Re: DELNI Strangeness


John Leong (leong@andrew.cmu.edu)
Thu, 11 Sep 86 19:58:36 edt


Clive,

I don't really know what specifically is your problem with the DELNI.
However, one thing you may care to check is this :

The DELNI (and for that matter, almost all multiport tranceiver in the
market) does not handle "heart beat" correctly. For all practical purposes,
heart beat is a kind of acknowledgment from the tranceiver to the station.
Within a very short period of time after the tranceiver finished sending the
packet into the coax cable, it issue a heart beat signal on the collision
pairs of the tranceiver cable - essentially saying "all's well". In the case
of the DELNI, this signal is (mistakenly) fanned out to ALL attached
stations. The station doing the transmit will know it is a heart beat.
However, the other attached stations will treat it as an asynchonous
collision notification. Most station will ignore it but I don't know about
the DEC-20.

However, definitely, do not attached a repeater to a DELNI. Repeater will not
toss away heart beat. It will act on it and generate collision reinforcement
(jam) on the other side!!! The result is excessive collisions. Very bad news
to the overall band width availability.

John Leong

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