Frame formats are similar between Ethernet and IEEE 802.3. Figure 36.8 depicts the similarities
and differences between the two. The frame structures are read from right to left. Starting at the
right, you see that both frames begin with a preamble. The
Preamble
is a seven-byte field.
(Notice that we have moved from bits to bytes to specify field lengths.) The preamble consists
of alternating 1s and 0s.
The next field is the
SOF
, the start-of-frame delimiter. It is used to synchronize the framereception
portions of all the machines on the segment. This field is only one byte long.
The two fields following the SOF are six bytes each; they are the
Destination
and Source
MAC addresses of the receiving and sending stations. Each MAC address is unique.
Up to this point, the frames are exactly the same. Starting with the next field, they are different.
The next field is a two-byte field in both frame structures. Ethernet defines the field as a Type field;
IEEE 802.3 defines it as a Length field. Ethernet uses this field to specify which upper layer protocol
will receive the packet. IEEE 802.3 uses the field to define the number of bytes in the payload
(802.2 header and data) field. One easy method of observing the difference between an Ethernet
and 802.3 frame is to look at the Type/Length field. If this value is 1500 (0x05DC) or less, then
it is an IEEE 802.3 frame. If it is greater than 1500, it is an Ethernet frame.
Next is the Data field, in both Ethernet and 802.3 formats. The only difference between the
two versions of this field is that Ethernet uses a variable byte size, between 46 and 1500 bytes,
for data. This data is what will be handed to the upper layer protocols. IEEE 802.3 uses a
46–1500 variable byte size, as well, but the information here contains the 802.2 header and the
encapsulated data that will eventually be passed to an upper layer protocol that is defined
within the Data field.
Finally, the last field is the Frame Check sequence (FCS) field. It is four bytes and stores
information that will be used for calculating the CRC after the data has been sent or received.