1 / 35

Papers

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”

tadita
Download Presentation

Papers

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related