Medium Access Control MAC

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Medium Access Control (MAC)

• EECE 542

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General Categories of Medium Access

• Random Access
• Controlled Access

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Random Access Traits

• Attempt to ensure equal access to the medium for
  all stations
• No scheduling or polling
• Susceptible to collisions
• Two or more stations trying to access the medium
  at the same, resulting in their frames
  interfering with each other.

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Random Access Issues

• When can a station access the medium?
• What should a station do if the medium is
  occupied?
• What will determine the success or failure of a
  given transmission?
• What should a station do if a collision occurs?

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Random Access Protocols

• Aloha
• Slotted Aloha
• CSMA
• CSMA/CD
• CSMA/CA

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Delay-Bandwidth Product

• Determines the efficiency that two stations can
  share a common medium.
• Equals the product of the propagation time tprop
  and data rate R
• Can also be though of as the length of the medium
  in bits.

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• 2 tprop is required to coordinate access for each
  packet transmitted

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• 2 tprop is required to coordinate access for each
  packet transmitted
• Efficiency of the channel
• Where a is the ratio of the (one way)
  delay-bandwidth product to the average packet
  length
• Also considered the ratio of medium length to the
  frame length in terms of bits.
• Note an error in Ch. 16 of your text (no factor
  of 2 in front of term a)

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ALOHA

• Developed by the University of Hawaii to network
  the Hawaiian islands in 1970
• Wireless LAN at 9600 bps
• Simple
• Station sends frame as soon when it has a frame
  to send
• It does not check the medium for availability
• It does not check for collisions
• If an acknowledgement is not received within two
  propagation periods, it resends the frame

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ALOHA Throughput

• Assume frames arrive at a rate G which is called
  the total arrival rate or total load
• Includes both original frames and re-transmited
  frames due to collisions
• Throughput S is the product of G and the
  probability of no collision

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• The vulnerability window for a transmission is
  shown to be 2X seconds long
• Assuming frames are generated by a Poisson
  distribution at rate G, the probability of k
  transmissions occurring within the 2X second
  interval is
• Therefore, the probability of no collision equals
  the probability of 0 transmissions within the 2X
  second interval is
• So throughput for ALOHA is given by

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Slotted ALOHA

• Improves the performance of ALOHA by reducing the
  probability of collisions
• Time is divided into slots of X (Tframe) seconds
• Stations wishing to transmit can only begin a
  transmission at the beginning of a time slot

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• Vulnerability period is reduced to X seconds
• Pno collision
• Throughput

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ALOHA vs. Slotted ALOHA
S
G
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Carrier Sense Multiple Access (CSMA)

• Reduces the probability of collision by sensing
  the medium for the presence of a frame prior to
  transmitting
• Listen before talking
• Collisions can still occur
• If a frame is sensed on the medium, then a
  persistence strategy is employed

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• Vulnerability time tprop

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Persistence

• Persistence refers to the action a station takes
  if it senses a carrier on a CSMA system.
• Three persistence strategies
• Nonpersistent
• 1-Persistent
• p-Persistent

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Nonpersistent

• A station with a frame to send senses the line
• If the line is idle, the frame is sent w/o delay
• If the line is busy, the station waits a random
  amount of time before sensing the line again
  (backoff algorithm)
• Probability of collision is reduced but the
  efficiency is decreased
• Medium has significant idle periods even if
  stations have frames to send

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Nonpersistent
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1-Persistent

• A station with a frame to send senses the line
• If the line is idle, the frame is sent w/o delay
• If the line is busy, the station continues to
  sense the channel until the line is idle and then
  transmits
• If more than one station is waiting a collision
  will occur
• Relatively high collision rate

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1-Persistent
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p-Persistent

• After a station senses an idle line it may or may
  not send the frame
• Sends with probability p
• Refrains with probability 1-p
• A random number generator is used to determine
  whether the station sends
• If the station refrains from sending, it waits
  one time slot before sensing again
• Fewer collisions than 1-Persistent

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p-Persistent
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Non-persistent CSMA
S vs. G for different values of a
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1-Persistent CSMA
S vs. G for different values of a
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CSMA-CD

• Carrier Sense Multiple Access with Collision
  Detection
• When a frame is sent the line is monitored to
  determine if a collision occurs
• No collision successful transmission
• Collision the station sends a jam signal and
  waits a random amount of time (backoff algorithm)
  before sensing the medium again
• 2 tprop is required before determining a
  transmission occurred without collision
• Used in Ethernet

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Backoff Algorithm

• IEEE 802.3 standard specifies the truncated
  binary exponential backoff algorithm
• A frame undergoing its nth retransmission
  attempt determines the delay by selecting an
  integer between the range of 0 and 2k-1 and
  multiplying by 2tprop, where k min(n, 10)
• Typically a maximum of 15 retransmissions are
  attempted before the frame is aborted

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CSMA-CD
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CSMA-CD Performance

• Maximum normalized throughput
• Average normalized delay

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Maximum Achievable Throughput
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CSMA-CD Avg. Delay
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CSMA-CA

• CSMA with Collision Avoidance
• Collisions are completely avoided by using
  appropriate delays, acknowledgements, and
  time-out values.
• Backoff algorithm is also employed
• Method similar to that used in IEEE 802.11
  (Wireless LAN)

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CSMA-CA
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Controlled Medium Access

• Most common method is Token Passing
• A token is passed around a physical or logical
  ring
• Only the station with the token may transmit
• Other methods
• Reservation Systems
• Polling