Date: Mon, 29 Jan 2001 19:40:59 -0600 From: David Kelly <dkelly@hiwaay.net> To: Tony Landells <ahl@austclear.com.au> Cc: "John Bolster" <j.bol@gte.net>, "Freebsd-Questions@Freebsd. Org" <freebsd-questions@FreeBSD.ORG> Subject: Re: collisions Message-ID: <200101300140.f0U1exG06901@grumpy.dyndns.org> In-Reply-To: Message from Tony Landells <ahl@austclear.com.au> of "Tue, 30 Jan 2001 09:05:58 %2B1100." <200101292205.JAA20561@tungsten.austclear.com.au>
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Tony Landells writes: > As Kevin said, this is all pretty normal. > > To give you some indication, most people consider anything up to > between 5 and 10% collisions (compared to output packets) normal. > > Although, as one network course instructor said--too many collisions > is when the users start complaining about slow network performance. To give you some kind of indication, this person considers anything under 150% to be normal and 200% is only an eyebrow lifter. As to "too many collisions is when the users start complaining about slow network performance", users will complain about any numbers they can find. Such as a network statistic labeled "collisions"... :-) There are two kinds of collisions, early and late. Those reported by "netstat -in" are "early". Some ethernet chipsets don't report early collisions at all. Late collisions are very bad and reflect serious network problems. When your ethernet card is in half duplex mode and transmitting it is also listening. If it fails to copy the bits its sending then it knows there is a problem. If within the first 64 octets of an ethernet packet then this is an "early collision". Responds by backing off a random time and waiting for a clear net to try again. An early collision simply means two or more hosts decided to transmit at the roughly the same time. The speed of light is what limits the length of a network segment. It takes a while to get from one end to the other. So a time of 64 octets is given for overlap. Anything beyond 64 octets is a late collision indicating your network is too long or a host's network stack is badly broken. File transfer between two fast hosts on a half duplex network will result in a collision for most every packet. The sending host will be dumping packets on the wire tail-up with no pauses between. The receiving host (being another fast computer) will calculate checksum on incoming packet, queue an ACK, and try to send it on the next time frame. But the sender is already sending the next data packet. Collision. One for most every packet. An SGI employee used to have a web page addressing the issue with real mathematical analysis but I've lost the reference and not been able to find it again. The number that comes to mind is for collision rate less than 100% (or 150% or 200%, I forgot what threshold) the 10baseT network thruput dropped only 8%. -- 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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