Motorola PowerQUICC II MPC8280 Series Reference Manual page 1307

Chapter 36
Fast Ethernet Controller
The Ethernet IEEE 802.3 protocol is a widely-used LAN based on the carrier-sense
multiple access/collision detect (CSMA/CD) approach. Because Ethernet and IEEE 802.3
protocols are similar and can coexist on the same LAN, both are referred to as Ethernet in
this manual, unless otherwise noted. Ethernet/IEEE 802.3 frames are based on the frame
structure shown in Figure 36-1.
Start Frame
Preamble
Delimiter
7 Bytes 1 Byte
Note: The lsb of each octet is transmitted first.
The elements of an Ethernet frame are as follows:
• 7-byte preamble of alternating ones and zeros.
• Start frame delimiter (SFD)—Signifies the beginning of the frame.
• 48-bit destination address.
• 48-bit source address. Original versions of the IEEE 802.3 specification allowed
16-bit addressing, which has never been used widely.
• Ethernet type field/IEEE 802.3 length field. The type field signifies the protocol used
in the rest of the frame, such as TCP/IP; the length field specifies the length of the
data portion of the frame. For Ethernet and IEEE 802.3 frames to exist on the same
LAN, the length field must be unique from any type fields used in Ethernet. This
requirement limits the length of the data portion of the frame to 1,500 bytes and,
therefore, the total frame length to 1,518 bytes.
• Data
• Four-bytes frame-check sequence (FCS), which is the standard, 32-bit CCITT-CRC
polynomial used in many protocols.
When a station needs to transmit, it waits until the LAN becomes silent for a specified
period (interframe gap). When a station starts sending, it continually checks for collisions
MOTOROLA
Freescale Semiconductor, Inc.
Frame Length is 64–1,518 Bytes
Destination Source
Address Address
6 Bytes 6 Bytes
Figure 36-1. Ethernet Frame Structure
Chapter 36. Fast Ethernet Controller
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Type/
Data
Length
2 Bytes 46–1500 Bytes
Frame Check
Sequence
4 Bytes
36-1