Date: Fri, 25 Feb 2000 11:25:59 -0500 (EST) From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu> To: "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net> Cc: wollman@khavrinen.lcs.mit.edu (Garrett Wollman), cwasser@v-wave.com (Chris Wasser), freebsd-current@FreeBSD.ORG Subject: Re: dc0 wierdness with Compex Freedomline Message-ID: <200002251625.LAA30533@khavrinen.lcs.mit.edu> In-Reply-To: <200002250913.BAA69782@gndrsh.dnsmgr.net> References: <200002241907.OAA27437@khavrinen.lcs.mit.edu> <200002250913.BAA69782@gndrsh.dnsmgr.net>
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<<On Fri, 25 Feb 2000 01:13:51 -0800 (PST), "Rodney W. Grimes" <freebsd@gndrsh.dnsmgr.net> said: >> [I wrote:] >> quite right. In a CSMA/CD medium access protocol, like that used by >> Ethernet, the actual capacity of the link is always(*) somewhat less than >> 100%; the exact value depends on the precise parameters of the >> transmissions at both ends.(**) >> (*)In non-trivial conditions; i.e., when actual work is being done. >> (**)I've heard numbers between 70% and 95%. > And the major set of parameters that effect the higher side of this > number are MTU(Maximum Transmission Unit) and IFG (Interframe Gap) > and the protocol overhead of what ever proto you are using. Nope. Those two values are defined by the protocol, and are not parameters at all. (It's only a parameter if it could conceivably be varied in an experiment.) The relevant parameters are the transmission schedules of the end stations, which are of course dynamic in most real-world applications, but not necessarily so. These can be boiled down into a single number P(coll), which is the probability that two stations will cause a collision by transmitting at precisely the same time. Although this seems unlikely, certain kinds of network protocols can unintentionally synchronize end-stations to the extent of causing pessimal behavior. Back in the mists of ancient time, this was considered one of the major arguments against CSMA/CD protocols and for token-passing protocols, and many academic papers were written about it. How horrifying, that your dearly-bought 10-Mbit/s (or even 3-Mbit/s) Ethernet channel could be wasting 30% of its capacity on collisions! Nowadays, we recognize that the sort of protocols which show pathological behavior are a bad idea for lots of reasons, and the normal operation of TCP and IP over an Ethernet channel results in significantly better than worst-case behavior. -GAWollman -- Garrett A. Wollman | O Siem / We are all family / O Siem / We're all the same wollman@lcs.mit.edu | O Siem / The fires of freedom Opinions not those of| Dance in the burning flame MIT, LCS, CRS, or NSA| - Susan Aglukark and Chad Irschick To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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