On Thu, 2 Mar 2000, der Mouse wrote:

[snip]

> What *is* this SQE, heartbeat, whatever it's properly called?  In
> particular, what does it have to do with connecting to a real host vs
> connecting to the station port of a multiport device?

SQE is "Signal Quality Error", originally CPT or "Collision Presence
Test" in the original DIX spec.

Basically, it's meant for ethernet cards talking to a transceiver.  When
the card sends a packet out, the transceiver "acknowledges" the
reception (and transmission on the wire) by sending a short collision
signal back to the host (_only_ the host - it doesn't go out on the
wire).  This way the host can tell that the transceiver is
attached and functioning (powered, etc...).

The problem is this: if you put a tranceiver on a hub (or repeater,
etc...), this short SQE collision signal is taken by the hub as an
_actualy_ collision.  Since the hub can't really tell an SQE from a
collision, this "false" collision propigates over the network, and
generally dampens the performance (and people's moods :-).

Also, most software doesn't deal correctly with the SQE, and treats it as
a real collision.

A quick, but more in depth explaination is at:

http://www.stratacom.com/warp/public/86/12.html

-Jon
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