On Thu, 21 Sep 2006 18:03:43 -0400, "Paul"
<paule-nospam@mindspring.com> wrote:
>We've been asked to provide wireless laptop access for a group of people
>attending a conference. All users will be in one conference room approx
>30ft x 50ft. Estimate 50 wireless users. They will be doing medical
>imaging type application over the web (i.e. needing lots of bandwidth).
>We're thinking of putting 3 Linksys WAP54G's or WRT54G's in the room roughly
>in a triangular pattern.
Argh. 50 full time users is going to require at least 3 access
points. Try to stagger your demos and downloads or you're going to
kill the backhaul. Better yet, get some kind of web cache device and
buffer the traffic locally. If the web content can be downloaded,
save it on a local web server and have the users grab it from there
instead of from the internet.
I once did a system like this for a seminar at a local hotel. Only 30
attendees but still a big mess. Just about everything went wrong. The
hotel was using a mix of 802.11b and 802.11g access points. Everything
on one SSID. The backhaul was a 1500/256Kbit/sec DSL line. As soon as
everyone turned on their laptops, most of them decided that this was
the perfect time to update their virus scanner, run Windoze update,
automatically check for email, and synchronize their calendars. The
DSL was totally staturated for at least 30 minutes until the traffic
settled. There may have been a virus or worm running among the
attendees, but it probably gave up when faced with the constipated DSL
pipe.
The mix of access points lacked "client isolation" or "ap isolation"
features. This is to prevent clients from connecting to each other or
attacking each other with worms. Some had it, some didn't. That
resulted in users creating a few ad-hoc networks, which generated LOTS
of interference as they passed home movies between attendees instead
of paying attention to the speakers.
With all the same SSID traffic was not exactly balanced. The users
with 802.11b radios really slowed down the 802.11g access points, but
they couldn't turn off 802.11b compatibility because there were some
users with 802.11b only hardware.
During the seminar, one of the speakers passed out a cdrom that
included online registration in order to activate the demo software.
Everyone plugged in about the same time causing the router to detect
30 connections to the same IP address. The hotel IDS declared this to
be spam or an attack (I forgot which) and blocked furthur connections
to this IP. Most of the registrations failed for about 30 minutes
until someone disarmed the IDS.
Part of the demo included streaming media from the internet. 30
laptops all went to the same web pile, from the same routeable IP, and
caused the site dispensing the streaming media to declare that it was
being somehow attacked. It turned off for about 10 minutes causing
substantial head scratching.
The worst problem was that several idiots (including me) decided to
"do something" about the traffic problem. Much of our tweaking with
QoS and blocking sites did more damage than good. Repeated access
point reboots along with corresponding client disconnects, were not
very popular. We would have been better off leaving everything alone
than trying to twiddle and tweak blindly.
Are you sure you want to do this?
>Should we put each one on a different channel (1, 6, and 11) ?
Yes. 3 access points, 3 channels.
>If they all have the same SSID's, what would be the odds of relatively even
>user distribution across the APs in a room that small?
In the past, I've always suggested using the same SSID so that it's
easier for users to roam. I changed my mind. The typical wireless
client is stupid. There's no algorithm for selecting the strongest or
best access point. Most will pick the first one it hears and stick
with it almost forever. The signal could turn to garbage but the
client will not go searching for a better access point until it
literally disconnects. This is not true for all clients. Intel
Proset is rather smart about roaming for the best access point.
However, this is the exception, not the rule. However, even with
Intel Proset, you're still not going to get an even distribution of
users per AP. If one of your AP's is a bit stronger or more
aggressive than the others, it could easily get ALL the users to
connect to it. Seperate SSID's may take a bit of explaining, but at
least you won't have as much crowding.
>Perhaps we should use 2 APs with 2 different SSID's and tell people on one
>side of the room to connect to network A and the other side to connect to
>network B?
Three AP's with 3 SSID's on 3 different channels (1, 6, and 11).
Good luck and try not to repeat my mistakes.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
jeffl@comix.santa-cruz.ca.us
#
http://802.11junk.com jeffl@cruzio.com
#
http://www.LearnByDestroying.com AE6KS
Best way to handle high usage in a small area 
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