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From: koning@koning.enet.dec.com (Paul Koning)
Newsgroups: comp.dcom.lans
Subject: Re: Repeater count on DEC ethernet and Isolan EtherConnect
Message-ID: <18350@shlump.nac.dec.com>
Date: 19 Dec 90 15:38:00 GMT
References: <1990Dec19.144011.45@lincoln.ac.nz>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: koning@koning.enet.dec.com
Organization: Digital Equipment Co., distributed systems architecture
Lines: 30


|>
|>Does anyone know why Digital allows only 2 repeaters in the signal path between
|>any two stations on the network, whereas the 802.3 standard allows 4 repeaters
|>between any two stations ?

Both allow the same thing.  The difference is ONLY in the way it is
described: the DEC/Intel/Xerox "blue book" uses one set of terminology,
the 802.3 standard a different one.

In the blue book, "repeater" is something with connections to the coax
(or equivalent) on each side.  Something that connects to a fiber point
to point link is a "half-repeater".  The rules allow 2 repeaters, where
each repeater can be replaced by a pair of half-repeaters.

In the 802.3 standard, both "repeater" and "half-repeater" are called
"repeater set".  It says you can have up to 4 of these, but IN ADDITION
adds the rule that out of the 5 resulting segments, only 3 can be coax
segments.  The remaining ones must be link segments.

If you translate this into blue book terms, you will conclude that you
can have either two repeaters ("2 repeater sets, 3 coax segments, 0 link
segments"), or one repeater and two half-repeaters ("3 repeater sets,
3 coax segments, 1 link segment") or four half-repeaters ("4 repeater
sets, 3 coax segments, 2 link segments").

So while this unfortunate terminology change makes the words different,
there IS NO DIFFERENCE in the topologies that are permitted.

	paul