Article Title           : heart_sqe
Creation Date           : unknown
Author                  : unknown
Last Update             : 6-1-93
Last Update By          : NCD Technical Support
Expiration Rules        : 
Location		: NCD-Articles/Networking
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				HEARTBEAT/SQE
				==============

Here is what heartbeat/SQE is supposed to do:


SQE/ heartbeat fixes the problem in earlier versions of ethernet
where a host does not know if a transceiver is connected.  It does
this by providing about 1 usec after the end of a transmission 1 usec
of collision. The ethernet controller in the host will record this
even and set a flag where the host can check on it.   

Heartbeat/SQE is only returned from the transceiver to the host or 
unit it is connected to. 

It is not put on the net.


Most systems never check on heartbeat. Either the driver programmers
do not know what to do with it, the host hardware was improperly 
designed and heartbeat does funny things,  the programmer does not 
care or want to be bothered with it, or the users nets are messed  up
and SQE is just one more thing.

The previous "don't care" ONLY applies to hosts that have ethernet 
controllers, something with smarts.

For repeaters:

Heartbeat MUST be disabled or the net WILL go down.

The reason is that a repeater is dumb.  It just repeats carrier. It
does not care what kind of carrier, it just repeats it.  What happens
if heartbeat is on:


	1. repeater transmits to port A


	2. repeater stops transmit to port A


	3. port A returns heartbeat


	4. repeater says carrier


	5. repeater repeats carrier to all other ports and
	   does a fragment extend which expands the 1usec to 9.6usec





This in its self is not too bad.  It just eats bandwidth. Some
repeaters will count the heartbeat as a collision and just partition 
after 32 transmits to that segment.  And sometimes a host will record
this even as a collision, framing error or slot time  violation and
think the packet will have to be resent, eating more bandwidth, It
might even think there are no good  packets, all of them have errors
associated with them.

If there  is two with heart beat on, then that extended fragment will
collide with each other and after 32 transmits to the pair  both
segments will shut down due to auto partition.  This type will happen
almost immediately after the first packet since they will be echoing
back and forth the same fragment extend until one  or both segments
partition.

And sometimes a host will record the returned fragmented extended
heartbeat as a collision, framing error or slot time violation and
thinks it will have to retransmit, eating more bandwidth.

All the above information is based on mostly a good net. If there are
dribble bits, bad wires and connections, flaky parts then things get
really worse faster.    

To summarize:


Since the heartbeat is local only between the transceiver and some
sort of host system. If the host system is a computer, bridge,
router, etc. something with intelligence, then heartbeat is a
function of whether driver software/hardware/ user wants it enabled.

If the host system is a repeater, hub, concentrator, etc, then
heartbeat must be off. This also includes concentrators with
bridges/routers internal to them.