Re: Atheros, hardware access layer, collisions

From: David Malone (dwmalone_at_maths.tcd.ie)
Date: 07/21/05

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    Date: Thu, 21 Jul 2005 07:36:06 +0100
To: Sam Pierson <samuel.pierson@gmail.com>

    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?

            David.
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