Re: Atheros, hardware access layer, collisions

From: David Malone (dwmalone_at_maths.tcd.ie)
Date: 07/26/05

  • Next message: Scott Long: "Re: how to use the function copyout()" 
    To: Sam Pierson <samuel.pierson@gmail.com>
Date: Tue, 26 Jul 2005 15:13:27 +0100

    > I just had a lengthy discussion with a couple of guys about the 802.11
    > protocol. One had said that the random delays inserted before
    > transmission was one of the *IFS delays (can't remember which
    > now), and that it was a standard 802.11 number, not a random
    > delay.

    Yep - in 802.11b CWmin is fixed at 32 and the random number is
    chosen between 0 and CWmin-1 (unless you have a collision). The
    recent Atheros cards support adjusting CWmin as part of their
    WME/802.11e support.

    > The thing he said was that if carrier sensing "sensed" that the channel
    > was busy, it would not decrement the CW, effectively NOT transmitting
    > this packet until the channel is clear.

    That's correct, but it probably takes a few microseconds for the
    carries sense to kick in (if there wasn't a delay there would
    be almost no need for the random backoff). That's why you'll
    also have to have your transmissions synchronised very closely.

    > Is the carrier sensing something done in the HAL, or is it embedded
    > in the hardware itself?

    I'm afraid I don't know - Sam might.

            David.
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  • Next message: Scott Long: "Re: how to use the function copyout()"

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