Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!rutgers!ncs.dnd.ca!jstewart
From: jstewart@ncs.dnd.ca (John Stewart)
Newsgroups: comp.dcom.lans
Subject: Re: SQE and strange behavior
Keywords: SQE
Message-ID: <732@ncs.dnd.ca>
Date: 17 Jan 90 18:21:27 GMT
References: <408@skipper.dfrf.nasa.gov>
Reply-To: jstewart@ncs.dnd.ca (John Stewart)
Organization: DREO, Ottawa, Ontario
Lines: 59

In article <408@skipper.dfrf.nasa.gov> rando@skipper.dfrf.nasa.gov (Randy Brumbaugh) writes:
>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?

SQE (sometimes called heartbeat) sends a signal back to the controller
on successful transmission of a packet. The signal pair that it uses is
the collision detect pair, so the controller has to know when to
interpret the pulse on the pair as a collsison or SQE.

Now; one of the design criteria of repeaters is that if they see a
collision on any segment, then they must then jam the other segments to
simulate the collision on any other connected networks. (read the specs
if you don't believe me; they have to do this to keep the signals on 
all connected networks close)

So; we have a repeater that does not understand SQE. If a packet is sent
to it (and there will be many :-), the repeater will take that packet
and propogate it to all connected networks. If it just happens to send
to the tranceiver with SQE, then it will receive a collision detect.

It will then jam all the connected networks to copy the perceived
collision.

I am not sure if there are repeaters that use SQE or not; but I know
that any that I have worked with do not. Off the top of my head, I can
not think of a reason why it would not be possible for a repeater to 
work with SQE.

>
>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.

Good question. Timing? Luck? Maybe the 3-com card does funny (or
correct) things after seeing a collision? This would be an interesting
one to follow up.
>
>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?
>
If you turn off SQE, and the controller needs it, then the controller
will inform  your driver (which will inform... etc) that it can not
transmit.

It's safe to bet that it can be disabled for most, if not all devices.


John Stewart.