From owner-freebsd-current Fri Feb 25 1:14: 8 2000 Delivered-To: freebsd-current@freebsd.org Received: from gndrsh.dnsmgr.net (GndRsh.dnsmgr.net [198.145.92.4]) by hub.freebsd.org (Postfix) with ESMTP id 9259537BEBF for ; Fri, 25 Feb 2000 01:14:03 -0800 (PST) (envelope-from freebsd@gndrsh.dnsmgr.net) Received: (from freebsd@localhost) by gndrsh.dnsmgr.net (8.9.3/8.9.3) id BAA69782; Fri, 25 Feb 2000 01:13:52 -0800 (PST) (envelope-from freebsd) From: "Rodney W. Grimes" Message-Id: <200002250913.BAA69782@gndrsh.dnsmgr.net> Subject: Re: dc0 wierdness with Compex Freedomline In-Reply-To: <200002241907.OAA27437@khavrinen.lcs.mit.edu> from Garrett Wollman at "Feb 24, 2000 02:07:40 pm" To: wollman@khavrinen.lcs.mit.edu (Garrett Wollman) Date: Fri, 25 Feb 2000 01:13:51 -0800 (PST) Cc: cwasser@v-wave.com (Chris Wasser), freebsd-current@FreeBSD.ORG X-Mailer: ELM [version 2.4ME+ PL54 (25)] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG > < said: > > > > The theoretical maximum for 100BaseT-FDX (which is 200Mbps) is 25MB/s > > (megabytes per second), 100BaseT-TX is 12MB/s [FYI: Mbps->MB/s you divide > > by 8] I realize my punctuation may be off, but there you are. > > Assuming you mean ``100BASE-T (half duplex)'' here... This is not Just to put a final nit pick on this... 100BASE-T is not defined by the standards. 100baseTX, 100baseFX, and 100baseT4 are. > 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.(**) > > -GAWollman > > (*)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. For those really interested in the nuts and bolts of the calculations for this I refer you to ``Switched, Fast and Gigabit Ethernet Third Edition'' Robert Bryer & Sean Riley, ISBN 1-57870-073-6 chapter 7, ''Bandwidth: How Much Is Enough?''. Also chapter 2 in the same book for some of the data used below and for a more indepth explination of what all these things are, for that matter, 802.* :-) Some interesting numbers: (In a format that might help some people understand where the magic numbers 1500, 1518 and 1538 come from in ethernet documentation (though few folks but ethernet framer geeks would ever deal with the 1538 number :-)). Layer 2 maximal data rate: (ie, raw ethenet packets, no protocol) data / ( IFG + ( Preamble + SFD ) + DA + SA + type + data + CRC ) = 1500 / ( 12 + ( 7 + 1 ) + 6 + 6 + 2 + 1500 + 4 ) = 97.53% 1500 / ( 12 + ( 8 ) + 1518 ) = 97.53% 1500 / ( 1538 ) = 97.53% So infact the Layer 2 maximal data rate of 100BaseTX is 97.5929Mb/s or 12.1912MB/s. I'll leave the Layer 3 to 7 calculation up to the reader, as I am a hardware geek and I showed you how to do the calculations at the hardwire layer, you software geeks can go figure out the overhead for the software layers :-). Please note that ``maximal'' is a key word in the above discussion, so please don't tell me that not all frames are 1500 bytes of data, because if they are not 1500 bytes of data you didn't solve the problem for the maximal. The first hardware that I know of that was ever capable of doing the 95.53% at the link layer was the i82586 by Intel on 10base, and you had to lock the sucker up in continues ring search mode with a good shared memory access mechanism to make it hit that. You needed very large chunks of shared memory (MB's) to sustain a very long burst of traffic at wire speed. You can not use ethernet in a shared mode and achive these numbers (point to point or effectivly point to point via switching only). Anyone quoteing ethernet or any other MAC layer data rates without atleast as much detail as above is practicing abuse of statistics and not presenting hard factual data which should be treated as such. -- Rod Grimes - KD7CAX @ CN85sl - (RWG25) rgrimes@gndrsh.dnsmgr.net To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message