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
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.
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.
Hardware.
Sam
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