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S3C2501X
ETHERNET CONTROLLER
7-37
7.5 ETHERNET OPERATIONS
7.5.1 MAC FRAME FORMAT
Table 7-29 lists the eight fields in a standard (IEEE 802.3/Ethernet frame).
Table 7-29. MAC Frame Format Description
Field Name
Field Size
Description
Preamble
7-byte
The bits in each preamble byte are 10101010, transmitted from left
to right.
Start frame delimiter
(SFD)
1-byte
The SFD bits are 10101011, transmitted from left to right.
Destination address
6-byte
The destination address can be an individual address or a multicast
(or broadcast) address.
Source 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'.
Length or type
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.
Logical link control
(LLC) data
46 to 1500-byte
Data bytes used for logical link control.
PAD
0 to 46-byte
If the LLC data is less than 46-byte long, the MAC transmits pad
bytes of all zeros.
Frame check
sequence (FCS)
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:
'X
32
+ X
26
+ X
23
+X
22
+ X
16
+ X
12
+ X
11
+ X
10
+ X
8
+ X
7
+ X
5
+ X
4
+
X
2
+ X
1
+ 1'.
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