Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!ut-emx!spurgeon
From: spurgeon@ut-emx.UUCP (Bud Spurgeon)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet max length problems
Keywords: repeaters? ethernet too long
Message-ID: <18908@ut-emx.UUCP>
Date: 28 Sep 89 14:15:11 GMT
References: <634@elan.elan.com> <2829@orion.cf.uci.edu> <16069@umn-cs.CS.UMN.EDU>
Reply-To: spurgeon@emx.UUCP (Bud Spurgeon)
Organization: UTexas Computation Center, Austin, Texas
Lines: 25

In article <16069@umn-cs.CS.UMN.EDU> peiffer@umn-cs.cs.umn.edu (Tim J. Peiffer) writes:

>Why did our repeater not partition under a large impedance mismatch?  I expect
>that the repeater should have seen many collisions...  

If your wire had been completely unterminated the repeater would have
stayed partitioned just fine.  But as soon as the repeater can get ONE
good packet onto the failing wire, it will stop partitioning.  Then
after >32 consecutive packet failures it will partition again (but
still keep trying to send packets onto the partitioned segment) and if
the segment isn't a *complete* lose it will succeed in transmitting a
packet onto it and stop partitioning.

Depending on how bad your wire mismatch/damage is this can happen
quite rapidly and make it appear that the repeater hasn't partiioned
the segment at all.  As far as the hapless users on the "good" side of
the repeater are concerned, it might as well not have tried since the
effect is tons of collisions as the repeater jams each failing packet
and keeps counting till >32 is reached, then goes back into
transmission after one good packet gets through, then starts jamming
again, over and over.

The moral seems to be that auto-partitioning is good for isolating
nice solid wire failures, but if your failure is marginal then the
auto-partitioning feature can't really protect you from it.