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