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Draytek VigorSwitch G1240 Manual

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VigorSwitch G1240 User’s Guide
22
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 there is signal on the medium, the MAC defers the traffic to avoid a
transmission collision and waits for a random period of time, called backoff time, then
sends the traffic again.
After the frame is assembled, when transmitting the frame, the preamble (PRE) bytes are
inserted and sent first, then the next, Start of frame Delimiter (SFD), DA, SA and through
… 28 …


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