From: hyder@niwot.scd.ucar.edu (Paul Hyder)
Subject: Query and note on short fiber optic Ethernets
Organization: Scientific Computing Divison/NCAR Boulder, CO
Date: Wed, 29 Jul 1992 16:09:17 GMT


After three months tracking down a problem that ended up being 
overdriven fiber connections I'd like to get some feedback from anyone
out there that has or is planning similar configurations.

This new network used fiber as the major interconnect in preparation for
full scale FDDI and beyond implementation.  Details below, in short we
used short fiber runs (3 to 500ft) to connect between close buildings,
between floors, and to in-room hardware.  We have been seriously
impacted by the lack of minimum length information.

QUESTIONS:
	1.  If you have short fiber optic Ethernet runs how did you
	    solve the problems?  Where did you get attenuators?  What
	    did your solution cost?

	2.  Are there vendors out there that don't have this problem?
	    (Products would need to include fiber transceivers (FOTs)
	    and fiber repeaters.)
	
	3.  Has anyone seen overdriven FDDI?

As always TIA.  Email to me please and I'll post a synopsis.
	Paul Hyder
	NCAR
	Boulder, CO
	hyder@ncar.ucar.edu

 ---problem detail follows---

SYNOPSIS
If you are about to use or currently use fiber optic Ethernet on
short fiber links you are extremely likely to need to purchase and
install lots of attenuators!

It is very very easy to cause major network problems when you overdrive
the receivers! Runs shorter than 500-1000ft need to be checked.

None of the specifications I've found say anything about minimum fiber
length.  They all simply give you length or budget values as X or 
less.  This isn't the whole truth.  It is possible to have the fiber
to short, particularly if you have low loss termination.

BACKGROUND

We just completed tracking down what looked like a severe network
cabling problem or multiple unit HW failure.

Our configuration is a newly installed network that uses Ethernet
on fiber optic cable to interconnect buildings and to do the floor
to floor runs.  We used Fiber Optic Transceivers (FOTs) with fiber
repeaters that finally fan out to multi port transceivers and
multi port thin Ethernet repeaters.  These fiber runs are from 
about 3 feet to about 300 feet.  [We do our own termination and
make extensive use of OTDR to check our fiber.  Our cables are
as clean as is humanly possible.]

The symptoms were:
	1. Massive numbers of CRC errors
	2. High collision rates
	3. Lots of RUNTS

All of this was load related, you don't notice much until the load goes
over 25% and then the net response is dismal.  Note that if you monitor
something with good counters (like a Cisco router interface) you see an
unusually high rate of CRC errors, collisions, and runts all the time.

Strangely when you capture the CRC packets they look more like very
long collisions or garbled packets.  The only way I can describe them
in general is that they look like nothing I've run into before.

After three months of checking, monitoring, and trying to find a net
design problem news postings from last May were found on this topic.
(With our thanks to Lon Stowell and Steven Lendt.)

The problem traces to the use of repeaters and FOTs that are designed
to push a signal up to 3Km.  They actually overdrive enough to introduce
error and garble.  Some of this noise looks like CRC errors and the
rest looks like strange collisions.  In the garble caused collision
case hosts correctly turn off transmission and that provides the RUNTS.

The solution is to examine all of the short links with a power meter
and attenuate until the error goes away.  In our tests we introduced
loss to get a final reading of about -25 dB, this is about 10 dB drop.
[Our FOTs start out with a power reading of -14.3 dB]

Until you find a vendor that provides an adjustable FOT you should
be very very careful when using short runs of Ethernet on fiber.
[And please let me know if you DO find a vendor that has such a
FOT.]
From: pat@hprnd.rose.hp.com (Pat Thaler)
Date: Thu, 23 Jul 1992 18:01:57 GMT
Subject: Re: 10bT patch panneling
Organization: HP Roseville Networks Division