Path: utzoo!mnetor!uunet!husc6!think!bloom-beacon!oberon!cit-vax!mangler
From: mangler@cit-vax.Caltech.Edu (Don Speck)
Newsgroups: comp.dcom.lans
Subject: Re: Thinwire ethernet: Question about drop cables
Message-ID: <5948@cit-vax.Caltech.Edu>
Date: 26 Mar 88 02:38:03 GMT
References: <4491@june.cs.washington.edu> <13328@uflorida.cis.ufl.EDU> <421@ontenv.UUCP>
Organization: California Institute of Technology
Lines: 29
Summary: stub lengths add

Simple transmission line theory.  When the wave travelling down the
Ether hits a tee connector, the energy splits, with 4/9 of it going
down the drop cable, 4/9 continuing on the main cable, and 1/9
reflecting back.  The waveform loses 1/3 of its voltage.
When the energy going down the drop cable reflects off the end and
rejoins the main cable, the voltage goes back up.  Thus the waveform
has a notch in it, whose duration is twice the round trip time of
the stub.  Each time the waveform passes another stub, the notch
gets wider and deeper.	When the width of the notch approaches half
a bit time (i.e. 50ns for Ethernet, 500ns for Starlan), the signal
will have reversed itself by the time the reflections add back in
(destructive interference), and things will go downhill fast from
there.

That means the combined length of all stubs is limited to about 3 meters.
If you have 100 taps on the cable, that's 3cm per stub (about what you
get from a single tee and BNC connector).  With only 10 stations, you can
have one-foot drop cables.

It might help slightly to use high-impedance drop cables (video cable
or IBM 3270 cable) to reduce the depth of the notch if you're going to
cheat by hanging cables from a tee.

If you need a long drop cable someplace, make it one end of the main
cable and terminate it at the workstation.  (Fortunately, cables have
two ends, so even if all your drop cables need to be long, you should
still be able to get two stations per thin ethernet segment).

Don Speck   speck@vlsi.caltech.edu  {amdahl,ames!elroy}!cit-vax!speck