Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!bloom-beacon!gatech!hao!oddjob!nucsrl!morrison
From: morrison@nucsrl.UUCP (Vance Morrison)
Newsgroups: comp.dcom.lans
Subject: Quest: Ethernet cable restrictions?
Message-ID: <3880004@nucsrl.UUCP>
Date: 8 Feb 88 14:50:22 GMT
Organization: Northwestern U, Evanston IL, USA
Lines: 35


Hello,

	I have a question for the ethernet experts.  I have heard some
people mention that the restrictions on segment lengths and transceiver
placement (on the black lines), and I really do not see the rational
behind it.

	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.

	First of all, we are not sending a single frequency down our
transmission line, for ethernet, the signals can be anywhere in the
range of 5-10MHZ (because of the manchester encoding), depending on
the bit pattern being transmitted.  

	Now certainly we will have reflections off couplers, transceivers
and any other discontinuity, but whether these reflections add or cancel
depends on the bit pattern, and thus is unknown.  The ONLY way to 
insure that these reflections have no adverse effect is to insure that
they are small (not by hoping they cancel).  Thus it seems to me that
there is no benefit in the cabling restrictions.

	If anyone knows that rational, please let me know, because
the restrictions do not make sense to me.

					Vance Morrison
					morrison@accuvax.nwu.edu
					morrison@nuacc.bitnet
					morrison@northwestern.arpa

PS. Along the same lines could someone please tell me the reasoning
behind the minimum transceiver distance.