Path: utzoo!yunexus!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!cs.utexas.edu!usc!ucsd!ames!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: <61353@sgi.sgi.com> Date: 1 Jun 90 04:12:46 GMT Article-I.D.: sgi.61353 References: <145340@felix.UUCP> <2230087@hprnd.HP.COM> <1990May31.123149.2718@hellgate.utah.edu> Sender: rpw3@rigden.wpd.sgi.com Reply-To: rpw3@sgi.com (Rob Warnock) Organization: Silicon Graphics, Inc., Mountain View, CA Lines: 41 In article <1990May31.123149.2718@hellgate.utah.edu> haas@cs.utah.edu (Walt Haas) writes: +--------------- | In article <2230087@hprnd.HP.COM> pat@hprnd.HP.COM (Pat Thaler) writes: | >It is correct that heartbeat should not be sent to repeaters. | Could you clarify this? Clearly the problem with heartbeat to multiports | is that the heartbeat signal will be repeated inappropriately to N-1 ports. | In the case of a repeater connected directly to a transceiver which generates | SQE, the SQE signal should only appear at the appropriate time as a function | of the packet being forwarded through the repeater. +--------------- Think a little more about the configuration. Suppose some other station on the DELNI sends a packet, which the DEMPR dutifully repeats (and by the way later gets a SQE at the end of the *transmitted* packet on the other side, which is just fine) and then sees a SQE coming in the *receive* side [due to the brpken DELNI] immediately behind this packet it just received and repeated! "Oops! Collision. Gotta jam!", sez he, and the jam he sends out may or may not cause (some) receivers on one or the other sides to barf on the perfectly good preceding packet, because the jam came in the inter-packet interval. (I have observed this "partial" failure mode.) Repeaters must do receiver-based collision detection, to avoid repeating garbage fragments. Since the data is AC-coupled, a collision may cause parts of bits to "cancel", breaking up the collision interval into micro- intervals of carrier/no_carrier. That's why "carrier-detect" in the station is defined to be the "OR" of carrier-detect and collision. Getting a SQE while *receiving* (on the "front" or DELNI side) is not a good thing. No, the DELNI does it wrong, and DEC is perfectly correct in telling you not to put a DEMPR behind a DELNI. You *could* successfully put a DEMPR behind a "multiport" that did it right... [other vendors' names ommitted] -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