From: cctr143@csc.canterbury.ac.nz (Jerry Schave)
Subject: Re: Who says DELNIs and DEMPRs alter the specs.
Organization: University of Canterbury, Christchurch, New Zealand
Date: Fri, 22 Jan 1993 01:42:36 GMT

In article <C169LH.Mr4@unixhub.SLAC.Stanford.EDU>, streater@unixhub.SLAC.Stanford.EDU (Tim Streater) writes:
> I think that those who have responded to Marty's question have missed the 
> point of what he is trying to do. We have RTFM-ed, in fact I have a copy of
> the 92/93 DEC Network Buyers Guide right here. On page 2-39 it says, and I
> quote, "When a DELNI/DEMPR combination is connected to a standard baseband
> cable segment, the following restrictions apply: The standard segment must
> be less than 300 meters ......"
> 
> The issue for us is that this leaves a host of questions unanswered, such as:
> 
> 1) Why is this?
> 2) Suppose I have 2 of these things thusly connected. DO I knock another 200m
>    off my maximum cable length, do I reduce the allowed length by a factor of
>    300/500 again, or what?
> 3) We in fact are using Cabletron equivalnets of DELNI/DEMPR units. Do the
>    same restrictions apply to these boxes as to the DEC units? If not, why not?
> 4) If I use a DELNI without a DEMPR connected, does this restriction apply?
>    (Looks like it doesn't).
> 
> I am sure I could think of other questions, but right now the info we have is
> no more than a recipe to be followed in one given situation, that of a DEMPR
> cascaded on a DELNI. It appears not to apply to a DECRepeater 90C, for example.
> I don't like designing networks according to arbitrary rules that don't
> generalise to other situations. I don't mind there being a rule; I need to
> know what's behind it so I can understand if I *need* to apply it to other
> situations.
> 
> Tim Streater / SLAC Networking Development
-- 
Very good point! Right now I'm trying to add 10BaseT hubs on the end of a 10Base2
repeater (DEMPR) connect to a 10Base5 segment.  Now all the older documentation
talks about the 2 repeater rule (No 2 stations shall be separated by more than
2 repeaters unless a router or bridge is also in the path).  Last year I came
across the 5-4-3 rule which states you are allowed 5 segments of which 3 can be
populated with 4 repeaters.  Now this is contradictory to the 2 repeater rule.
So is the below configuration valid with older repeaters such as DEMPR's as well
as newer repeaters or has the electronics inside the repeaters gotten faster to
reduce the delay?  Besides for the constant per meter delay of the various medium
what other delays must be accounted for to stay within the 46.4 us maximum delay?

			10Base5
  ==================================================
     |                   |                        |
  10Base2              10Base2                 10Base2
  Repeater             Repeater                Repeater
     |                    |                       |
     |-unpopulated        |-unpopulated           |
     |                    |                       |
  10Base2              10BaseT                 10Base2
  Repeater             Repeater                Repeater
  | | | |              | | | |                 | | | |
-----------------------------------------------------------------------
| Jerry Schave                 E-Mail   j.schave@csc.canterbury.ac.nz |
| Computer Services Centre                                            |
| University of Canterbury              Phone +64 3 364-2329          |
| Christchurch, New Zealand             Fax   +64 3 364-2332          |
-----------------------------------------------------------------------