Re: DELNI characteristics


Edward J Cetron (cetron@utah-cs.ARPA)
Fri, 12 Sep 86 10:48:21 MDT


        David Roode writes:

>It is also against the rules to plug one DELNI into another one
>unless that DELNI is NOT connected to an Ether[net].

WRONG!!!!!!!!!!!!!!!! Nowhere does it say that one DELNI cannot be
cascaded to another ethernet connected DELNI! And I've check every manual,
tech and sales, Digital has produced....

        The above is a very common misconception...the real configuration
rule has to do with the fact the propagation delays build through the
ethernet having to do with things like cable lengths, numbers of repeaters
etc....

        The one rule that is most apropos is the 'no more than two repeaters
between any two nodes....' This is (among other things) to prevent the
propagation delay between any two nodes to be less than the max... For very
conservative purposes, the DELNI acts with a propagation delay of 1 repeater
and this sort of implies the above misconception.

        In reality:
        1. A DELNI actually appears to introduce the delay of about .6-.7 that
of a repeater.
        2. I have seen systems with 2, 3, and 4 cascaded DELNI's that work and
DO maintain in-spec propagation delays.
        3. It has been pointed out that with the appropriate configuration, one
could even make 4 repeaters in a row work.
        4. The configuration rules (like the rs232 max line length rules) are
very conservative to allow 100% probability of a network working when they are
strictly adhered to. This means that quite often, for networks not in a very
critical area (i.e. hospital ICU's controlling patient therapy) they are only
to be considered guidelines BY PEOPLE WHO UNDERSTAND WHY THE RULES ARE THERE.
Rules are there for a reason, but should be tested and verified before blindly
obeying them without knowing why

-ed cetron
Center for Engineering Design
Univ of Utah
cetron%utah-cbd@utah-cs.arpa



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