Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!wuarchive!uwm.edu!src.honeywell.com!msi.umn.edu!noc.MR.NET!jhereg!andrew
From: andrew@jhereg.osa.com (Andrew C. Esh)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet Collisions
Keywords: What is the capacity of an ethernet? Is "this" normal?
Message-ID: <1991Apr15.231744.10399@jhereg.osa.com>
Date: 15 Apr 91 23:17:44 GMT
References: <4150@gmuvax2.gmu.edu>
Organization: Open Systems Architects, Inc., Mpls, MN
Lines: 84

In article <4150@gmuvax2.gmu.edu> rauletta@gmuvax2.gmu.edu (R. J. Auletta) writes:
>We have been having some problems with an ethernet installation
>that our Computer Network Services organization seems unwilling
>to resolve. I am looking for some insight from those who might
>have a sense of whether what we are seeing is normal.
>

Unwilling to resolve? Pardon me, but this sounds like an attitude problem.
It's their job to resolve just this sort of thing. Maybe they are stuck,
and can't think of an approach. Keep at them.

>The problem is characterized as follows.
>
>1) Interactive sessions (typing etc) tends to get periodically
>interrupted every couple of seconds for a tenth of a second or
>more (the echo time becomes longer than the time to type a 5-10 character word.)
>(This seems directly related to any burst of ethernet traffic
>over about 10,000 bytes/sec as reported by etherd on a Sun.)
>(When the ethernet load is low, the ethernet is very
>responsive.)
>
>2) The following indications on an American Network Connections
>ANC-80 8-port fanout transceiver 802.3 while the problem is present.
>
>RCV light intermittently active
>REM COL blinks as the RCV goes out [everytime]. (Remote Collision)
>LOC COL off
>TRVR PRES light is dimly on.        (?)
>SQE is off.

Collisions! There's most of it right there!

>
>3) Traffic on the ethernet is (as reported by etherd on a
>Sun workstation) about 25K-75K bytes per second running
>about 50-150 packets per second when (1) is observed.
>Most of the traffic is between just two Sun workstations.
>
>4) Running netx (a tcp exerciser) on a Vax3600 to a VS2000
>showing a network load of about 10% shows almost continuous collisions even
>when only the two machines are active on the network.
>(Every blink of the RCV light on the fanout unit results in the REM COL
>blinking, the LOC COL stays off.)
>
>Is this normal? At the described load would one expect to experience
>poor interactive response that ethernets are known for? Might
>this be due to the "supposed" problem with Sun's interpretation
>of the ethernet standard in regards to back to back packets?
>
>The general form of the ethernet is a thick riser with one thinnet
>transceiver and several AUI transceivers with a bridge to
>a fiber-optic segment.
>
>What I am looking for are some suggestions as to what we might
>look for to isolate the problem (such as "this sounds like a noise
>problem", or "excessive reflections", or "load is just too high").
>
>Characterized but still confused,
>
>R J Auletta
>rauletta@sitevax.gmu.edu

I would suggest checking everything between the ANC-80 and the main
backbone (or whatever it connects to). I would concentrate on the cable,
but check transceivers too. If you can get a cable or a TDR, that will
probably show you that the cable is bad. Check the end connectors, and try
it with a VOM, testing for a short between the shield and the conductor.
Wiggle and twist the ends as you do this test, since it could be
intermittent. Also find a convenient ground and see if there is any voltage
potential between the shield and ground. A ground faluted sheild make
sending a signal down the wire like trying to blow a marble through a
garden hose full of holes. Bad cable will usually give you the kind of
reflactions that cause the collisions you seem to be seeing. Even mediocre
cable will run 20% with less than one collision per second.

Also, could you say more about this ANC-80 thing? Is it a Multiport
Repeater, or a Bridge, or what? 8 port fanout transciever?  Not sure what
that might be.

-- 
Andrew C. Esh			andrew@osa.com
Open Systems Architects, Inc.
Mpls, MN 55416-1528		Punch down, turn around, do a little crimpin'
(612) 525-0000			Punch down, turn around, plug it in and go ...