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From: rpw3@redwood.UUCP (Rob Warnock)
Newsgroups: net.lan,net.dcom
Subject: Re: advice wanted on Ethernet wiring
Message-ID: <150@redwood.UUCP>
Date: Sat, 2-Feb-85 01:07:19 EST
Article-I.D.: redwood.150
Posted: Sat Feb  2 01:07:19 1985
Date-Received: Mon, 4-Feb-85 04:59:59 EST
References: <1093@ulysses.UUCP>
Organization: [Consultant], Foster City, CA
Lines: 38
Xref: watmath net.lan:646 net.dcom:848

+---------------
| We're about to wire our building with Ethernet coax...
| One solution, of course, is to use vampire taps.  But the Ethernet package
| for a 3B2 includes a 3Com transceiver, which uses barrel connectors.
| So we're contemplating putting connectors every 12.5 meters or thereabouts,
| allowing for easy access... Is that likely to add too much noise?  All
| of the wiring will be in the ceiling, which is moderately accessible...
+---------------

Two cautionary suggestions:

First, make very sure that unused connectors (male-barrel-male), in fact
ALL connectors and terminators, are insulated from any metal contact, such
as ground, cable trays, light fixtures, etc. (The coax should be grounded to
a solid earth ground in EXACTLY one place, maybe in your "main" machine room.)
Ground loops (multiple ground connections) can be a cause of damage, both due
to A/C currents (most buildings are NOT at the same potential at both ends!)
and due to lightning (even nearby strikes induce large voltages). Use rubber
"boots" rather than tape, as tape gets "sticky/gunky" after a while.
(See the Ethernet 2.0 spec, section 7.6.3, p.82-83)

Second, it would be best if your coax were a single length of cable that
was cut AFTER it was installed. Since that is hard (putting connectors on
in the ceiling is rough!), you can equivalently specify that the sections
be cut from a single piece of cable, or failing that, from the same manu-
facturer and lot. This will lower the impedance variations in the cable.
(See the Ethernet 2.0 spec, section 7.6.1, p.81-82)

Otherwise, sounds fine. (If you begin also using vampire taps, remember to
keep them on 2.5m spacings from your connectors.)


Rob Warnock
Systems Architecture Consultant

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