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From: rpw3@redwood.UUCP (Rob Warnock)
Newsgroups: net.lan
Subject: Re: Stretching the Ethernet specs
Message-ID: <148@redwood.UUCP>
Date: Thu, 31-Jan-85 06:02:12 EST
Article-I.D.: redwood.148
Posted: Thu Jan 31 06:02:12 1985
Date-Received: Fri, 1-Feb-85 21:37:33 EST
References: <425@mcvax.UUCP>
Organization: [Consultant], Foster City, CA
Lines: 54

+---------------
| To my suprise I'm offered two different brands of repeaters, which claim they
| can have more then two...
| Sension claims a maximum of six, and Isolan (Manufacturer BICC data networks
| LTD) 4.
| Anyway, the big question is of course "Can this be true? Has anyone experience
| with the named products?"
+---------------

(Ah... the stuff consultants thrive on! "Fee, Fie, Foe, Fum, I smell the
blood\\\\\ money of a naive one..." ;-} )

What we have here is a little "version skew" in the spec, methinks.

While it is true that the Ethernet 2.0 spec still says a maximum of
two repeaters across the "diameter" of the net, that limit seems to
be a holdover [read: "bug"] from the 1.0 spec which did NOT require
that repeaters regenerate the preamble back to 64 bits. In 1.0, two
repeaters was all you could have, because the various allowed "bit
eatings" (bits lost at the beginning of a packet) of transceivers and
coder/decoders along the path wouldn't leave enough preamble bits if
you had more than two repeaters.

But 2.0, like 802.3, requires repeaters to regenerate a full 64 bits of
preamble. (Compare section 7.6.4.1 in both E/1.0 and E/2.0). Therefore,
the real limit on repeaters is the round-trip propagation delay limit.

There is a certain allowed "start up" delay of a transceiver, a PLL,
a decoder, an encoder, and a transceiver. Each repeater would thus
shorten the permitted physical "diameter" of the network, to ensure
worst-case end-to-end prop delay of 46.4 usec. Unfortunately, this
makes the whole issue configuration dependent, as less cable means
more allowed repeaters.

(If I recall, 802.3 just said "maximum of four repeaters".)

I expect that the only way people are going to keep it straight is if
repeater manufacturers label their products with the "effective cable
length", so you can subtract the sum of your repeaters delays (less the
two repeaters "included" in the standard) from 2500 meters to get the
allowed "diameter".

So assuming your repeaters are 2.0 conforming, and the distance was not
too great, you COULD use two "full" repeaters and a piece of Ethernet
cable (or fibernet) to make one "pair of half-repeaters with included
point-to-point link". (Good luck. Let me know if it works.)


Rob Warnock
Systems Architecture Consultant

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