Atheros, hardware access layer, collisions

David Malone dwmalone at
Tue Jul 26 14:13:30 GMT 2005

> 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.


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