Motorola MPC8260 User Manual
MOTOROLA
Chapter 30. Fast Ethernet Controller
30-1
Chapter 30
Fast Ethernet Controller
Fast Ethernet Controller
300
300
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 30-1.
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 30-1.
Figure 30-1. Ethernet Frame Structure
The elements of an Ethernet frame are as follows:
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7-byte preamble of alternating ones and zeros.
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Start frame delimiter (SFD)ÑSigniÞes the beginning of the frame.
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48-bit destination address.
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48-bit source address. Original versions of the IEEE 802.3 speciÞcation allowed 16-
bit addressing, which has never been used widely.
bit addressing, which has never been used widely.
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Ethernet type Þeld/IEEE 802.3 length Þeld. The type Þeld signiÞes the protocol used
in the rest of the frame, such as TCP/IP; the length Þeld speciÞes the length of the
data portion of the frame. For Ethernet and IEEE 802.3 frames to exist on the same
LAN, the length Þeld must be unique from any type Þelds 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.
in the rest of the frame, such as TCP/IP; the length Þeld speciÞes the length of the
data portion of the frame. For Ethernet and IEEE 802.3 frames to exist on the same
LAN, the length Þeld must be unique from any type Þelds 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.
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Data
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Four-bytes frame-check sequence (FCS), which is the standard, 32-bit CCITT-CRC
polynomial used in many protocols.
polynomial used in many protocols.
When a station needs to transmit, it waits until the LAN becomes silent for a speciÞed
period (interframe gap). When a station starts sending, it continually checks for collisions
period (interframe gap). When a station starts sending, it continually checks for collisions
Preamble
Start Frame
Data
Delimiter
Destination
Address
Type/
Length
Source
Address
Frame Check
Sequence
7 Bytes
1 Byte
6 Bytes
6 Bytes
2 Bytes
46Ð1500 Bytes
4 Bytes
Frame Length is 64Ð1,518 Bytes
Note: The lsb of each octet is transmitted first.