On Wed, 17 Nov 2004 01:03:27 -0500, Rôgêr <(E-Mail Removed)> wrote:
>Correct me if I'm wrong (no big leap in logic there) but it seems as
>though you are discussing more theoretical connections, even if you
>actually were generating connections. I mean, 253 real world users
>actually attempting to use the net at the same time? No hidden node
>problems? No extremely high collisions? All this without polling software?
I was generating 253 simulated connections (associations) with
corresponding DHCP delivered IP addresses. However, there was only
traffic from one IP address at a time. That's not a very realistic
test for real world conditions. Conventional wisdom and rule of thumb
is that the average WISP access point can handle per channel:
100 casual users checking email and light web browsing.
10 business users doing whatever business users do.
1 teenage Napster/Kazaa/Bearshare/Morpheus/whatever user.
As you may notice, it's not the number of connections, but what
they're doing (i.e. traffic pattern).
I've done traffic load tests in a laboratory environment using about
40 computahs. It was quite a mess and I spent much of my time dealing
with issues that had nothing to do with wireless loading. The plan
was to optimize some timing, flow control, and retrans algorithms. I
spent the first hour dealing with two of the machines that apparently
had a worm and were generating lots of junk traffic. The next hour
was spent changing the SSID to something that wasn't duplicated
elsewhere in the lab. Another 30 minutes or so was spend disarming
the screen blankers and power save features that kept shutting down
the laptops and PC's at inconvenient times. This type on nonsense
went on all afternoon. I finally got about 2 hours of testing done.
Not enough to do a decent job, but enough to check for obvious
problems. No real problems. Bandwidth distributed itself equally
between active computahs as expected.
I also got the chance to test the effectiveness of CTS/RTS flow
control. Didn't make any difference in aggregate thruput since there
were no hidden nodes in the lab. Everyone could hear everyone else so
there were no collisions. So much for that test.
I once did some modeling to see at what point polling (i.e. Karlnet
TurboCell, 802.11e (draft spec), 802.11 PCF (point coordination
function, etc) schemes were beneficial. My seat of the pants
guess(tm) is that about when about 20% of the active clients qualify
as hidden nodes, polling is highly beneficial. It doesn't take many
collisions to screw things up. Probably somewhat beneficial at
smaller number of hidden nodes. In general, outdoor networks should
use polling (or flow control) while indoor networks can get away with
CSMA/CA. For point to multipoint, where the clients all have
directional antennas, flow control is manditory and polling is highly
benificial, since none of the clients can hear any of the other
clients.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
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