414Lecture18

414Lecture18 - Lecture 18 Link Layer Random Access...

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Lecture 18 Link Layer Random Access Protocols, MAC Addressing, ARP ECSE 414 – Fall 2010
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ECSE 414, Lecture 18 2 Announcements Homework Assignment #4 Due Thursday Nov 11 2010 Michael Rabbat
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ECSE 414, Lecture 18 3 Multiple Access protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time Multiple access protocol Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit Communication about channel sharing must use channel itself! No out-of-band channel for coordination 2010 Michael Rabbat
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ECSE 414, Lecture 18 4 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. simple 2010 Michael Rabbat
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ECSE 414, Lecture 18 5 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning (TDMA, FDMA) divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions “recover” from collisions “Taking turns” nodes take turns, but nodes with more to send can take longer turns 2010 Michael Rabbat
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ECSE 414, Lecture 18 6 Random Access Protocols When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes Two or more transmitting nodes “collision” , • Random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA 2010 Michael Rabbat
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ECSE 414, Lecture 18 7 Slotted ALOHA Assumptions: all frames same size time divided into equal size slots (time to transmit 1 frame) nodes start to transmit only at slot beginning nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation: when node obtains fresh frame, transmits in next slot if no collision: node can send new frame in next slot if collision: node retransmits frame in each subsequent slot with probability p until success 2010 Michael Rabbat
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ECSE 414, Lecture 18 8 Slotted ALOHA Pros single active node can continuously transmit at
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This note was uploaded on 05/09/2011 for the course ECSE 414 taught by Professor Rabbat during the Fall '10 term at McGill.

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414Lecture18 - Lecture 18 Link Layer Random Access...

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