| When stations transmit to each other
on a LAN, they format the data in a structured manner so that
devices know what octets signify what information. Various frame
formats are available. When you configure a device, you must define
the format that your station will use, realizing that more than one
format might be configured, as is the case for a router.
Figure illustrates four common frame
formats for Ethernet. Some users interchange the terms packets and
frames
rather loosely. However,
according to RFC 1122, a significant difference does exist. Frames
refer to the entire message, from the data link layer (Layer 2)
header information through and including the user data. Packets
exclude Layer 2 headers and include only the Layer 3
protocol header through and including user data.
The frame formats developed as the
LAN industry and associated protocol requirements evolved. When
Xerox developed the original Ethernet (which was later adopted by
the industry), a frame format like the Ethernet frame in Figure
was defined. The first six octets contain the destination MAC
address, and the next six octets contain the source MAC address. Two
bytes following that indicate to the receiver the type of Layer 3
protocol encapsulated within the data portion of the frame. For
example, if the frame encapsulates an IP packet, then the type
field value is 0x0800. Figure
lists several common protocols and their associated type values.
Following the type value, the
receiver expects to see a protocol header. For example, if the type
value indicates that the packet is IP, the receiver expects to
decode IP headers next. If the value is 8137, the receiver decodes
the encapsulated packet as a Novell packet.
IEEE defined an alternative frame format. In
the IEEE 802.3 formats, the source and destination MAC addresses
remain, but instead of a type field value, the packet length is
indicated. Three derivatives of this format are used in the
industry: raw 802.3, 802.3 with 802.2 Logical Link Control (LLC),
and 802.3 with 802.2 and Subnetwork Access Protocol (SNAP).
A
receiver recognizes that a packet follows 802.3 formats rather than
Ethernet formats by the value of the 2-byte field following the
source MAC address. If the value falls within the range of 0x0000
and 0x05DC (1500 decimal), the value indicates length; protocol type
values begin after 0x05DC, in which case the frame type is
Ethernet Version II. Further, if the 16-bit value following
the type/length field is 0xAAAA, then the frame is a SNAP
(or IEEE 802.3 SNAP) frame; if this value is 0xFFFF, then
the frame is a raw 802.3 (or Novell 802.3 raw)
frame; otherwise, it is a 802.3 with 802.2 LLC (or IEEE
802.3
) frame.
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