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S3C2500B
7.5 ETHERNET OPERATIONS
7.5.1 MAC FRAME FORMAT
Table 7-2 lists the eight fields in a standard (IEEE 802.3/Ethernet frame).
Field Name
Preamble
Start frame delimiter
(SFD)
Destination address
Source address
Length or type
Logical link control
(LLC) data
PAD
Frame check
sequence (FCS)
Table 7-53. MAC Frame Format Description
Field Size
7-byte
The bits in each preamble byte are 10101010, transmitted from left
to right.
1-byte
The SFD bits are 10101011, transmitted from left to right.
6-byte
The destination address can be an individual address or a multicast
(or broadcast) address.
6-byte
The MAC does not interpret the source address bytes. However, to
qualify as a valid station address, the first bit transmitted (the LSB
of the first byte) must be a '0'.
2-byte
The MAC treats length fields greater than 1500-byte as type fields.
Byte values less than or equal to 1500 indicate the number of
logical link control (LLC) data bytes in the data field. The MAC
transmits the high-order byte first.
46 to 1500-byte
Data bytes used for logical link control.
0 to 46-byte
If the LLC data is less than 46-byte long, the MAC transmits pad
bytes of all zeros.
4-byte
The FCS field contains a 16-bit error detection code that is
computed as a function of all fields except the preamble, the SFD,
and the FCS itself. The FCS - 32 polynomial function is as follows:
32
'X
2
X
+ X
26
23
22
16
+ X
+ X
+X
+ X
+ X
1
+ 1'.
ETHERNET CONTROLLER
Description
12
11
10
8
+ X
+ X
+ X
+ X
7
5
4
+ X
+ X
+
7-37
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