Path: utzoo!mnetor!uunet!husc6!bu-cs!kwe
From: kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England))
Newsgroups: comp.dcom.lans
Subject: Re: Quest: Ethernet cable restrictions?
Message-ID: <19816@bu-cs.BU.EDU>
Date: 10 Feb 88 16:36:11 GMT
References: <3880004@nucsrl.UUCP>
Reply-To: kwe@buit13.bu.edu (Kent England)
Followup-To: comp.dcom.lans
Organization: Boston Univ. Information Tech. Dept.
Lines: 47

In article <3880004@nucsrl.UUCP> morrison@nucsrl.UUCP (Vance Morrison) writes:
>
>	Oh, they mumble something about reflections off the transceivers
>and the couplers, but this to be is a simple minded answer.  I happen
>to be an EE and my understanding of transmission lines makes the answer
>to the cabling restrictions far from obvious.
>
	Let's see if I can help answer this without mumbling.  I am an
EE and I studied transmission lines and antenna theory, but I don't
want to try to recall the theory and fill the page with differential
equations.  I don't think that's what you want anyway.

	First, as engineering students we studied analog signals or
modulated analog signals and we didn't worry about the instantaneous
time histories of these spectra.  Ethernet uses baseband (square wave)
signals with spectra that vary instantaneously, but for the purposes
of understanding the reflections and interference patterns we can use
average spectra or instantaneous spectra to good effect.  By that I
mean take a sequence of a few bits in a packet, figure the spectrum,
and see what happens on a given transmission line (the Ethernet
cable).  In general, there will be a 1st, 3rd, and 5th harmonic for a
uniform square wave.  These will set up varying standing waves on the
cable.  If you are located at an interference point (which, of course,
varies in location instantaneously) your spectrum will be corrupted
and your received waveform will not be "square" and, according to the
rules for decoding Manchester signals, you might not find the right
spot for sampling a stable level.  In addition, the rules for
detecting collisions depend on average (I don't know how the average
is computed) power levels at your transceiver.  If the instantaneous
power levels received average out a "quantum" higher than expected due
to the instantaneous interference received over the averaging
interval, a collision will be declared and the packet aborted.
	Now this instantaneous behavior must hold true on average for
the general rules about cable lengths and transceiver spacings to be
valid.  You only have to corrupt a bit or two to lose the packet, so
if the instantaneous interference point location wanders over your
location on the cable, you will experience bit errors.  Perhaps a DEC
engineer can explain how their lab tests prove out their
recommendations on cabling.
	Otherwise, you can try systematically violating the rules and
then we can put a TDR and a LAN analyzer on your net and see what's
what.  :-)
	This explanation wouldn't help you do RF R&D, but it's only
intended to be a rough model (which is still allowable in the spirit
of scientific inquiry).  Is it helpful without being too inaccurate?

	Kent England