b43 terribly slow

[Stefan Monnier <monnier@iro.umontreal.ca>]

I'm using the b43 driver with a OpenWRT's 2.6.25 kernel (which includes
changes from compat-wireless).  I'm happy to say that it works on my WPA
network, but the bandwidth it offers is cometimes terrible.
(like less than 50kB/s over NFS).

The affected machine (an asus WL-700gE) is not overloaded at all, and
while it serves those files over NFS, `top' says it's 94% idle.

Has anybody seen this or have some idea what might be the trouble?

It's not always like that, so it might be due to interference, but 
I have an old WL-500g right next to this box (i.e. similar b43
hardware, but running a 2.4 kernel with broadcom's proprietary driver),
and I always get around 700kB/s bandwidth, so the problem is likely to
be on the driver side.

This afternoon, the problem lasted for a while, so I experimented a bit.
While investigating, I noticed that a likely culprit is that `iwconfig'
says it's rate is set to 1M.  So I tried to change the rate.

Typical link quality during this test:
Link Quality=70/100  Signal level:-40 dBm  Noise level=-63 dBm
tho I've seen link quality reported as high as 105/100
(and as low as 60/100).

- with a 1M transfer speed, I see a bandwidth around 80kB/s
- setting the rate manually to 2M improved the transfer speed to
  around 140kB/s.
- setting the rate to 5.5M improved the speed to around 280kB/s,
  but with periods where nothing gets through.
- with 11M, the periods where nothing get through are much more
  significant, so connections are becoming very difficult.
- with 54M, same thing.
- if I "iwconfig wlan0 rate auto" it goes back down to 1M fairly quickly.

Any idea what might be going on?  Or what I could do to try and figure
it out?  It seems that the rate choice is a bit conservative (2M works
as well as 1M, just faster), but it's probably not the culprit.
So could someone explain to me why I see these drops in connectivity
even though the WL-500g that sits right next stays connected "full
blast" (at least 500kB/s, so at least 11M I guess)?


        Stefan