Freescale Semiconductor MPC8260 - Manual (Page 1197)

MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 35-1
Chapter 35
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 35-1 .
Figure 35-1. Ethernet Frame Structure
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 on the LAN. If a collision is
detected, the station forces a jam signal (all ones) on its frame and stops transmitting. Collisions usually
occur close to the beginning of a frame. The station then waits a random time period (backoff) before
attempting to send again. When the backoff completes, the station waits for silence on the LAN and then
begins retransmission on the LAN. This process is called a retry. If the frame is not successfully sent within
15 retries, an error is indicated.
Preamble
Start Frame
Data
Delimiter
Destination
Address
Type/
Length
Source
Address
Frame Check
Sequence
7 Bytes 1 Byte 6 Bytes 6 Bytes 2 Bytes 46–1500 Bytes 4 Bytes
Frame Length is 64–1,518 Bytes
Note: The lsb of each octet is transmitted first.