MAC

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MAC
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MAC

• Now consider broadcast channels. In broadcast
  channels/multiaccess channels/random access
  channels, multiple sources may compete for a
  shared channel.
• The biggest problem now is how to determine who
  gets the channel.
• With point-to-point link, this problem does not
  exist.
• After figuring this out, you can run some
  point-to-point protocol to make sure that things
  got there right or, you dont.

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How would you design a MAC protocol?

• The simplest. Got data, send immediately.
• But, what if there is collision?
• Immediately resend?
• Wait for a constant time and resend?

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ALOHA

• Each station sends when data is available.
• Each station can detect if the frame it sent
  collided with frames from other stations. If yes,
  wait for a random time, resend.

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ALOHA

• What is the performance likely to be?
• At low traffic load, everything will get through.
• At high traffic load, very few frames can get
  through.

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Slotted ALOHA

• Time is divided into slots and each station sends
  at the beginning of time slots.
• Intuitively, why this is better?
• Can we do even better than that?

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Beyond Aloha Carrier Sense Protocols

• CSMA Carrier Sense Multiple Access
• Why call it carrier?

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Carrier Sense

• 1-persistent. Station listens to the channel, if
  busy, wait until idle and send immediately
  (1-persistent). If collision, wait for a random
  time and starts over again.
• Better than ALOHA because at least if someone is
  sending, wont send
• Problems
• Two stations wait for channel to be idle and will
  send at the same time
• If propagation delay is long, B does not know A
  has started sending

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Carrier Sense

• Non-persistent. If no one else is sending, send.
  Else, wait for a random time and check the
  channel again.

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Carrier sense

• P-persistent. Slotted channel. If busy, wait to
  the next time slot. If idle, send with
  probability p. With probability q1-p, defer to
  the next slot. Until he sends or he senses that
  another station has started sending (considered a
  collision). If collision (either real or
  considered), wait for a random time and start
  the process again.

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What next?

• What can we do beyond carrier sense?

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CSMA with collision detection

• CSMA/CD. Improves CSMA by listening to the
  channel and abort immediately when there is a
  collision.
• Used by Ethernet.
• What is the worst case in detecting a collision?

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Worst case