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From: koning@koning.enet.dec.com (Paul Koning)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet cabling/repeater specifications?
Message-ID: <19450@shlump.nac.dec.com>
Date: 25 Jan 91 17:01:02 GMT
References: <1991Jan23.050705.5029@tmsoft.uucp>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: koning@koning.enet.dec.com
Organization: Digital Equipment Co., distributed systems architecture
Lines: 40


|>
|>I know the cabling limitations with Thick/Thin ethernet.  500M and 185M
|>respectively.  BUT I AM IN NEED OF REPEATER LIMITATIONS!
|>
|>I have gotten 2 or 3 different references.  DEC says no more than 2 repeaters
|>between nodes.  Black Box says no more than 2 inter repeater links which I
|>would take to mean 3 repeaters with 2 links between the three?  Any others 
|>out there?
|>

Unfortunately, the SAME rules are described by different terminology
in different places.  That doesn't change the rules themselves, though.
I'd suggest you get the IEEE 802.3 standard and the DEC/Intel/Xerox 
"Blue Book" (Ethernet spec).  Those are the actual authority.  If you
read them side by side you will see that in fact they describe the same
rules, but by different words...

When DEC mentions the "two repeater" rule, that means two local repeaters.
There are also fiber repeaters, with an inter-repeater link between them;
the traditional term for those is "half-repeaters".  A pair of those with
its inter-repeater link counts for one repeater.

The IEEE 802.3 spec calls each half-repeater a "repeater set".  It then
gives a maximum of 5 segments, of which at most 3 can be coax segments,
and 4 repeater sets.  So therefore:

a. If you have fiber repeaters ("half-repeaters") you're allowed four
   of those; 3 coax segments, and 2 inter-repeater links.  You're at the
   limit for all 3 limits simultaneously.
b. If you have only local repeaters, you're allowed two.  What limits you
   here is the rule that you can have at most 3 coax segments.

Again, the problem is differences in wording, NOT differences in the
actual rules.  If you care to go through the effort, it's instructive
to draw several configurations, both legal and illegal ones, and see
why each one of them does (or does not) meet each of the possible
restatements of the rules...

	paul