Media Access Control
DLL
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Data Link Layer
• Logical Link Control
• Media Access Control
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Media Access Control
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Random Access/Contention Methods
• station that has data to send uses a procedure
defined by the protocol to make a decision on
whether or not to send.
• State of the medium (idle or busy).
• No scheduled time for a station to transmit.
• Transmission is random among the stations
• Stations compete with one another to access
the medium.
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Random Access/Contention Methods
(Contd..)
• Collision - If more than one station tries to send,
there is an access conflict.
• The frames will be either destroyed or modified.
• When can the station access the medium?
• What can the station do if the medium is busy?
• How can the station determine the success or
failure of the transmission?
• What can the station do if there is an access
conflict?
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Random Access/Contention Methods
(Contd..)
• ALOHA (simple protocol)
• CSMA (station will be force to sense the state
of the medium)
– CSMA/CD
– CSMA/CA
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ALOHA
• University of Hawaii (1970)
• It was designed for a radio (wireless) LAN, but
it can be used on any shared medium.
• Pure ALOHA
• Slotted ALOHA
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Pure ALOHA
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Pure ALOHA (Contd..)
• pure ALOHA protocol relies on
acknowledgments from the receiver.
• When a station sends a frame, it expects the
receiver to send an acknowledgment.
• If the acknowledgment does not arrive after a
time-out period, the station assumes that the
frame (or the acknowledgment) has been
destroyed and resends the frame.
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Pure ALOHA (Contd..)
• A collision involves two or more stations.
• Each station waits a random amount of time
before resending its frame - backoff time TB
• After a maximum number of retransmission
attempts Kmax, a station must give up and try
later.
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Pure ALOHA Procedure
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Simple Task
•
•
•
•
Distance between station is 600Km
Signal Propagates at 3 X 108 m/s
Take No. of attempts=2
Compute the Backoff time
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Vulnerable time (Pure ALOHA)
• Length of time in which possibility of collision.
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Throughput (Pure ALOHA)
• G - average number of frames generated by
the system during one frame transmission
time.
• Average number of successfully transmitted
frames for pure ALOHA is
S = G × e−2G
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Task 2
• A pure ALOHA network transmits 200-bit
frames on a shared channel of 200 kbps. What
is the throughput if the system (all stations
together) produces
a. 1000 frames per second?
b. 500 frames per second?
c. 250 frames per second?
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Slotted ALOHA
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Vulnerable Time
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