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Path: utzoo!watmath!clyde!cbatt!ihnp4!ihlpa!normt
From: normt@ihlpa.UUCP
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet Compatibility...
Message-ID: <2930@ihlpa.UUCP>
Date: Tue, 10-Feb-87 18:26:48 EST
Article-I.D.: ihlpa.2930
Posted: Tue Feb 10 18:26:48 1987
Date-Received: Wed, 11-Feb-87 20:28:26 EST
References: <4312@columbia.UUCP> <144000002@tiger.UUCP>
Organization: AT&T Bell Labs, Naperville, IL
Lines: 55

> 
> /* Written  2:41 pm  Feb  5, 1987 by desimone@tom.columbia.edu.UUCP in tiger.UUCP:comp.dcom.lans */
> /* ---------- "Ethernet Compatibility..." ---------- */
> 
> Hi...We've been successfully running an ethernet LAN here for the past
> 	couple of years.  Recently, however, we've received a number
> 	of products that conform to the IEEE 802.3 standard.  Since
> 	our network is a few years old, some of our hardware was
> 	designed according to the original ethernet standard (Version 1)
> 	and some according to Version 2.  I haven't really been able to
> I'd like to give you a couple of hearsay incompatibilities.  I read in
> DATA COMMUNICATIONS magazine that ethernet 1.0's voltage level at 
> "idle" on the baseband cable is 0.7 Volts DC, whereas IEEE 802.3's 
> idle voltage leve is 0.0.  I had an Entre salesman (take this with a
> grain of salt) tell that some ethernet boards incorrectly interpret
> an IEEE 802.3 "length" field as a "type" field.  I have not been able to
> determine the accuracy of the latter statement.

Both of these statements are true. The "Ethernet" network idles "with the
transmit+ positive with respect to transmit-".  The ethernet versionII and
IEEE802.3 idles at a zero differential voltage.
This is the only hardware incompatibility and depending on the interface you
are using there may be a switch to select which format you want to run. I don't
know if or how well these will talk to each other, but my intuition says "not
well."

With regard to the type field, this is a two byte field that comes after the
destination and source address and before the data field. In IEEE802.3 it is
defined as the length of the valid data in the data field. This normally will
not include any padding bytes that are added to the data to get the 46 minimum
byte data field. This number is only used by higher level protocols. The actual
transmitter and receiver should not (and will not in any compatible product)
use this field for length. It is simply passed to the higher level protocols as
data. In The original ethernet, this was a type field that informed higher
levels what the message protocol was. For example if the data field is formatted
with TCP/IP format the type field is 0800 if the formatting is the defense
department's XNS this field is 0600. Both of these if taken as hex numbers
would be illegal length fields.  i.e. no confusion. Historically this comes
from the indended purposes and users of the network, this I won't go into
since it does not add to the discussion. Basically the hardware problem
exists and the two have problems talking to each other. The type/length field
is something that is ignored from the hardware point of view and so if
updated software is running on both ends, you don't really care what is in
this field as long as both ends assume the same.

As far as address assignment, this has passed onto the IEEE standard board, 
however if you want to register a type field for your higher level protocol, this
is still done through Xerox corp. If anyone is interested I have the address
and contact name for both of these places.

	Norm Tiedemann
	AT&T Bell Labs
	Naperville, IL 60566
	(312) 979-3535
	...!ihnp4!ihlpa!normt