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Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!ihnp4!fortune!rpw3
From: rpw3@fortune.UUCP
Newsgroups: net.lan
Subject: Re: Ethernet cable length query - (nf)
Message-ID: <2832@fortune.UUCP>
Date: Sun, 25-Mar-84 04:05:48 EST
Article-I.D.: fortune.2832
Posted: Sun Mar 25 04:05:48 1984
Date-Received: Sun, 25-Mar-84 20:58:58 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
Lines: 51

#R:cbosgd:-111500:fortune:5900005:000:2640
fortune!rpw3    Mar 25 00:14:00 1984

The reason the rules are less stringent for single lot cable is
that the rules are primarily concerned with step discontinuities
in the characteristic impedance. Different lots of cable can vary
several ohms (even a single piece can wander up and down a little).
But Mark, if you are concerned primarily about maintainability of
the installed cable, buy your cable in one big roll, cut it up just
about any way you want, and don't worry. Buy as much now as you can
afford and that you think you will use for several years.

To be real picky, put your cuts on 2.5 meter boundaries (at one of the
little annular ring markings). This way, your cuts look like 3com
transceivers with nothing in them! ;-}  [Besides, you can always
put in-line transceivers (such as 3com's) at the cut-points later.]
Be sure to use the standard N-type males one the cable, with barrel
females between.

When we wired up our the Twin Dolphin building before we moved in,
we did that -- bought one big piece but installed it as several
concatenated sections. (Not enough experience yet to know the effect.)
You have to start pushing (or exceeding) the 100-transceiver/500-meter
(not feet) limits before you're going to see any problems. Friends at
Xerox/PARC tell me they've got one 10 Mb/s link running with 160 transceivers
on it at 600+ meters, and it's o.k. No guarantee; your mileage may differ.

Other tips: MAKE SURE you get a blueprint with the routing of the cable
on it, preferably (as we did) with the 2.5 meter spots marked on it
(actually, the print only shows every third one). That way, when you have
problems, a TDR from one end can pinpoint the trouble to within a couple
of ladder shiftings (yes it's a new technical term... ;-} "a couple of turns
at climbing UP the ladder, then DOWN the ladder, then MOVE the ladder").

Let's see... remember that transceivers can be 50 METERS from the station,
so don't be to worried about snaking cable into every little corner.

Also, loop the cable around so that both ends are in the machine room, or
the comm lab, or someplace where you can get at them easily. Make sure the
joins in the cable are centered over open corridor, etc.. Basically, sit
down and think for 3 or 4 minutes about how you are going to wish you had
done it, a year or two from now. Imagine some horrid scenario of the Big Boss
coming upstairs with the Important Visitors and you just got a shorted
transceiver "somewhere" in the net. There. Go build it like that.

Rob Warnock

UUCP:	{sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3
DDD:	(415)595-8444
USPS:	Fortune Systems Corp, 101 Twin Dolphin Drive, Redwood City, CA 94065