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CS 15-849E: Wireless Networks (Spring 2006) MAC Layer Discussion Leads: Abhijit Deshmukh Sai Vinayak Instructor : Srinivasan Seshan. Papers. “An Energy-Efficient MAC Protocol for Wireless Sensor Networks” Wei Ye, John Heidemann, Deborah Estrin “The Case for Heterogenous Wireless MACs”

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Papers

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  1. CS 15-849E: Wireless Networks (Spring 2006)MAC LayerDiscussion Leads:Abhijit Deshmukh Sai VinayakInstructor: Srinivasan Seshan

  2. Papers “An Energy-Efficient MAC Protocol for Wireless Sensor Networks” Wei Ye, John Heidemann, Deborah Estrin “The Case for Heterogenous Wireless MACs” Chung-cheng Chen, Haiyun Luo “Design and Evaluation of a new MAC Protocol for Long-Distance 802.11 Mesh Networks” Wei Ye, John Heidemann, Deborah Estrin

  3. Outline • Motivation • MAC – Wireless Sensor Networks • Heterogenous Wireless MACs • MAC for Mesh Networks • Take Aways • Similarities and Differences • Q & A

  4. Motivation • Last Lecture • MACAW, Carrier Sense, Idle Sense • Basic Terms, Algorithms • Major Focus on Fairness • Very Generic • Special Requirements for • Sensor Networks • Heterogeneous • Mesh Networks

  5. MAC for Sensor Networks • Sensor Networks • Sensors, Embedded processor, Radio, Battery • Ad hoc fashion • Proximity, short-range multi-hop communication • Committed to One or few applications • MAC Protocol • Energy Efficiency • Scalability • Accommodate network changes • Fairness, Latency, Throughput and Bandwidth

  6. Sensor Networks • Sources of Energy Waste ? • Collision • Overhearing • Control packet overhead • Idle Listening • Tradeoff of fixing these • Reduction in per-hop fairness and latency. How? • Message Passing, Fragment long message • Why not a big concern in Sensor Networks? • Application-level performance

  7. Related Work • PAMAS • Avoid overhearing among neighbors • Two independent radio channels • Suffers from idle listening • TDMA • Natural Savings • Scheduling • Static • Piconet • Periodic Sleep

  8. Sensor-MAC Protocol Design • Periodic Listen and Sleep • Message Passing • Collision and Overhearing Avoidance

  9. Periodic Listen and Sleep • Basic Scheme • Turn off Radio, set timer to wake up, sleep • Clock Drift • Sync using relative timestamps • Long listen period • Reduce Control Overhead • Sync with neighbors, exchange schedules • Advantage over TDMA ? • Looser Synchronization • Disadvantage? • Latency due to switching, RTS/CTS

  10. Periodic Listen and Sleep • Choosing and Maintaining Schedules • Schedule Table • Synchronizer • Follower Rebroadcast SYNC Listen Wait (random) Wait (random)

  11. Periodic Listen and Sleep • Maintaining Synchronization • SYNC packet • Listen Interval • SYNC + RTS

  12. Collision & Overhearing Avoidance • Collision Avoidance • NAV • Virtual vs. Physical Carrier Sense • Overhearing Avoidance • Listening to all transmissions • Who all should sleep? • All neighbors of sender and receiver x x E C A B D F

  13. Message Passing • Long vs. Short Message Length • Stream of Fragments, single RTS-CTS • Problem? • No Fairness • 802.11 Methodology? • Why send ACK after each fragment? • Prevent hidden terminal problem

  14. Implementation • Rene Motes + Tiny OS • Simplified IEEE 802.11 • Message Passing (overhearing avoidance) • S-MAC (Message Passing + Periodic Sleep) • Topology used

  15. Results • Low performance for high loads? • Synchronization overhead (SYNC packets) • Latency

  16. Heterogeneous Wireless MACs • Basic Service Set (BSS) • Careful Channel Assignment • Wireless interference • Limited orthogonal channels

  17. Motivation • Exposed Receiver – Hidden Sender CTS / RTS ? data data x Blocked S1  R1 ? ACK

  18. 4-way Handshake? • Hidden Receiver • Exposed Sender

  19. Incomplete vs. Inconsistent • Channel status at sender • Incomplete estimate of receiver • Inconsistent at multiple competing senders • Incomplete channel status == high packet loss • Inconsistent channel status == unfair channel sharing

  20. Intra-BSS Interference Mitigation • When to use 4-way handshake? • Client detecting data transmission vs. Client’s data transmission being detected • Access point to initiate channel access? • BSS in center • Less chance of interference from other BSS

  21. Inter-BSS Interference Mitigation • RTR (Request to receive) • RTR-DATA vs. RTS-CTS-DATA • ACK in form of next RTR • Stateless Approach • Alternating between MAC protocols • Simple Design and Implementation • Low Channel Utilization

  22. Fairness • Why is flow 23 getting unfair treatment? • Client 3 is exposed receiver • Receiver 1 is not interfered by 23 • How to solve it ? • Switch to receiver initiated protocol • Increase power levels of CTS/RTS

  23. MAC for Long Dist. 802.11 Mesh • Motivation • Extend 802.11 for long haul • Challenges • Use off-the shelf hardware • Low cost

  24. Overview • Basic Principle • SynRx & SynTx

  25. Design and Implementation • Design decisions driven by • Low cost considerations • Usage of off-the-shelf 802.11 hardware • Achieving SynOp • Get rid of immediate ACKs • Get rid of carrier sense backoffs

  26. Design and Implementation (contd.) Immediate Acks • Use IBSS mode of operation • Convert IP unicast to MAC broadcast • No ACKs for broadcast packets in IBSS mode • Broadcast = Unicast since link is 1-1 • ACKs can be implemented at the driver level Carrier Sensed Backoffs • Make use of feature provided by Intersil Prism chipsets

  27. 2P Operation on Single Link • Marker acts as a token • Loose Synchrony

  28. 2P Operation on Single Link (contd.) • Need to handle 2 scenarios • Temporary loss of synchrony (loss of marker) • Link recovery after failure • 2P handles both using timeouts • Advantages • Link-resync process is quick • CRC errors do not cause timeout (other than marker) …. Why ?

  29. 2P Operation on Single Link (contd.) • Two ends of a link get out of synchrony at the same time and timeout together …. So? • They would not hear each others marker packets since both SynTx coincides … So? • Repeated Timeouts … !!! Solution …? • Staggered timeouts  Bumping

  30. Topology Formation • What are the topologies in which 2P? • Bipartite ? • A tree is trivially bipartite • Bad in terms of fault tolerance • Add redundancy but turn on only one tree at a time (Morphing) • 3 Heuristics • Reduce length of links used • Avoid short angles between links • Reduce hop-count

  31. Evaluation • Goal is threefold • Measure impact of step by step link establishment • Study effect of 2P in a large topology • Study performance of TCP over 2P • Link Establishment • 12.9 ms for first case (delay due to bumping) • 4.9 afterwards

  32. Throughput

  33. 2P vs TCP

  34. Similarities and Differences Similarities • MAC protocol implementations • Extend 802.11 for a specific environment • Others? Differences • Deployment scenarios • Energy Saving, Long haul, Heterogeneity • Writing Style • Others?

  35. Q & A

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