cx82100 Conexant Systems, Inc., cx82100 Datasheet - Page 100

• Manufacturer Part Number: cx82100
• Description: Home Network Processor Hnp
• Manufacturer: Conexant Systems, Inc.

7.1
Figure 7-2. Ethernet MAC Frame Format
7-2
2 or 6 Bytes
2 or 6 Bytes
(variable)
(variable)
7 Bytes
2 Bytes
4 Bytes
1 Byte
MAC Frame Format
Fram e Check Sequence (FCS)
Defined in Standard
Start Fram e Delim iter (SFD)
Destination Address (DA)
Source Address (SA)
Conexant Proprietary and Confidential Information
Figure 7-2 shows the MAC frame format supported by the HNP (see Section 3.1.1 of
Reference [4]). As depicted in the figure, the bytes of a frame are transmitted from top to
bottom. The bits of each byte in each field (with the exception of the FCS) are
transmitted from the LSb to MSb (i.e., LSb transmitted first).
At the head of the MAC frame is the 7-byte preamble 0xAA AA AA AA AA AA AA
followed by the 1-byte Start of Frame Delimiter (SFD) 0xAB. The MAC receiver must
detect the SFD pattern 0xAB. After SFD the MAC must begin receiving the frame
(assuming the Carrier Sense signal is asserted).
The address fields are the next fields in the frame. First is the 48-bit destination address
followed by the 48-bit source address. Then comes the 2-byte length field. Then comes
the LLC data and any pad bits required so the frame size is the minimum allowable
(which is 64 bytes not including FCS).
Finally, we have the 4 byte frame check sequence (FCS) which is a CRC to check for
frame errors due to Ethernet PHY (EPHY) transmission impairments. All fields except
the preamble, SFD, and FCS are used to compute the CRC. The CRC generating
polynomial is G(x) = X
The 32 bits of the CRC value are placed in the FCS field so that the X
leftmost bit of the first byte of the field, and the X
byte of the field. The bits of the FCS field are thus transmitted in the order X
X
Pream ble
LLC Data
Length
2
PAD
, X
CX82100 Home Network Processor Data Sheet
1
, X
0
.
32
+X
26
+X
23
(variable)
(variable)
+X
7 Bytes
6 Bytes
6 Bytes
2 Bytes
4 Bytes
1 Byte
22
+X
16
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+X
Fram e Check Sequence (FCS)
12
Start Fram e Delim iter (SFD)
Destination Address (DA)
+X
0
Source Address (SA)
term is the rightmost bit of the last
Implem ented
11
+X
Pream ble
LLC Data
Length
10
PAD
+X
8
+X
7
+X
31
5
+X
term is the
4
+X
31
2
, X
+X+1.
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30
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