On Wed, Jun 05, 2002 at 12:10:43AM +0100, Stuart Henderson wrote:
> > I'll be swapping my channels and no doubt interrupting my users service
> > in about an hour and I'll report back if I learn anything new.
>
> Wlanexpert (on a prism2 card) is a very useful tool for scanning each
> channel for the best signal:noise ratio.
>
> > if he disabled broadcasting he'd be even more invisible!
>
> Depends which survey tool you use. Sniffing raw frames you'd see someone
> with a DSSS 802.11b in the way even without broadcast. Though there are of
> course other possibilities, like FHSS, video senders, etc.etc. which
> wouldn't show up that way (but presumably would affect the RSSI).
>
> Interference is, of course, one of the things you just have to live with or
> work around if you're using unlicensed spectrum.
Trying to determine noise with an 802.11 radio will miss a lot of
interference that may be causing your link to be reliable. It also
is not a very good tool to do things like dish alignments.
Matt, Dan and I tried to align a 20+ mile link using the signal and
noise figures on a Cisco 350 card. There is very high latency in
the reporting on these cards as well as numbers that bounce all
over the place. We have come to the conclusion that you really
need good analog test equipment to do this right.
For instance, using a decent spectrum analyzer for interference
issues. Many signals that would interfere with 802.11 will be
hard to track with 802.11 cards. A analyzer will have a much better
chance in catching an tracking down signals that are impairing your
links.
We also should be using a carrier wave (CW) transmitter on the
center channel and an analog receiver or spectrum analyzer for
antenna alignment. Changes will be much faster to resolve as you
sweep through the beam and a CW spike is much easier to see in the
noise of the band.
Tim
--
Snail: Tim Pozar / LNS / 1978 45th Ave / San Francisco CA 94116 / USA
POTS: +1 415 665 3790 Radio: KC6GNJ / KAE6247
"It's a damn poor mind that can only think of one way to spell a word."
- Andrew Jackson
"What is wanted is not the will to believe, but the will to find out,
which is the exact opposite." - Bertrand Russell, "Skeptical_Essays"
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