Publication date: Mar. 2016
33600 Rev B
24
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.
How does a MAC work?
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.
Frame transmission
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 the data field and FCS field in turn. The followings summarize
what a MAC does before transmitting a frame.
1.
MAC will assemble the frame. First, the preamble and Start-of-Frame
delimiter will be put in the fields of PRE and SFD, followed DA, SA, tag
ID if tagged VLAN is applied, Ethertype or the value of the data length,
and payload data field, and finally put the FCS data in order into the
responded fields.
2.
Listen if there is any traffic running over the medium. If yes, wait.
3.
If the medium is quiet, and no longer senses any carrier, the MAC
waits for a period of time, i.e. inter-frame gap time to have the MAC
ready with enough time and then start transmitting the frame.
4.
During the transmission, MAC keeps monitoring the status of the
medium. If no collision happens until the end of the frame, it transmits
successfully. If there is a collision happened, the MAC will send the
patterned jamming bit to guarantee the collision event propagated to
all involved network devices, then wait for a random period of time, i.e.
back off time. When backoff time expires, the MAC goes back to the
beginning state and attempts to transmit again. After a collision
happens, MAC increases the transmission attempts. If the count of the
transmission attempt reaches 16 times, the frame in MAC’s queue will
be discarded.
