Draytek VigorSwitch P1085, Руководство пользователя, Страница: 261 / 267

VigorSwitch P1085 User’s Guide 253
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.
It is created by the sending MAC and recalculated by the receiving MAC to check if the packet
is damaged or not.
H
H
o
o
w
w
d
d
o
o
e
e
s
s
a
a
M
M
A
A
C
C
w
w
o
o
r
r
k
k
?
?
The MAC sub-layer has two primary jobs to do:
1. Receiving and transmitting data. When receiving data, it parses frame to detect error;
when transmitting data, it performs frame assembly.
2. Performing Media access control. It prepares the initiation jobs for a frame transmission
and makes recovery from transmission failure.
F F r r a a m m e e t t r r a a n n s s m m i i s s s s i i o o n n
As Ethernet adopted Carrier Sense Multiple Access with Collision Detect (CSMA/CD), it detects
if there is any carrier signal from another network device running over the physical medium
when a frame is ready for transmission. This is referred to as sensing carrier, also “Listen”. If