Atheros, hardware access layer, collisions

David Malone dwmalone at
Thu Jul 21 06:36:09 GMT 2005

On Wed, Jul 20, 2005 at 10:03:49PM -0500, Sam Pierson wrote:
> I think there is still collision detection happening on the hardware 
> level.  I think I have to disable the retransmission of frames 
> which are lost due to collisions.  Here's my reasoning:  In the lab, two
> hosts are sending packets to the middle guy at the same time.
> After examining the traffic on the middle guy, one packet will
> arrive before the other one (sometimes in different order) and
> the second packet comes 500-1200us after the first.  From this,
> I think some retransmission is happening because of collision,
> since the results are seemingly random.

Since introducing random delays before transmission and using carrier
sensing are basic features of the 802.11 MAC, I'd be suprised if
you can stop the hardware doing it. To reduce the effects as much
as possible, you could try trying to reduce the number of retransmission
attempts and changing the cwmin parameter to be small. Even if you
do this, you'll still need to transmit the packets quite close to
one another (probably within 20us) to avoid the carrier sense stuff
kicking in.

What effect are you trying to achieve?


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