Signamax
™
065-7921PoE 12-Port 10/100/1000BaseT/TX Web Smart PoE Switch
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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.
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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.
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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.
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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
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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.
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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.
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It is created by the sending MAC and recalculated by the receiving MAC to
check if the packet is damaged or not.
