Atheros, hardware access layer, collisions

Sam Leffler sam at
Tue Jul 26 18:50:23 GMT 2005

David Malone wrote:
>>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
> 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.

Well "recent" is any 5212 or 5211 card so you're talking about any 
Atheros card except the 5210--which is not so recent.  It may also be 
possible to program the 5210 but given how few of them are in use it's 
hardly worth the effort (and they'll never support WME).

>>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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