From owner-freebsd-hackers@FreeBSD.ORG Thu Jul 21 06:36:09 2005 Return-Path: X-Original-To: freebsd-hackers@freebsd.org Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 34BD316A41F for ; Thu, 21 Jul 2005 06:36:09 +0000 (GMT) (envelope-from dwmalone@maths.tcd.ie) Received: from salmon.maths.tcd.ie (salmon.maths.tcd.ie [134.226.81.11]) by mx1.FreeBSD.org (Postfix) with SMTP id 63A0A43D45 for ; Thu, 21 Jul 2005 06:36:07 +0000 (GMT) (envelope-from dwmalone@maths.tcd.ie) Received: from walton.maths.tcd.ie ([134.226.81.10] helo=walton.maths.tcd.ie) by salmon.maths.tcd.ie with SMTP id ; 21 Jul 2005 07:36:06 +0100 (BST) Date: Thu, 21 Jul 2005 07:36:06 +0100 From: David Malone To: Sam Pierson Message-ID: <20050721063606.GA33233@walton.maths.tcd.ie> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.6i Sender: dwmalone@maths.tcd.ie Cc: FreeBSD Hackers Subject: Re: Atheros, hardware access layer, collisions X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 21 Jul 2005 06:36:09 -0000 On Wed, Jul 20, 2005 at 10:03:49PM -0500, Sam Pierson wrote: > I think there is still collision detection happening on the hardware > level. I think I have to disable the retransmission of frames > which are lost due to collisions. Here's my reasoning: In the lab, two > hosts are sending packets to the middle guy at the same time. > After examining the traffic on the middle guy, one packet will > arrive before the other one (sometimes in different order) and > the second packet comes 500-1200us after the first. From this, > I think some retransmission is happening because of collision, > since the results are seemingly random. Since introducing random delays before transmission and using carrier sensing are basic features of the 802.11 MAC, I'd be suprised if you can stop the hardware doing it. To reduce the effects as much as possible, you could try trying to reduce the number of retransmission attempts and changing the cwmin parameter to be small. Even if you do this, you'll still need to transmit the packets quite close to one another (probably within 20us) to avoid the carrier sense stuff kicking in. What effect are you trying to achieve? David.