I can confirm this bug on a Toshiba A110 using Kubuntu Jaunty with a
2.6.28-15-generic kernel.

The driver used is the iwl3945 version 1.2.26ks which loads the lbm-
iwlwifi-3945-2.ucode,  (standard the code ....-1.ucode is loaded but I
used a backport from Ubuntu hoping it would improve the results using a
newer driver). The router used is a Linksys WRV200 (firmeware 1.0.39).

Some remarks:
1. changing the channel in the router to minimize interference with other 
routers improved stability and speed a lot (making it workable).
2. in Windows XP the wireless connection without problems measured speed about 
15 Mb/s (when connection is 54 Mb/s) via WPA2 connection. My Internet 
connection is practically about 28 Mb/s (using a PC with cabled 
networkconnection). Speedtests all via my provider from internet.
3. in Linux the connection direct near the router is about 12 Mb/s (best, 
sometimes only about 7 or 8 Mb/s), at a little distance behind a wall the 
connection speed drops 2,5 - 5 Mb/s while Windows still holds a 12 Mb/s (being 
connected still at 54 Mb/s). I also noticed a big performance drop (compared to 
Windows) with other routers on different locations.
4. when downloading a big iso-file via nfs the connection stops after one or a 
few minutes, sometimes the connection reestablishes and sometimes not. Message 
in syslog: kernel: wlan0: beacon loss from AP f60516b8 - sending probe request.
5. THE ROUTER REALLY GETS IN TROUBLE: when the connection between the router 
and laptop is broken sometimes also the connection between the router and PC 
(with a cable) is broken (a while or permanent). Sometimes the router has to be 
reseted by switching it on and off. This never occurs in Windows.
6. In the past (before Jaunty or even Hardy?) things worked  in earlier 
versions well (or only had minor problems which I did not notice).

My suggestions:
1. I think, as others do, this is a driver problem, I think something quite low 
level.
2. It seems to me somehow the router is flooded when used intensely (with 
errors, fragments of maybe a corrupted WiFi protocol; but that's all beyond my 
knowledge). At least the router gets very busy or confused.
3. It seems to me that there is a connection between this problem and the 
intensity in which the WiFi is used by other WLAN's.
4. I think that disturbing the router is quite a bad thing.

Details about advanced settings of the router (I did not change them): 
Beacon interval 100
DTIM interval 1
Fragmentation threshold 3446
RTS threshold 3447

-- 
[Jaunty] Intel wireless 3945ABG is unstable and disconnects frequently
https://bugs.launchpad.net/bugs/348204
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