Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!aplcen!haven!ames!skipper!rando
From: rando@skipper.dfrf.nasa.gov (Randy Brumbaugh)
Newsgroups: comp.dcom.lans
Subject: SQE and strange behavior
Keywords: SQE
Message-ID: <408@skipper.dfrf.nasa.gov>
Date: 16 Jan 90 19:46:53 GMT
Organization: NASA Ames-Dryden FRF, Edwards, CA
Lines: 45

We recently observed some bizarre behavior on our net.
The problem is solved, but we still don't fully understand
the cause.  It may be very clear to someone who understands
repeaters and heartbeat-SQE.

First, some info on our set up.  A thick IEEE 802.3 cable runs
around our building.  At strategic locations, multi-port
repeaters tap this thick cable and branch out into thinwire
cabling to individual office areas.  On our thinwire segment,
we have 3 Suns, 1 Masscomp, 1 Kinetics Fastpath and 2 386-PCs
with 3c503 boards.

This setup worked fine, EXCEPT that the PCs couldn't communicate
with anybody.  Not telnet, ftp, PCNFS.  Watching traffic on the
LAN showed that for a telnet the PC would ARP 3 times. Each time
a reply was sent, but apparently ignored by the PC.  The problem
seemed to be the 3c503 receiver.  Then we discovered that if 
the thick interface on the repeater was disabled, everything worked -
The PCs could talk (through the repeater) to other thinwire
segments as well as hosts on the same segments.  After a lot of
theories were discarded, we found that the SQE was enabled on the
transciever connected to the thick cable and the repeater.  
Disabling SQE fixed the problem.

I have three questions:

1-  What exactly is SQE?  I think it is supposed to be turned off
 on repeater transcievers.  Is this true?  Why?

2-  Why did everything work fine EXCEPT the 3c503 cards on the PC?
 Note- I'm not knocking the 3 Com card- if the SQE was set wrong, 
 it was the only one to notice- maybe that makes it better. Maybe.
 The cards seemed to transmit, but not receive.  The symptoms were
 completely repeatable.

3-  If IEEE 802 says SQE should be off for repeater transcievers,
 does it say it should be on for others?  Is it required?  Can
 similar bad things happen if SQE is accidentaly disabled on a 
 workstation transceiver?

 Thanks,
  Randy Brumbaugh
  rando@skipper.dfrf.nasa.gov