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From: ian@mulga.OZ (Ian Richards)
Newsgroups: net.dcom,net.lan
Subject: Re: DEC's broadband Ethernet tranceivers
Message-ID: <605@mulga.OZ>
Date: Sun, 6-Jan-85 18:25:35 EST
Article-I.D.: mulga.605
Posted: Sun Jan  6 18:25:35 1985
Date-Received: Sun, 13-Jan-85 08:50:55 EST
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Organization: Comp Sci, Melbourne Uni, Australia
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Xref: watmath net.dcom:779 net.lan:597

> DEC has announced Ethernet tranceivers for use on broadband networks.  They
> implement the standard 10 M bps in 18MHz of bandwidth.  There are
> configurations for either single or double cable broadband.  Maximum
> distance is 3800 meters.  Available in the spring.
> Darrel J. Van Buer, PhD

Is the 3800 metres correct? If so, how do they do it?
If they are still using CSMA/CD then the maximum distance between
nodes is limited by the contention period (46.4 microseconds) which in
turn dictates the minimum frame size (512 bits). The DEC/Xerox/Intel
"standard" suggests that after you allow for all kinds of switching
and hardware delays you have about 23 of the 46 microseconds left over
for propogation time which at 0.7c is pretty close to 5000 metres.
This of course is the "round trip" time which means a maximum node
separation of 2500 metres. Now if you use a broadband cable surely
this ought be halved again.
This is because the worst case contention problem is two nodes near to
each other but furthest from the head end. One begins transmission.
Its signal propogates via the head end and all the way back until it
almost reaches the other node which then begins a transmission. This
signal (which is now a collision) must propogate all the way via the
head end and back to the first station so it can detect the collision.
The first station must continue transmitting for all of this time.

Now perhaps they can avoid the double propogation delay by doing the
collision detection on the transmission channel/cable. That brings it
back to the baseband situation. (But how does a station upstream from
a transmitting station find out that if it transmits it will cause a
collision?) Perhaps they can get rid of some of those aforementioned
delays or perhaps their signals go faster than 0.7c. I would be
interested to hear the answer if anyone knows.

Ian Richards					decvax!mulga!ian