Slow wireless

Bryan Kadzban bryan at kadzban.is-a-geek.net
Mon Oct 2 18:28:31 EDT 2006


Greg Stark wrote:
> However they're all on channels 6 and 11 so I thought I was safe with
> channel 3.

Surprise!  You're not...  ;-)

> The other networks are 54Mb/s networks, does that affect things?

It doesn't make any difference AFAIK, no.  They're still putting out
energy in the same frequency band as you are, and that's causing some
interference.  (Whether that's what's causing the low speeds is another
question, though.  I'd suspect it is, but I don't know for sure.)

> And 22Mhz bandwidth, does that mean 11Mhz on either side so at least 
> 5 channels separation?

Yep, that 11MHz number is (one) problem with spread-spectrum signalling.

> So only channels 1, 6, and 11 are actually useful?

Yep.  Well, some people claim 1, 4, 7, and 11, but that's pushing it.

And you'll get even better results if you can get rid of 6.  There isn't
a lot of interference between 1 and 6, or between 6 and 11, but there is
some.  (The interference more-or-less exponentially backs off as you
move away from the center frequency.)

(I've seen a paper by some guy that claims much better throughput if he
actually uses the *same* channel as surrounding APs, though.  IIRC the
reason for the better throughput was that if he uses the same channel,
his card can use RTS/CTS to negotiate airwave usage with the other
clients.  There may have been something with his card seeing 802.11
headers or decoding the preamble from other clients' frames, as well.)

> I wonder what else could be causing interference though. The router's
> case is off, can the motherboard or other cards cause interference?

It may be a microwave.  (It's probably more likely a microwave than a
motherboard, unless the CPU is 2.4GHz and its clock is radiating a lot
of energy.)  Microwaves use the same approximate frequency range, and
AFAIK there isn't much on a motherboard that's running at that high a
frequency.  (Other than the CPU.)
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