lrtherond@gmail.com hath wroth:
>I have been reading this newsgroup for several hours and I saw that
>you consistently provide your help to those who need it.
>Your knowledge and willingness to help others are admirable and
>commendable.
Thanks. I realize that I spend too much time answering questions.
I'll make an effort not to do it as often.
>I am trying to decide if I should allow CTS/RTS to kick in, but I find
>it hard to make this decision without knowing whether interferences/
>hidden nodes are causing an excessive amount of collisions.
Interference and hidden nodes are handled differently. CTS/RTS flow
control is very useful for hidden nodes. Your access point should
orchestrate the transmission times of the various clients including
those that can't hear each other. It works best with at least 8 or
more clients and does next to nothing for a smaller number of clients.
It will slow things down considerably, but does help reduce
collisions. The problem is that a few vendors just don't seem to
correctly honor flow contol packets or DTIM (Delivery Traffic
Indication Message) broadcasts. The result is that they tend to
monopolize the channel (airtime).
Interference is handled in several ways. One of the older schemes is
to the reduce the packet size so that your packets have a greater
chance of making it through the interference intact. This works very
well for periodic interference like microwave ovens and impulse noise.
Just shrink the packet until it fits in between the noise hits. That's
done with the "fragmentation threshold". The catch is that more
smaller packets mean more overhead and therefore slower thruput.
Beam steering and beam forming antennas (a form of MIMO) are other
popular methods of reducing the effects of interference. Just punch a
hole in the antenna pattern in the direction of the interference
source. Read all about it at:
<http://www.ruckuswireless.com/technology/beamflex.php>
The catch is that you can't substitute external antennas.
Frankly, I don't think any of these are going to do very much for your
massive RF polluted environment. The best you can do is install
highly directional antennas on both the access points and clients in
an attempt to not hear the nearby garbage. That's very difficult to
do if the interference is moving or difficult to locate. Perhaps some
RF shielded wall paper:
<http://www.newscientist.com/article.ns?id=dn6240>
More seriously, I've had good results by simply making an effort to
keep the wireless antennas away from outside windows and therefore
away from interference. If the AP's and clients can directly see the
sources of interference, the chances of them having an effect are
reduced (but not eliminated).
>I am clearly new to 802.11 standards and I am reading everything I can
>in order to get a clue.
Warning... reading 802.11 standards will turn your brain to mush.
<http://standards.ieee.org/getieee802/802.11.html>
Don't say I didn't warn you.
Actually, the reviews and tutorials on Tom's Hardware (Small Net
Builder) are quite good and instructive:
<http://www.smallnetbuilder.com/content/view/85/106/>
See the tiny "Browse All Wireless" box menu on the left side.
Also, lots of interesting stuff at Wi-Fi Planet:
<http://www.wi-fiplanet.com/tutorials/>
The problem here is that the writing and information quality are
highly variable.
Also, try the FAQ at:
<http://wireless.wikia.com/wiki/Wi-Fi>
>My other recent post in this group will tell you more about the
>environment that raised all those questions.
Nope. I don't want to go hopping all over the place answering
question fragments and trying to assemble a picture of what you're
doing. Please spend the time to get organized. Disclose what you're
trying to accomplish and EXACTLY what you have to work with in the way
of hardware and software. Add anything that you've done and what
happened. Put it in one question, and you'll get decent answers. If
there are any unrelated questions, keep them separate.
--
Jeff Liebermann
jeffl@comix.santa-cruz.ca.us
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
How to tell if excessive collisions occur? 
Linear Mode
