Contention MAC protocols reminder

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(No Transcript)
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Contention MAC protocols (reminder)

• CSMA/CD (Carrier Sensing Multiple
  Access/Collision Detection)
• It listens to the channel in advance to avoid
  collisions, but when they happen it resends the
  frame (used by Ethernet LAN)
• 1-persistent the station resends with
  probability 1 when the channel is idle
• p-persistent when the channel becomes idle, it
  transmits with probability p
• non-persistent when the channel idle waits a
  random time before transmitting

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Unifying view and adaptive solution

• Mixed strategies are adaptive
• Division of stations into groups every group
  may contend for a different slot
• Limiting case only one member in group
  (collision free protocol)
• Limiting case everybody for one slot (slotted
  ALOHA)
• Intermediate case they choose the group size
  depending on the load (adaptive protocol)
• For example at first all are entered, then if
  there is a collision they are reduced in half and
  half and so on.

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Summary table of MAC solutions
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Wireless networks
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Wireless LAN (1)

• In a wireless LAN not all the stations are
  within the range of one another
• On one hand, this is an advantage because
  multiple transmissions can be executed at one
  time
• On the other hand, some problems may arise

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No difference
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Wireless LAN (2)

• Hidden station problem
• When A transmits to B, C senses the medium as
  free and starts the transmission
• Exposed station problem
• When B transmits to A, C senses the medium as
  busy and does not transmit to D

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MACA (Multiple Access with Collision Avoidance)

• RTS (Request To Send)
• Any station hearing the RTS is close to A and
  must keep silent until a CTS is sent back to A
• CTS (Clear to Send)
• Any station hearing the CTS is close to B and
  must keep silent during the upcoming data
  transmission

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MACA for Wireless

• ACK data
• Carrier sense to avoid RTS collision
• Info on congestion
• Back-off algorithm for each data stream