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From: cliff@violet.berkeley.edu (Cliff Frost)
Newsgroups: comp.dcom.lans
Subject: Re: Do DELNI's cause collisions?
Message-ID: <1990May30.181740.14203@agate.berkeley.edu>
Date: 30 May 90 18:17:40 GMT
References: <145340@felix.UUCP> <11702@blia.BLI.COM>
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In article <11702@blia.BLI.COM> ted@blia.BLI.COM (Ted Marshall) writes:
>
>If a DELNI is in "global" mode and the transceiver connected to the global port
>is wired for SQE test (heartbeat), the heartbeat signal is passed down to ALL
>of the local ports. In other words, when any of the stations finishes a
>packet transmision, all of the stations on the DELNI see the heartbeat "blip"
>on collision detect pair.
>
>This condition will cause few, if any, problems because most Ethernet stations
>don't care about collision signals when they aren't trying to transmit. Also,
>because the blip comes early within the required 9.6us inter-packet gap,
>stations waiting to transmit shouldn't be affected either.

But the 9.6us gap is per transmitter.  You've got one station that has just
sent a packet, and it won't try to send another for 9.6us, but you've got
7 (or 15) other stations that don't know anything about the first station's
transmission and may decide to transmit themselves.  One of them could
easily see the heartbeat "blip" as a false collision.

From all the responses I got back to my question I believe this is a
compelling reason NOT to use SQE with multiports.  Maybe this is why
multiports aren't defined in the IEEE specs.

	Cliff Frost
	UC Berkeley