Newsgroups: comp.dcom.lans Path: utzoo!henry From: henry@utzoo.uucp (Henry Spencer) Subject: Re: ANC connectors Message-ID: <1988Sep25.014454.426@utzoo.uucp> Organization: U of Toronto Zoology References: <157@ernie.NECAM.COM> <22961@amdcad.AMD.COM> <920@ncrcan.Toronto.NCR.COM> Date: Sun, 25 Sep 88 01:44:54 GMT In article <920@ncrcan.Toronto.NCR.COM> brian@ncrcan.Toronto.NCR.COM (The Super User) writes: >I'm not disputing the idea of putting transceivers every 6 inches or so, >but what kind of havoc does this wreak on the ethernet? I thought the >2.5 meter separation rule was there to prevent/minimize reflections on the >medium? Yes, but reflections matter only if the total end-to-end-and-back propagation delay is some significant fraction of the rise and fall times of the signals involved. If the cable is only a meter or so long, the delay will be only a few nanoseconds -- not significant at Ethernet speeds. Loosely speaking, reflections are the process by which the various parts of the system agree on how they will respond to the signal. When things are close together, agreement is reached more quickly than the signal can change, and essentially one has consensus at all times. (Much of what you get taught in an elementary-electronics course quietly assumes this.) When delays are long, though, consensus breaks down and the responses of distant parts trickle in slowly after a signal change. The trouble is that all this back-and-forth can be confusing to parts in intermediate places. So when delays are long, one has to take precautions to try to ensure that all parts respond the same way, and that any remaining nonuniformities cause minimum disturbance. But most of these precautions are irrelevant if the delays are slight. -- NASA is into artificial | Henry Spencer at U of Toronto Zoology stupidity. - Jerry Pournelle | uunet!attcan!utzoo!henry henry@zoo.toronto.edu