LevelOne GEP-0950 - Manual (Page 24)

18
PRE SFD DA SA Type/Length Data Pad bit if any FCS
7 7 6 6 2 46-1500 4
- Preamble (PRE) —The PRE is 7-byte long with alternating pattern of ones
and zeros used to tell the receiving node that a frame is coming, and to
synchronize the physical receiver with the incoming bit stream. The
preamble pattern is:
10101010 10101010 10101010 10101010 10101010 10101010 10101010
- Start-of-frame delimiter (SFD) — The SFD is one-byte long with
alternating pattern of ones and zeros, ending with two consecutive 1-bits. It
immediately follows the preamble and uses the last two consecutive 1s bit to
indicate that the next bit is the start of the data packet and the left-most bit in
the left-most byte of the destination address. The SFD pattern is 10101011.
- Destination address (DA) — The DA field is used to identify which
network device(s) should receive the packet. It is a unique address. Please
see the section of MAC addressing.
- Source addresses (SA) — The SA field indicates the source node. The SA
is always an individual address and the left-most bit in the SA field is always
0.
- Length/Type — This field indicates either the number of the data bytes
contained in the data field of the frame, or the Ethernet type of data. If the
value of first two bytes is less than or equal to 1500 in decimal, the number
of bytes in the data field is equal to the Length/Type value, i.e. this field acts
as Length indicator at this moment. When this field acts as Length, the
frame has optional fields for 802.3/802.2 SNAP encapsulation, 802.3/802.2
encapsulation and Netware 802.3 RAW encapsulation. Each of them has
different fields following the Length field.
- If the Length/Type value is greater than 1500, it means the Length/Type
acts as Type. Different type value means the frames with different protocols
running over Ethernet being sent or received.
For example,
0x0800 IP datagram
0x0806 ARP
0x0835 RARP
0x8137 IPX datagram
0x86DD IPv6
- Data — Less than or equal to 1500 bytes and greater or equal to 46 bytes.
If data is less than 46 bytes, the MAC will automatically extend the padding
bits and have the payload be equal to 46 bytes. The length of data field
must equal the value of the Length field when the Length/Type acts as
Length.
- Frame check sequence (FCS) — This field contains a 32-bit cyclic
redundancy check (CRC) value, and is a check sum computed with DA, SA,
through the end of the data field with the following polynomial.