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From: jqj@svax.cs.cornell.edu (J Q Johnson)
Newsgroups: net.lan,net.dcom
Subject: thin Ethernet advice wanted
Message-ID: <436@svax.cs.cornell.edu>
Date: Sat, 2-Aug-86 08:14:50 EDT
Article-I.D.: svax.436
Posted: Sat Aug  2 08:14:50 1986
Date-Received: Tue, 5-Aug-86 06:00:24 EDT
Reply-To: jqj@systems.cs.cornell.edu (J Q Johnson)
Organization: Cornell Univ. CS Dept, Ithaca NY
Lines: 45
Xref: mnetor net.lan:923 net.dcom:1186

I'm planning Ethernet wiring for a new building (about 150 stations).
I'd been planning a traditional scheme with thick backbone+xcver in
hallway raceways and drops into each office, similar to my existing
offices.  However, I am investigating using thin Ethernet instead.

I'd appreciate advice from one of you who has installed thin Ethernet
in a multiple-individual-office environment.  The environment is quite
multivendor, with a few VAXes, 50 Xerox Dandelions, perhaps a dozen
each of SUNs and Symbolics Lisp Machines, and lots of other vendors'
hardware.  Also, most offices currently contain only a single
workstation, though this may change in the next few years.

A thin ether configuration would presumably consist of a DEC DEMPR or
equivalent, with one RG58 leg running down each hallway, looping into
each office (say 8 offices) for a 300' maximum length.  In each office
the RG58 loop would appear at one point on a wallplate.  This might be
the base of a T-connector (allowing a cheapernet xcver like the DEC
DESTA to be attached directly to the wall) or a BNC connector with a
small external loop of RG58, allowing extension of the RG58 into the
office.  Does this latter scheme introduce too many connectors and
hence reflections on the ether?  Does having connectors in each office
make it likely that users will accidentally disconnect the Ethernet
cable?  Are there better plans?

I'm also interested specifically in the DEC thin-ether xcver (DESTA).
Does anyone know how compatible it is with non-DEC interfaces?  Does it
provide (disableable) heartbeat like the H4005?  Are there preferable
alternatives from other vendors?  Can I hook a DELNI to a DESTA?

I don't completely understand the configuration issues for thin ether.
RG58 of course has greater attenuation and slower propogation than does
thick ether.  What is the delay budget, etc. for a multiport repeater
like a DEMPR (is it just like a standard repeater)?  DEC's "networks"
guide states that a thick segment connected to a DEMPR must be less
than standard length and have no other repeaters; why?

Are lots of BNC connectors in the thin segments a problem?  How about
transitions between thick and thin ether cable (would I do well to
replace long straight runs of RG58 with transitions and thick ether)?

Anything else I should beware of?

Probably best to reply by mail unless you think your answers are of general 
interest:	jqj@systems.cs.cornell.edu      [preferred]
		jqj@cornell.arpa OR jqj@crnlcs.bitnet OR jqj@cornell.uucp