Ref: 99980073
Title: Compatibility Among Ethernet Vers 1.0, 2.0, and IEEE 802.3
Date: 10/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

Ethernet was initially introduced by Digital Equipment
Corporation, Intel Corporation, and Xerox Corporation in 1980
with what is now called Ethernet Version 1.0 specification.  In
1982, after the IEEE 802.3 committee identified the required
changes, an Ethernet Version 2.0 specification was released.  The
IEEE 802.3 standard was published in 1985.

This section outlines the compatibility issues among Ethernet
Versions 1.0, 2.0, and IEEE 802.3.

The physical and data link layers of the ISO Reference model can
be viewed as 3 sublayers:

1.  The electrical functions of the physical layer include
    carrier sense, collision detection, bit transmission,
reception, and jabber control.  These functions are split
between the transceiver and the controller.

2.  The frame format at the data link level includes addressing
and checksum.

3.  The logical link level packet format (encapsulated in the
data link format) is used for demultiplexing of packets
within a station.

Following is a brief description of the major differences among
Ethernet Versions 1.0, 2.0, and IEEE 802.3 at each of these
sublayers:

1.  Electrical Functions

Electrically, the signals produced on the coaxial cable by
Ethernet Versions 1.0, 2.0 and IEEE 802.3 are identical.  A
controller that has been properly connected to the coaxial
cable with the appropriate transceiver and transceiver cable
can communicate to another properly attached controller
regardless of the version of either controller.

However, Ethernet Version 2.0 and IEEE 802.3 have additional
features that are not available on Ethernet Version 1.0 such
as jabber control in the transceiver, tighter tolerances, and
the heartbeat collision signal test.

2.  Frame Format

The frame format is the main incompatibility among Ethernet
Versions 1.0, 2.0 and IEEE 802.3.

   ________________________________________________________
   |            |       |     |          |         |       |
   | Ethernet   |  DA   |  SA |   TYPE   |  DATA   |  CRC  |
   |____________|_______|_____|__________|_________|_______|
   |            |       |     |          |         |       |
   | IEEE 802.3 |  DA   |  SA |  LENGTH  |  DATA   |  CRC  |
   |____________|_______|_____|__________|_________|_______|

Ethernet uses the TYPE field to determine which client
protocol the frame is using.  IEEE 802.3 uses the LENGTH
field to do additional error checking and this leaves
protocol demultiplexing to the 802.2 layer.  The IEEE 802
standard states that if the LENGTH field is greater than 1500
bytes, it may be viewed as a TYPE field.  Thus, IEEE 802.3
and Ethernet units will be able to coexist but not
necessarily communicate.

3.  Logical Link Control

IEEE 802 has defined a protocol called Logical Link Control
(LLC) (802.2) on top of the frame format which provides a
similar function to the TYPE field in an Ethernet packet.
Thus, a complete 802.2/802.3 packet is as follows:

 --------------------------------------------------------------
|  DA |  SA |  Length |  DSAP |  SSAP |  CONTRL  | DATA  | CRC |
 --------------------------------------------------------------

Ethernet, however, does not define a LLC-like protocol.

4.  Interface

Although the interface between controller and transceiver for
Ethernet Versions 1.0, 2.0, and IEEE 802.3 is similar, the
potential incompatabilities between Ethernet Versions 1.0,
2.0, and IEEE 802.3 include:

 -   The grounding pinout is different for Ethernet Versions
     1.0 and 2.0 when compared to IEEE 802.3

 -   For safety and noise reasons, Ethernet Versions 1.0 and
     2.0 transceivers use a different transceiver cable than
     the IEEE 802.3 transceiver.

 -   Due to differences in the Ethernet Version 1.0 electrical
     signals compared with Ethernet Version 2.0 and IEEE
     802.3, an Ethernet Version 1.0 controller may not work
     properly with an Ethernet Version 2.0 or IEEE 802.3
     transceiver.  Conversely, Ethernet Version 2.0 and IEEE
     802.3 controllers may not work properly with an Ethernet
     Version 1.0 transceiver.

In most Bridge products, a server with an EC/2 controller
(based on IEEE 802.3) works properly with Ethernet Version
1.0, 2.0, or IEEE 802.3 transceivers and transceiver cables.
However, a server with an EC/1 controller (based on Ethernet
Version 1.0) will not work with Ethernet Version 2.0 or IEEE
802.3 transceivers and transceiver cables.

Most companies with Carrier Sense Multiple Access/Collision
Detection (CSMA/CD) LAN products that run XNS or TCP/IP are
shipping Ethernet Version 1.0 and 2.0 products and not IEEE 802
(i.e., not 802.2).  Most ISO protocol products will be 802.2 and
802.3 compatible.

List of Abbreviations

.br;DA      =   Destination Address
.br;SA      =   Source Address
.br;CRC     =   Cyclic Redundancy Check
.br;DSAP    =   Destination Service Access Point
.br;SSAP    =   Source Service Access Point

Note that the CS/100 servers that were shipping prior to March,
15, 1985, have an Ethernet Version 1.0 interface.  The CS/100
servers that shipped after March 15, 1985, have an IEEE 802.3
interface.  The CS/200, GS/300, IVECS, and NCS/150 servers also
have an IEEE 802.3 interface.

For the GS/3, if it is running SW/3 Version 14020 or  earlier, it
only supports the EC/1 controller (based on Ethernet Version
1.0); if it is running SW/3 Version 15000 or later, it can
support both the EC/1 and EC/2 (based on IEEE 802.3) controllers.

Most Bridge servers are shipped with IEEE 802.3 transceivers and
transceiver cables except for the GS/3 that runs the SW/3 Version
14020 software.





