C.3 IEEE 802.3 and Ethernet Differences
In September 1980, Digital, Intel and Xerox (DIX) published 'the blue
book' Version 1.0: A Local Area Network, Data Link Layer and Physical
Layer specifications. It was later revised as Version 2 in November 1982.
Version 2.0 stated that it was an interim specification and the Ethernet
Specification would now evolve with the Standards bodies (IEEE). In June
1983 the IEEE published its specification for Ethernet under the banner of
IEEE 802.3.
The IEEE improved and modified the original specifications in the
following ways:
1. SQE Test. The SQE test or heartbeat in the transceiver is a collision
detection circuit which is turned on inside the transceiver after each
transmission. However, DIX version 1.0 does not specify this feature,
and therefore transceivers designed to that specification will fail to
operate with units designed to use IEEE standards.
2. Jabber Control Function. IEEE 802.3 provides a Jabber Control
feature that prevents stations on a network from continually
transmitting. It will disconnect the station if an attempt is made to
transmit for a longer period than a valid data packet would normally
take to transmit. DIX version 1.0 does not provide this protection.
3. Transformer Coupling. IEEE 802.3 specification provides a more
sensitive signal across the transceiver cable by specifying a half-step
differential voltage drop across the transceiver input, resulting in a zero
voltage drop in the idle state on the transceiver input.
4. Specific Range Values. IEEE 802.3 specification defines more
electrical tolerances than the DIX specification. These are input
impedance, input capacitance, bias current, and collision detect
thresholds.
5. AUI Cable Earthing. IEEE 802.3 requires the shield to be connected
only to connector shell. DIX 1 and 2 require tie pin 1 to connect to shell
and cable shield.
The above points do not prevent IEEE and DIX equipment operating on the
same network (as long as IEEE stations use IEEE transceivers). However,
there is a fundamental difference in the Data Link Layer of the DIX and
IEEE specifications.
X870-300351 Issue 1 C-7 Rev.0
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