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From: mccallum@nbires.UUCP (Doug McCallum)
Newsgroups: net.dcom,net.lan
Subject: Re: DEC's broadband Ethernet tranceivers
Message-ID: <363@nbires.UUCP>
Date: Mon, 14-Jan-85 11:38:37 EST
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Posted: Mon Jan 14 11:38:37 1985
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	> Is the 3800 metres correct? If so, how do they do it?
I am assuming that DEC followed the proposals put forth in the IEEE 802.3
committee for their implementation of a broadband CSMA/CD.
The proposed addition of an AUI Compatible Broadband to the IEEE 802.3
standard allows up to 4km. for the length with 0.87c cable.  There are
other differences in the spec as well.
	> 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
CSMA/CD doesn't impose the contention period, the "Ethernet" spec does.  By
changing many of the parameters to better fit the existing broadband media,
it is possible to get a CSMA/CD system with a larger area, different data rates,
etc.  The IEEE 802.3 standard is making allowances for CSMA/CD on different
media and data rates.
	> 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
The signal being used in the proposed broadband 10Mb system (proposed
by DEC, M/A COM and possibly others) uses 3 CATV channels and splits these
into a data channel and an outofband channel for collision detection.
If two stations start broadcasting at the same time, collision can be
detected by comparing what is received with what is transmitted.  The
comparison need only be done on the header through the source address.

I really don't know much more than that.  The above came from some notes
I took at the last IEEE 802 meeting.

		Doug McCallum
		{allegra,ucbvax,ut-sally}!nbires!mccallum