Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!ulysses!smb
From: smb@ulysses.homer.nj.att.com (Steven M. Bellovin)
Newsgroups: comp.dcom.lans
Subject: Re: Need configuration info for various DEC ethernet widgets
Message-ID: <10879@ulysses.homer.nj.att.com>
Date: 21 Nov 88 20:40:14 GMT
References: <3607@phri.UUCP>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 59

In article <3607@phri.UUCP>, roy@phri.UUCP (Roy Smith) writes:
  .....
> 	The question is, just what *is* a DESPR?  The Dec-Direct catalog I
> have says little of value about the various widgets they make other than to
> say that the DESPR is a repeater.

Yup, the DESPR is indeed a repeater, with all the benefits and drawbacks
appertaining thereto.

> What exactly does that mean?  If he shorts
> out his thin cable, will that do anything bad to my net?

Most modern repeaters implement ``auto-segmenting''.  That is, if they detect
a fault on one segment they effectively disconnect it from the other half,
thereby isolating the fault.

> Do I have to count
> the effective length of his thin segments and the number of taps he has
> against the maximum limits of my cable?

No, because each segment is separate.  On the thickwire end, you're
entitled to 500 meters of cable and 100 taps; on the thinwire end,
you get 185 m. and 30 taps.  More precisely, you're entitled to 60
discontinuities, where a T-connector counts as 2, and a barrel connector
counts as 1, on a thinwire segment.

Incidentally, when you connect the DESPR to your coax be sure you use a
transceiver with *heartbeat disabled*.  If you order DEC gear, you need
to get the H-4000-BA instead of the H-4000; with other vendors' gear, such
as Cabletron, you can field-configure the transciever with or without
heartbeat.  (DESTAs are also field-configurable, but that's only the thinwire
end.)

> What about the rule that says you
> can't have more than two repeaters between any pair of taps on the network?
> Does his DESPR count as a repeater in this sense?

Yes, the DESPR is a repeater.

> What about my DELNI and
> TCL multiport xceiver boxes, do they count as repeaters?

No, they're transceivers, and they count as part of the 50 meters of drop
cable allowed.  The DELNI has a propagation delay equivalent to 5 meters
of drop cable; the TCL box is probably in the same ballpark.

The best source I know for the configuration rules is a book published
by DEC called the ``Network Buyer's Information Guide'', or some such
thing; it's reissued a few times a year, and is obtainable from your local
DEC salescritter.  It goes into great detail about legal configurations,
though on occasion it describes a DEC restriction as if it were a generic
one.  (For example, it says that repeaters -- DEMPRS, DEREPs, DESPRs, or
other brands of repeater -- may not be hooked to a DELNI.  This isn't even
strictly true for a DELNI (as some of the sample configurations show), but
it's even less true for some other brands of multi-port transceivers -- but
you're not told that....)


		--Steve Bellovin