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Lecture 9: Multiple Access Protocols

Lecture 9: Multiple Access Protocols. Taxonomy of Multiple Access Protocols. Random Access Protocols Aloha Slotted Aloha Carrier sense multiple access (CSMA): Ethernet Group random access Controlled Access Protocols Predetermined allocation (TDMA) Reservation protocols

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Lecture 9: Multiple Access Protocols

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  1. Lecture 9: Multiple Access Protocols

  2. Taxonomy of Multiple Access Protocols • Random Access Protocols • Aloha • Slotted Aloha • Carrier sense multiple access (CSMA): Ethernet • Group random access • Controlled Access Protocols • Predetermined allocation (TDMA) • Reservation protocols • Token passing protocols

  3. Aloha • If you have a packet, just send it. • If multiple people try it and so there is collision, then try resending it later! • Theoretical analysis (based on Poisson distribution) shows a throughput of only 18%.

  4. Slotted Aloha • Synchronous, that is time is divided into slots • Slot size is equal to the transmission time of a packet • When you are ready, transmit at the start of the time slot. • Doubles the efficiency of Aloha (38% throughput) • But requires synchronization!

  5. Carrier Sense Multiple Access • CSMA • Listen to channel. If busy then wait for a random time and then listen again. If not busy then transmit • Collision may still happen

  6. p-persistent CSMA • Quite like CSMA. • But, when the channel is idle then you transmit with probability p. Otherwise, even when it is idle, you wait for a random time before you listen to the channel.

  7. CSMA/CD • CSMA with collision detection(CD): • Listen while you are sending packets • Stop sending when collision happens • Wait random time before you attempt to resend. • IEEE 802.3 standard • Used in coaxial cable. You do exponetial backoff.

  8. Group Random Access • Instead of random backoff, use a structured search to find one unit to transmit • First enable a group. • If collision happens, then divide the group into two parts and let one part try.

  9. Token Passing • Form a circular list. Pass a token around. Whoever has the token can transmit. • Only the station that wants to trasmit, seize the token and release it after successful transmission.

  10. Reservation Aloha • Channel is divided into time slots of equal size. • Each slot is large enough to transmit a packet. • Slots are arranged into frames of equal size. • Frame size is proportional to propagation delay. • Units compete for slots. Once a unit gets a slot, it retains the slot (across frames) until it no longer needs it. • Efficient for bursty data but no so for single packet.

  11. FIFO Reservation • Channel is divided into slots. Units compete and make reservation for these slots in FIFO manner. • Every one keeps track of the order. • After every M slots, one slot is broken into small reservation slots using which units try to reserve next M slots. • Efficient in handling bursty data. No frame size limitation. But requires tracking of queue!

  12. Round Robin • Channel is divided into equal slots where each unit is a owner of a slot. (Just like TDMA) • But, others can use the slot of a unit UNTIL the unit wants it by creating a collision. • No activity is a signal for others to contend for the slot. • Good for burst data. But contend using other protocols for unused slots.

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