TCP/IP Tutorial and Technical Overview - IBM Redbooks

2.1 Ethernet and IEEE 802 local area networks (LANs)
Two frame formats (or standards) can be used on the Ethernet coaxial cable:
► The standard issued in 1978 by Xerox Corporation, Intel® Corporation, and
Digital Equipment Corporation, usually called Ethernet (or DIX Ethernet)
► The international IEEE 802.3 standard, a more recently defined standard
See Figure 2-1 for more details.
Figure 2-1 ARP: Frame formats for Ethernet and IEEE 802.3
The difference between the two standards is in the use of one of the header
fields, which contains a protocol-type number for Ethernet and the length of the
data in the frame for IEEE 802.3:
► The type field in Ethernet is used to distinguish between different protocols
running on the coaxial cable, and allows their coexistence on the same
physical cable.
► The maximum length of an Ethernet frame is 1526 bytes. This means a data
field length of up to 1500 bytes. The length of the 802.3 data field is also
limited to 1500 bytes for 10 Mbps networks, but is different for other
transmission speeds.
► In the 802.3 MAC frame, the length of the data field is indicated in the 802.3
header. The type of protocol it carries is then indicated in the 802.2 header
(higher protocol level; see Figure 2-1). In practice, however, both frame
formats can coexist on the same physical coaxial. This is done by using
protocol type numbers (type field) greater than 1500 in the Ethernet frame.
However, different device drivers are needed to handle each of these
formats.
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