Ethernet Frame
Formats
Introduction
An understanding of the basics of the Ethernet Frame
Format is crucial to any discussion of Ethernet
technology.
In this section, we will discuss:
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The four different frame formats used in the
Ethernet world; the purpose of each of the fields in
an Ethernet frame; the reasons that there are so
many different versions of the Ethernet Frame Format
- Ethernet, Ethernet, Ethernet, or Ethernet?! When
somebody tells me that they are running Ethernet on
their network, I inevitably have to ask: "Which
Ethernet?". Currently, there are many versions of
the Ethernet Frame Format in the commercial
marketplace, all subtly different and not
necessarily compatible with each other.
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The explanation for the many types of Ethernet Frame
Formats currently on the marketplace lies in
Ethernet's history. In 1972, work on the original
version of Ethernet, Ethernet Version 1, began at
the Xerox Palo Alto Research Center. Version 1
Ethernet was released in 1980 by a consortium of
companies comprising DEC, Intel, and Xerox. In the
same year, the IEEE meetings on Ethernet began. In
1982, the DIX (DEC/Intel/Xerox) consortium released
Version II Ethernet and since then it has almost
completely replaced Version I in the marketplace. In
1983 Novell NetWare '86 was released, with a
proprietary frame format based on a preliminary
release of the 802.3 spec. Two years later, when the
final version of the 802.3 spec was released, it had
been modified to include the 802.2 LLC Header,
making NetWare's proprietary format incompatible.
Finally, the 802.3 SNAP format was created to
address backwards compatibility issues between
Version 2 and 802.3 Ethernet.
As you can see, the large number of players in the
Ethernet world has created a number of different
choices. The bottom line is this: either a particular
driver supports a particular frame format, or it
doesn't. Typically, Novell stations can support any of
the frame formats, while TCP/IP stations will support
only one although there are no hard and fast rules in
Networking.
Ethernet Frame Formats
The following sections will outline the specific fields
in the different types of Ethernet frames. Throughout
the section, we will refer to fields by referencing
their "offset" or number of bytes from the start of the
frame beginning with zero. Therefore, when we say that
the destination address field is from offset zero
through five we are referring to the first six bytes of
the frame.
The Preamble
Regardless of the frame type being used, the means of
digital signal encoding on an Ethernet network is the
same. While a discussion of Manchester Encoding is
beyond the scope of this page, it is sufficient to say
that on an idle Ethernet network, there is no signal.
Because each station has its own oscillating clock, the
communicating stations have to have some way to "synch
up" their clocks and thereby agree on how long one bit
time is. The preamble facilitates this. The preamble
consists of 8 bytes of alternating ones and zeros,
ending in 11.
A station on an Ethernet network detects the change in
voltage that occurs when another station begins to
transmit and uses the preamble to "lock on" to the
sending station's clock signal. Because it takes some
time for a station to "lock on", it doesn't know how
many bits of the preamble have gone by. For this reason,
we say that the preamble is "lost" in the "synching up"
process. No part of the preamble ever enters the
adapter's memory buffer. Once locked on, the receiving
station waits for the 11 that signals that the Ethernet
frame follows.
Most modern Ethernet adapters are guaranteed to achieve
a signal lock within 14 bit-times.
The Different "Flavors" of Ethernet
While the preamble is common to every type of Ethernet,
what follows it is certainly not. The major types of
Ethernet Frame Format are: |
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