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From: rpw3@amdcad.UUCP
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Ethernet versions
Message-ID: <19186@amdcad.AMD.COM>
Date: Wed, 18-Nov-87 17:49:40 EST
Article-I.D.: amdcad.19186
Posted: Wed Nov 18 17:49:40 1987
Date-Received: Sat, 21-Nov-87 06:55:05 EST
References: <8711141736.AA21062@topaz.rutgers.edu>
Reply-To: rpw3@amdcad.UUCP ()
Organization: [Consultant] San Mateo, CA
Lines: 61

In article <8711141736.AA21062@topaz.rutgers.edu> Ron Natalie writes:
+---------------
| On the coax there is no differenece electrically between Version I
| Version II, and IEEE 802.3.
+---------------

Weeeelll... almost. There WAS a teensy change in the electrical spec on
the coax between Ethernet Version 1.0 (Sep'80) and Version 2.0 (Nov'82),
having to to with tightening the specs on the drive current (or at least
changing the way the A.C. versus D.C. components were specified). See
Section 7.3.2 "Coaxial Cable Signaling" in each version (p.61 in ver 1.0,
p.72 in ver 2.0). The net effect was to change the shape of the AC/DC schmoo
slightly. Very slightly.

There IS one significant change to that section in the 2.0 spec. The following
sentence is added:

	"The transceiver shall be able to produce its specified output
	current onto the coaxial cable with at least one other transceiver
	transmitting simultaneously."

That sentence made it official that receiver-based collision detection
shall be possible, by requiring that the current source in a transceiver's
transmitter not wimp out until the cable voltage was AT LEAST twice the
normal max peak voltage.

All practical "current sources" have a "maximum compliance voltage" above
which they quit being true current sources. (A "perfect" current source
would increase its voltage without limit, even to the point of arcing over
if you tried to disconnect it!) All of the current sources in the popular
Version 1.0 transceivers had plenty of compliance; the 2.0 spec just made
it official.

Why all the trouble? Well, if you are going to build a repeater, it's
important that you be able to creat "carrier" on the "other" side of
the repeater whenever you see carrier on "this" side (or a "jam" on the
other side whenever you see "collision" on this side). But in the case
of several transceivers transmitting at once, the current sources will
saturate and all the A.C. signal will disappear in the large D.C. offset.
It is important that this not happen at a voltage lower than the repeater
could reliably detect as a collision, when it itself was not transmitting.

Furthermore, I've been told that the tightening of the A.C. versus D.C specs
I mentioned above helped solve a possible ambiguity: In the case where a
repeater is at one end of a maximally-loaded cable and there is a collision
between two wimpy transmitters at the far end, the tightened spec plus the
tightened "voltage compliance" guaranteed that the repeater would see it as
a collision, and not interpret it as a single nearby macho transmitter.

Again, it's no big deal. All (?) of the 1.0 vendors' transceivers worked
(and work) just fine on a mixed 1.0/2.0/IEEE_802.3 cable. It just needed
to be said explicitly in the spec.


Rob Warnock
Systems Architecture Consultant

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