Information Technology Reference
In-Depth Information
Figure4-2
Ethernet and IEEE 802.3 Frame Formats
Ethernet
7
1
6
6
2
46-1500
4
S
O
F
Field length,
in bytes
Destination
address
Source
address
Preamble
Type
Data
FCS
IEEE 802.3
7
1
6
6
2
46-1500
4
S
O
F
Destination
address
Source
address
802.2 header
and data
Field length,
in bytes
Preamble
Length
FCS
SOF = Start-of-frame delimiter
FCS = Frame check sequence
Both Ethernet and IEEE 802.3 frames begin with an alternating pattern of ones and zeros called a
preamble
. The preamble tells receiving stations that a frame is coming.
The byte before the destination address in both an Ethernet and an IEEE 802.3 frame is a start-of-frame
(SOF) delimiter. This byte ends with 2 consecutive 1 bits, which serve to synchronize the frame
reception portions of all stations on the LAN.
Immediately following the preamble in both Ethernet and IEEE 802.3 LANs are the destination and
source address fields. Both Ethernet and IEEE 802.3 addresses are 6 bytes long. Addresses are contained
in hardware on the Ethernet and IEEE 802.3 interface cards. The first 3 bytes of the addresses are
specified by the IEEE on a vendor-dependent basis, and the last 3 bytes are specified by the Ethernet or
IEEE 802.3 vendor. The source address is always a unicast (single node) address, whereas the destination
address may be unicast, multicast (group), or broadcast (all nodes).
In Ethernet frames, the 2-byte field following the source address is a type field. This field specifies the
upper-layer protocol to receive the data after Ethernet processing is complete.
In IEEE 802.3 frames, the 2-byte field following the source address is a length field, which indicates the
number of bytes of data that follow this field and precede the frame check sequence (FCS) field.
Following the type/length field is the actual data contained in the frame. After physical layer and link
layer processing is complete, this data will eventually be sent to an upper-layer protocol. In the case of
Ethernet, the upper-layer protocol is identified in the type field. In the case of IEEE 802.3, the
upper-layer protocol must be defined within the data portion of the frame, if at all. If data in the frame
is insufficient to fill the frame to its minimum 64-byte size, padding bytes are inserted to ensure at least
a 64-byte frame.
In 802.3 the data field carries a payload header in addition to the payload itself. This header serves the
logical link control sublayer of the OSI model and is completely independent of the MAC sublayer and
physical layer below it. This header, functionally known as 802.2 encapsulation, contains destination
service access point (DSAP) and source service access point (SSAP) information. This will notify higher
protocols what type of payload is actually riding in the frame. It functions like the “type” field in
traditional Ethernet and is used by upper-layer network protocols such as IPX. Network software
developed to support the TCP/IP networking suite uses the type field to determine protocol type in an
Ethernet frame. The type field and the LLC header are not replacements for each other, but they serve to
offer backward compatibility between network protocol implementations without rewriting the entire
Ethernet frame.
After the data field is a 4-byte frame check sequence (FCS) field containing a cyclic redundancy check
(CRC) value. The CRC is created by the sending device and is recalculated by the receiving device to
check for damage that might have occurred to the frame in transit.
