Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!decwrl!sgi!rpw3@rigden.wpd.sgi.com From: rpw3@rigden.wpd.sgi.com (Rob Warnock) Newsgroups: comp.dcom.lans Subject: Re: Do DELNI's cause collisions? Message-ID: <61269@sgi.sgi.com> Date: 31 May 90 07:25:43 GMT References: <145340@felix.UUCP> <11702@blia.BLI.COM> <1990May30.181740.14203@agate.berkeley.edu> Sender: rpw3@rigden.wpd.sgi.com Reply-To: rpw3@sgi.com (Rob Warnock) Organization: Silicon Graphics, Inc., Mountain View, CA Lines: 39 In article <1990May30.181740.14203@agate.berkeley.edu> cliff@violet.berkeley.edu (Cliff Frost) writes: +--------------- | In article <11702@blia.BLI.COM> ted@blia.BLI.COM (Ted Marshall) writes: | >If a DELNI is in "global" mode and the transceiver connected to the global | >port is wired for SQE test (heartbeat), the heartbeat signal is passed down | >to ALL of the local ports... | But the 9.6us gap is per transmitter. You've got one station that has just | sent a packet, and it won't try to send another for 9.6us, but you've got | 7 (or 15) other stations that don't know anything about the first station's | transmission and may decide to transmit themselves. One of them could | easily see the heartbeat "blip" as a false collision. +--------------- Such as a repeater, which is *required* to do "receiver-based" collision detection, so it can "jam" the other side of the net correctly whenever there's a collision on the front side. Spurious SQEs look like collisions to repeaters, which then proceed to jam up the net. (*Ugh!* *Gasp!* *Gag!*) +--------------- | From all the responses I got back to my question I believe this is a | compelling reason NOT to use SQE with multiports. Maybe this is why | multiports aren't defined in the IEEE specs. +--------------- What's really disgusting is that it takes maybe all of $0.37 worth of parts per port to direct the SQE back to the one port that sent the transmission. All you need is a (non-critical) one-shot and an "AND" gate per port. Other vendors (e.g. Cabletron) can do it; no reason why DEC couldn't have. [Except... they didn't.] -Rob ----- Rob Warnock, MS-9U/510 rpw3@sgi.com rpw3@pei.com Silicon Graphics, Inc. (415)335-1673 Protocol Engines, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94039-7311