Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!umich!yale!husc6!frooz!cfa.HARVARD.EDU
From: wyatt@cfa.HARVARD.EDU (Bill Wyatt)
Newsgroups: comp.dcom.lans
Subject: Re: SQE and strange behavior
Message-ID: <286@cfa.HARVARD.EDU>
Date: 18 Jan 90 15:07:00 GMT
References: <732@ncs.dnd.ca>
Sender: news@cfa.HARVARD.EDU
Lines: 53


 >>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 
 [...]
 > So; we have a repeater that does not understand SQE. 
 [...]
 >                     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.

No absolute reason. The DEC DEREP works fine with SQE-enabled
transceivers. They had to, as DEC's old H4000 transceiver had no way
to turn off SQE. I think there must be a good reason the 802.3
standard says repeaters shouldn't have SQE circuits. Maybe it
preserves a little more of the time budget, as there's no dead time
where the repeater has to ignore collision detect; it can then
jam the other segment that much sooner if one is detected.

 >>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.

While disabling SQE may work for some devices, it's not a good idea.
The use of SQE is necessary since data corruption and general havoc
will ensue if a node's collision detection has failed. Data integrity 
is a vital commodity on a shared bus!

Bill Wyatt, Smithsonian Astrophysical Observatory  (Cambridge, MA, USA)
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