Subject: Re: very strange ethernet problem
To: Tod Daniels , Jeremy Cooper <jeremy@baymoo.org>
From: Don Lewis <Don.Lewis@tsc.tdk.com>
List: port-sparc
Date: 10/30/2000 23:54:47
On Oct 30,  9:09pm, Tod Daniels wrote:
} Subject: Re: very strange ethernet problem
} 
} I'll agree with Jeremy on this (in fact, he finished typing just before I
} did... :-)
} 
} SQE was originally a way for a transceiver to determine the functional
} state of its collision-detection circuitry.  In essence, it occasionally
} transmits a "jam" pattern onto the medium.  This would be viewed as
} collision by all your ethernet nodes (including the hub).

This is incorrect.  SQE is the collision signal that is sent across
the transceiver to the MAC in the host across AUI cable.  If the SQE
signal isn't working, then the host will not detect any collisions that
will happen and will just keep transmitting, stomping on the packets
that any other host tries to send and not noticing that it's own packets
were stomped on, resulting in packet loss.

Because of the importance of SQE to the proper operation of Ethernet,
IEEE 802.3 defined SQE Test to be a test signal sent on the SQE pair
at the end of each transmission.  The MAC is supposed to look for the
presense of SQE Test at the end of each transmission and signal an
error if it doesn't see the SQE Test signal.  In theory, the device
driver is supposed to complain if SQE Test is missing so that the
network administrator knows to repair the network hardware because
collision sensing is also likely to be broken causing late collisions
and packet loss.  The SQE Test signal is only sent on the AUI cable
and not across the network.

Without SQE Test, the host would have no way to detect that the SQE
signal was broken, since SQE would not otherwise be asserted if the
host transmits into an otherwise quiet network.

There are a few cases where SQE Test should be turned off:

	When connecting a repeater (hub) to a transceiver over
	an AUI cable, SQE Test on the transceiver should be turned
	off.  If you don't do this, the SQE test signal will confuse
	the repeater into thinking that an actual collision has
	occurred, and it will send a jam pattern on its other ports.

	Transceivers connected to AUI fanout devices (DELNI) should
	probably have SQE Test turned off, since the non-transmitting
	devices on the other ports might get confused by seeing SQE
	asserted at odd times.

	Transceivers connected to ancient Ethernet version 1.0 hardware,
	which predates heartbeat/SQE Test.