Lectures13_14

Lectures13_14 - Lectures 13 & 14 Packet Multiple...

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Packet Multiple Access: The Aloha protocol Eytan Modiano Massachusetts Institute of Technology Eytan Modiano Slide 1
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Multiple Access Shared Transmission Medium a receiver can hear multiple transmitters a transmitter can be heard by multiple receivers the major problem with multi-access is allocating the channel between the users; the nodes do not know when the other nodes have data to send Need to coordinate transmissions Eytan Modiano Slide 2
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Examples of Multiple Access Channels Local area networks (LANs) Traditional Ethernet Recent trend to non-multi-access LANs satellite channels Multi-drop telephone Wireless radio Medium Access Control (MAC) Regulates access to channel Logical Link Control (LLC) All other DLC functions NET DLC PHY MAC LLC Eytan Modiano Slide 3
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Approaches to Multiple Access Fixed Assignment (TDMA, FDMA, CDMA) each node is allocated a fixed fraction of bandwidth Equivalent to circuit switching very inefficient for low duty factor traffic Contention systems Polling Reservations and Scheduling Random Access Eytan Modiano Slide 4
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Aloha Single receiver, many transmitters Receiver ... . Transmitters E.g., Satellite system, wireless Eytan Modiano Slide 5
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Slotted Aloha Time is divided into “slots” of one packet duration E.g., fixed size packets When a node has a packet to send, it waits until the start of the next slot to send it Requires synchronization If no other nodes attempt transmission during that slot, the transmission is successful Otherwise “collision” Collided packet are retransmitted after a random delay 1 3 4 5 2 Success Idle Collision Idle Success Eytan Modiano Slide 6
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Slotted Aloha Assumptions Poisson external arrivals No capture Packets involved in a collision are lost Capture models are also possible Immediate feedback Idle (0) , Success (1), Collision (e) If a new packet arrives during a slot, transmit in next slot If a transmission has a collision, node becomes backlogged while backlogged, transmit in each slot with probability q r until successful Infinite nodes where each arriving packet arrives at a new node Equivalent to no buffering at a node (queue size = 1) Pessimistic assumption gives a lower bound on Aloha performance Eytan Modiano Slide 7
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Lectures13_14 - Lectures 13 & 14 Packet Multiple...

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