From owner-freebsd-hackers@FreeBSD.ORG Thu Jul 21 13:22:25 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 4449116A420 for ; Thu, 21 Jul 2005 13:22:25 +0000 (GMT) (envelope-from samuel.pierson@gmail.com) Received: from wproxy.gmail.com (wproxy.gmail.com [64.233.184.205]) by mx1.FreeBSD.org (Postfix) with ESMTP id E335C43D5D for ; Thu, 21 Jul 2005 13:22:08 +0000 (GMT) (envelope-from samuel.pierson@gmail.com) Received: by wproxy.gmail.com with SMTP id 71so206950wri for ; Thu, 21 Jul 2005 06:22:07 -0700 (PDT) DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=gmail.com; h=received:message-id:date:from:reply-to:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=TetJ7qS/edNCM3SzdusCWTb3yumr8hbLYAGVasEO0HeQwlqUtMpH+vNqApSOM/K8WDlI4Yu63wOKyiSs4cjBoRXf6/IiNnoQk9gzeI2roTPKmTUFvNApK4i43KHfu6pya9cMHeSrzChdBdAZ5auCDODH9JrcayyOeSvTbJ0fVmg= Received: by 10.54.120.12 with SMTP id s12mr516466wrc; Thu, 21 Jul 2005 06:21:37 -0700 (PDT) Received: by 10.54.144.1 with HTTP; Thu, 21 Jul 2005 06:21:37 -0700 (PDT) Message-ID: Date: Thu, 21 Jul 2005 08:21:37 -0500 From: Sam Pierson To: David Malone In-Reply-To: <20050721063606.GA33233@walton.maths.tcd.ie> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline References: <20050721063606.GA33233@walton.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 Reply-To: Sam Pierson 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 13:22:25 -0000 On 7/21/05, David Malone wrote: > 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, tw= o > > 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. >=20 > 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. >=20 > What effect are you trying to achieve? I've got two computers synchronized to send one packet each to this machine sitting between them. This machine responds with a packet to each that it receives (on the application level, not in the control fram= e space), so if there is a collision, I don't want the middle machine to respond and I don't want the senders to retransmit when they don't=20 receive an ACK control frame after they send their data packet (again, this packet is up on the transport layer, so more control frames might be sent). Normal operation (regular connectivity) is not needed on the two senders, so screwing up their retransmission scheme isn't a problem. -Sam