Intel
IXF1002 Dual Port Gigabit Ethernet Controller
Datasheet
79
6.3
MAC Frame Format
Ethernet is the generic name for the network type implementing the IEEE 802.3 Standard. An
Ethernet frame has a minimum length of 64 bytes and a maximum length of 1518 bytes, excluding
the preamble and the SFD bytes.
An Ethernet frame format consists of the following fields:
Preamble
Start frame delimiter (SFD)
Two address fields
Length or type field
Data field
Frame check sequence (CRC value)
Figure 17
shows the MAC frame format.
Note:
Numbers in parentheses indicate field length in bytes.
VLAN tagged MAC frames contain an additional 4 byte Qtag prefix field, as described in
Section
4.2.2
.
Table 12
describes the Ethernet frame fields.
The CRC polynomial, as specified in the 802.3 Standard, is as follows:
FCS(X) = X
31
+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
The 32 bits of the CRC value are placed in the FCS field so that the X31 term is the right-most bit
of the first octet. The CRC bits are thus transmitted in the following order: X31, X30,..., X1, X0.
A frame octet is transferred on the serial line from the LSB to the MSB.
Figure 17. MAC Frame Format
A5469-01
Preamble
(7)
(1)
(6)
(6)
(2)
(46..1500)
(4)
Destination
Address
Source
Address
Length/
Type
Data+
Padding
SFD
FCS
Table 12. Ethernet Frame Description
Field
Preamble
Start Frame
Delimiter (SFD)
Destination
address
Source address
Description
A 7-byte field of alternating 1s and 0s: 10101010.
A single-byte field containing the value 10101011.
A 6-byte field containing either a specific station address, or the broadcast address, or a
multicast (logical) address, all of which indicate the frame
’
s destination.
A 6-byte field containing the specific station address of frame origin.
A 2-byte field indicating whether the frame is in the IEEE 802.3 format or the Ethernet
format. A field greater than 1500 is interpreted as a type field, which defines the protocol
type. A field smaller than or equal to 1500 (05-DC) is interpreted as a length field, indicating
the number of data bytes in the frame.
A data field consisting of 0 to 1500 information bytes. This data field is fully transparent
because any arbitrary sequence of bits can occur. A data field shorter than 46 bytes,
specified by the length field, is allowed. In its default mode, padding is enabled and up to 46
bytes are added to the data field by the IXF1002 when transmitting.
A 32-bit cyclic redundancy check (CRC), computed as a function of the destination address
field, source address field, type field, and data field. The FCS is appended to each
transmitted frame and is used during reception to determine if the received frame is valid.
Length/type
Data+Padding
Frame Check
Sequence (FCS)