CX82100 Home Network Processor Data Sheet
7.1
MAC Frame Format
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).
Figure 7-2. Ethernet MAC Frame Format
Defined in Standard
Implemented
7 Bytes
1 Byte
Preamble
Start Frame Delimiter (SFD)
Destination Address (DA)
Source Address (SA)
Length
7 Bytes
1 Byte
Preamble
Start Frame Delimiter (SFD)
Destination Address (DA)
Source Address (SA)
Length
2 or 6 Bytes
2 or 6 Bytes
2 Bytes
6 Bytes
6 Bytes
2 Bytes
(variable)
(variable)
4 Bytes
(variable)
(variable)
4 Bytes
LLC Data
LLC Data
PAD
PAD
Frame Check Sequence (FCS)
Frame Check Sequence (FCS)
101545_029
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) = X32+X26+X23+X22+X16+X12+X11+X10+X8+X7+X5+X4+X2+X+1.
The 32 bits of the CRC value are placed in the FCS field so that the X31 term is the
leftmost bit of the first byte of the field, and the X0 term is the rightmost bit of the last
byte of the field. The bits of the FCS field are thus transmitted in the order X31, X30, ...,
X2, X1, X0.
7-2
Conexant Proprietary and Confidential Information
101306C
CONEXANT [ CONEXANT SYSTEMS, INC ]
