- The active scanning is pessimistic, i.e. scanning takes longer time
than real world, because it assumes all channels are busy with traffic and
always spends 50 ms in each channel (real world cards are smarter and spend
just a few ms if they do not sense any traffic in that channel).  In any
case, it works, in spite of being rudimentary.

 3- The code has not been reviewed yet by Mathieu, the WiFi maintainer,
although I hope he will consider it ;-)

 4- I don't have a lot of time to maintain the code, so it would be
wonderful if someone could pick it up.  Maybe I might develop something more
on top of this, or maybe not, I am not sure yet...

Further things to implement:

  1- Switching channels should consume between 1 and 5 ms (hardware
limitations) [1];

  2- Probing a "free" channel should require a delay of only 1-2 ms (aka
"minimum channel time"), and 10 to 27 ms (aka "maximum channel time") for
busy channels [1];

  3- Due to inter-channel interference, additional channels may erroneously
appear busy and thus cause additional scanning delays (has to spend maximum
channel time, instead of minimum, for additional channels), so without the
inter-channel interference model mentioned by Mathieu scanning delays might
be too optimistic;

  4- The scanning machinery should somehow record the "signal quality" of
the received ProbeResponses, and report it back to the client, so that the
client can choose the best AP based on signal strength;

  5- The final piece of the puzzle would be to implement a MobilityManager
kind of object, attached to the node, which would select the best AP at all
times.  Right now NS 3 has very rudimentary mobility management.  An
experiment I have made: place two APs on the same channel, same SSID, spaced
by 100 meters;  move a STA to the space in the middle of the two APs; the
STA starts receiving beacons from both APs, and it keeps associating to one
and other AP, constantly flip-flopping its association between the two APs.

[1] Murray, D.<http://ieeexplore.ieee.org/search/searchresult.jsp?disp=cit&queryText=%28murray%20%20d.%3CIN%3Eau%29&valnm=Murray%2C+D.&reqloc%20=others&history=yes>
Dixon, M.<http://ieeexplore.ieee.org/search/searchresult.jsp?disp=cit&queryText=%28%20dixon%20%20m.%3CIN%3Eau%29&valnm=+Dixon%2C+M.&reqloc%20=others&history=yes>
Koziniec, T.<http://ieeexplore.ieee.org/search/searchresult.jsp?disp=cit&queryText=%28%20koziniec%20%20t.%3CIN%3Eau%29&valnm=+Koziniec%2C+T.&reqloc%20=others&history=yes>
, "Scanning Delays in 802.11 Networks", NGMAST '07. (
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=4343430)

2008/10/7 Mathieu Lacage <mathieu.lacage at sophia.inria.fr>

>
> On Tue, 2008-10-07 at 04:23 -0700, Ramon Bauza wrote:
> > Hi all,
> >
> > I have seen in the ns-3 manual, that although WiFi networks can be
> > simulated in infraestructure mode, the current ns-3 version does not
> > perfom channel scanning. My questions are then:
> >
> > - Can a NetDevice switch among different channels at execution time?
>
> You can change the frequency band used by a device but there is no
> support in the current PHY model to modelize interference among signals
> in overlapping frequency bands. A patch/model description to do so would
> be welcome.
>
> > - Is the channel scanning going to be implemented?
>
> yes, if you send a patch.
>
> regards,
> Mathieu
>
>
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-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert