Path: utzoo!attcan!uunet!seismo!ukma!rutgers!uwvax!umn-d-ub!umn-cs!peiffer
From: peiffer@umn-cs.CS.UMN.EDU (Tim J. Peiffer)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet max length problems
Keywords: repeaters? ethernet too long
Message-ID: <16069@umn-cs.CS.UMN.EDU>
Date: 22 Sep 89 02:50:55 GMT
References: <634@elan.elan.com> <2829@orion.cf.uci.edu>
Reply-To: peiffer@umn-cs.cs.umn.edu (Tim J. Peiffer)
Organization: CSci Dept., University of Minnesota, Mpls.
Lines: 30

We had a problem here that was interesting, and easy to fix, but I am confused
at what I did not see.

Why did our repeater not partition under a large impedance mismatch?  I expect
that the repeater should have seen many collisions...  We have segmented our 
net into several lines where the aggregate lengths are less than 600 ft.  In
the problem we had, one of our people were installing some portions of a
cluster and accidentally installed a 75 ohm at the end.

I expected that signals would be broadcasted equally across the back plane to
the indvidual repeater modules, and would be echoed back as a collision.  
Since the mismatch was not really severe, and the aggregate length is ~435ft,
was it that the mismatch related reflection was absorbed by the lossyness of 
the thin-net, and not seen by the repeater?  The PC's and the MacII's failed
to function as expected, but the repeater failed to partition.  I am a bit
confused.  I really do not think that the repeater is a problem, only my
understanding of what did not happen.


3com Multiconnect
+---+
|   | +-->> 50 ohm terminator
| o---|
|   | +---> 250 ft >-> 3x25ft >-> 18x6ft between MacII >-> PC/AT -> 75 ohm
+---+

Tim Peiffer			peiffer@cs.umn.edu  or
Comp Science Dept		...!rutgers!umn-cs!peiffer
University of Minn
Mpls, MN