[unisog] Wireless in residence building - Tricky problem

Bob Johnson bob at eng.ufl.edu
Wed Nov 5 20:46:32 GMT 2003



Mitch Collinsworth wrote:
> On Wed, 5 Nov 2003, Marc Jimenez wrote:
> 
> 
>>>Two signal strength-and-direction reading 10-15 foot apart in the hallway
>>>and some triangulation would do it.  It's the electromagnetic version
>>>of "which room has the annoyingly loud stereo in it?", which is also
>>>solvable without a search warrant.... :)
>>

Triangulation of a radio source inside a building can be a lot harder to 
do in practice than in theory.  Multipath signals and variable 
attenuation paths can make it difficult to determine which direction the 
signal is coming from (well, you can usually determine where it's coming 
from, the problem is that that isn't always the direction the 
transmitter is in).  It is frequently easier and just as accurate to map 
signal strength.  Which, by the way, tends to be how you determine which 
room the loud stereo is in once you get close to it.  "Yep, it's loudest 
at this door."

>>Absolutely true, we can certainly determine to *our* satisfaction that the
>>AP is in the room, but incontravertable proof (which is what our school's
>>disciplinary board requires) is a bit harder to come by. If you've got a
>>more ... reasonable ... set of proof requirements, more power to you. I
>>wish I did....
> 

Don't have much to contribute regarding proof, but as for policy: the 
FCC regulates the transmitter, but it doesn't regulate the property it's 
on (not necessarily true for everything the FCC regulates, but probably 
true for wireless networking stuff).  The general feeling around here is 
that the University can declare "we don't allow those transmitters on 
our property without individual approval" and it would be perfectly 
legal.  As far as I know they haven't gone that far yet.

> 
> I don't see why triangulation wouldn't be considered incontravertable,
> though it's safer to do it with 3 samples, since doing it with 2 only
> gets you to one of 2 possible locations.
> 

With either two or three bearings, you get two possible locations.  The 
second location is on the other side of the earth, so there is no 
ambiguity unless you are seriously lost.  What you gain from the third 
fix is an estimate of your error.

That aside, a fix obtained by a qualified operator should stand up as 
evidence in court.  The challenge is to convince the court (or 
disciplinary board) that you are qualified.  You need, for example, to 
be able to explain all of the possible causes of ambiguity in your 
measurements, what you did about them, and why your conclusion is the 
only reasonable conclusion to be reached from your data.  IANAL, but it 
seems to me that if the hearing board is considered judicial, or 
quasi-judicial, they are required to accept as accurate the testimony of 
a qualified expert unless the other side can present a qualified expert 
who disagrees with yours.  The catch in that is that it is usually up to 
the court/board to decide who is a "qualified expert".  Have you asked 
your campus legal department for guidance?


> If you want better than that, cut the power to the room in question and
> see if the AP signal goes away.  (Doesn't work if it's on a UPS
> obviously.)
> 

> -Mitch

- Bob



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