Date: Wed, 03 Feb 1999 02:59:55 -0800 From: David Greenman <dg@root.com> To: FreeBSD Questions <questions@FreeBSD.ORG> Subject: Re: Excessive collisions on Ethernet Message-ID: <199902031059.CAA01058@implode.root.com>
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> > Anything less than 50% collision rate is okay and doesn't reduce the > > throughput significantly. All of the numbers below are in the noise. > > You mean it's the noise that's causing the collisions? > > Seriously, it's clear that it's not affecting the performance, though > that surprises me; how long does a board hold off after a collision? > I haven't been able to find that info anywhere, but I once worked for > a nameless computer manufacturer whose net had about 50% collisions, > and the throughput was terrible. > > The real question was: why is this happening? I still suspect that > it's trying to tell me something, but I haven't a good feeling for > what. The collisions are caused by the packet acks in the opposite direction. Collisions are usually detected very early in the transmission and the backoff is usually very short. Modern hardware leaves no room for the acks to slip in without colliding, so you get a lot more collisions. A high collision rate with a small number of hosts involved is usually nothing to worry about. It's when you have a lot of machines all trying to talk that you get into trouble (...but then the collision rate in that case can easily be several hundred percent). In the nameless comp manufacturer case above, they probably had other problems with their net that caused CRC errors and dropped packets...very different from collisions which are retried very quickly and for up to 16 times before the packet is dropped. -DG David Greenman Co-founder/Principal Architect, The FreeBSD Project To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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