Date: Mon, 17 Mar 2003 09:47:05 -0600 From: David Kelly <dkelly@hiwaay.net> To: Olivier Dony <odony@student.info.ucl.ac.be> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: Too many collisions on network? Message-ID: <20030317154705.GA52181@grumpy.dyndns.org> In-Reply-To: <025601c2ebf5$5abc25f0$1502a8c0@blacktrap.net> References: <005d01c2ebcb$82b343b0$1502a8c0@blacktrap.net> <20030316152335.GA1434@zi025.glhnet.mhn.de> <002201c2ebd3$92d0a7d0$1502a8c0@blacktrap.net> <3E74B8BC.4030009@potentialtech.com> <025601c2ebf5$5abc25f0$1502a8c0@blacktrap.net>
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On Sun, Mar 16, 2003 at 08:50:47PM +0100, Olivier Dony wrote: > > Any ideas? And thanks again, I've learned a lot so far with your kind help :-) Once Upon A Time when SGI was great and not "sgi" one of their employees posted an excellent article on "collisions" on his personal company home page. The gist of that article was to the effect, "collisions are not bad" and went on to prove it. That up to 200% collision rate will only hurt total wire thruput by 12%. "Collisions" are unavoidable on half-duplex ethernet because when a host starts transmission it doesn't know if anyone else is doing the same. So it copies its data off the wire and if someone else is transmitting at the same time the data is corrupted, both hosts notice and back off and apply a random wait before retrying. Some, not all, NIC hardware click a collision counter. The collision occurs in the first 64 octets of the transmission so very little wire time (bandwidth) is lost. Problem with modern fast hardware is one can receive a packet and queue the ACK in time for the next ethernet "opening" and collide with the next incoming packet of the file transfer (or whatever). So with a large file transfer if you DON'T have 100% collision rate, your hardware isn't as fast as you thought. A quick glance at Olivier's data showed only 30% to 50% collision rate. Nothing to worry about unless you know you are connected to a full duplex hub. What you should worry about is a "late collision", which is the same thing but happening after the first 64 octets. Late collisions are due to defective hardware, software, or the local ethernet is too long for the current value of the speed of light. Notification of late collisions is routed thru the kernel log. -- David Kelly N4HHE, dkelly@hiwaay.net ===================================================================== The human mind ordinarily operates at only ten percent of its capacity -- the rest is overhead for the operating system. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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