full-disclosure - WiFi question

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Message-ID: <174E3EE7B530C4F790EB3454@utd49554.utdallas.edu>
From: pauls at utdallas.edu (Paul Schmehl)
Subject: WiFi question

--On Thursday, November 18, 2004 09:32:27 AM -0600 Paul Schmehl 
<pauls@...allas.edu> wrote:

> --On Wednesday, November 17, 2004 12:41:44 PM -0500 "Lachniet, Mark"
> <mlachniet@...uoianet.com> wrote:
>
>> Could also be RF interference.  One of my coworkers tracked down a
>> particularly interesting problem with motion sensor lights.  Turns out
>> the motion sensors worked at the 240mhz range, which has resonance at
>> 2.4ghz, or something like that.  Hence every time the motion sensor
>> worked, it would spew what the wardriving (site survey) apps thought was
>> a zillion different access points with widely varying MAC addresses.  I
>> would have though it was a FAKEAP program also.  I would assume the same
>> could happen with other interference.  Having a common SSID would seem
>> to indicate this is not the problem, but just thought I'd mention it.
>>
> Thanks for a particularly interesting and potentially useful bit of
> information, Mark.
>
After forwarding this to our wireless expert, he responded with this (which 
he has authorized me to forward to the list.)

I find it hard to believe that this is possible.  2.4Ghz is the 9th 
harmonic.  By the time you get to the 4th harmonic of a signal, even in 
very very noisy radiators, the strength of the harmonic component of the 
signal is extremely minute.  And, given the fact that one of those sensors 
(which most likely does *not* truly operate in the 240MHz portion of the 
spectrum) will have a very low output (Part 15 device), the 10th harmonic 
of that signal will be undetectible as it will be at or below the level of 
background noise.

Finally, if a device managed to get past all of the improbabilities above, 
the chances of it *accidentally* creating a signal that looked like an 
802.11 beacon packet, complete with preamble, header, etc is so off the 
charts as to be laughable.

One other thing...  If that device truly was operating at 240MHz, then the 
first harmonic would be 480MHz.  I'm pretty sure that frequency lies in the 
public service bands (ie fire/police).  If not, its very close.  Given that 
and the fact that the first harmonic would be much stronger than the 9th 
harmonic, I'm pretty sure someone in those bands would have complained 
loudly to the FCC as they don't take intereference issues in those bands 
lightly.

Paul Schmehl (pauls@...allas.edu)
Adjunct Information Security Officer
The University of Texas at Dallas
AVIEN Founding Member
http://www.utdallas.edu