Date: Tue, 26 Jul 2005 15:13:27 +0100 From: David Malone <dwmalone@maths.tcd.ie> To: Sam Pierson <samuel.pierson@gmail.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Atheros, hardware access layer, collisions Message-ID: <200507261513.aa10533@salmon.maths.tcd.ie> In-Reply-To: Your message of "Mon, 25 Jul 2005 22:05:34 CDT." <d9204e4c05072520052082b27@mail.gmail.com>
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> 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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