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CMPE 259

CMPE 259. Sensor Networks Katia Obraczka Winter 2005 Transport Protocols II. Announcements. Feedback on project proposals. Project resources. Transport protocols (cont’d). RMST CODA Summary. RMST. RMST. Reliable Multi-Segment Transport. Where to do reliability? MAC. Transport.

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CMPE 259

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  1. CMPE 259 Sensor Networks Katia Obraczka Winter 2005 Transport Protocols II

  2. Announcements • Feedback on project proposals. • Project resources.

  3. Transport protocols (cont’d) • RMST • CODA • Summary

  4. RMST

  5. RMST • Reliable Multi-Segment Transport. • Where to do reliability? • MAC. • Transport. • Application.

  6. MAC reliability • 802.11. • RTS/CTS, Data, Ack. • Basic stop-and-wait ARQ. • No ARQ when in broadcast or multicast modes. • Random slot selection. • Options: • No ARQ. • AEQ always. • Selective ARQ.

  7. MAC reliability (cont’d) • Without ARQ: • Use broadcast mode. • For unicast: address screening at routing layer. • +’s: no overhead. • With ARQ: • Unicast transmissions. • For broad- & multicast, use multiple unicast. • Number of retries is configurable. • Selective ARQ: • Unicast uses ARQ. • Broad- and multicast use no ARQ. • E.g., route discovery.

  8. Transport reliability • Strictly e2e. • Initiated by sink. • Local recovery. • Intermediate nodes trigger repair when loss is detected. • Nodes cache packets. • NACK-based.

  9. Application-layer reliability • Directed-diffusion based. • Sink sends out request (“interest”). • When complete data received, sink removes request.

  10. Question? • Benefits of lower-layer reliability? • Additional overhead?

  11. RMST overview • Functions: • Fragmentation/reassembly. • Guaranteed delivery. • Unique identifiers: • “No fragments”. • Fragment id’s and number of fragments. • Loss detection and repair: • Sequence # holes and timers. • Loss detection at either sinks or intermediate nodes. • NACKs.

  12. Preliminary analysis • Demonstrate the benefits of hop-by-hop reliability.

  13. RMST evaluation • MAC-only reliability. • Local recovery. • With and without MAC reliability. • End-to-end reliability. • With and without MAC reliability.

  14. Observations • When there is no transport reliability: • MAC reliability critical in lossy links. • Hop-by-hop transport reliability: • Adds little to reliable MAC. • But, hop-by-hop transport reliability only more efficient than adding MAC reliability. • MAC ARQ overhead incurred in every packet. • E2E transport reliability: • When no MAC reliability is used, simulation does not terminate: hop-by-hop recovery is critical. • If MAC reliability used, hop-by-hop and e2e transport reliability are equivalent.

  15. Observations (cont’d) • Experiments with high error rates: • Hop-by-hop transport reliability without MAC reliability. • Hop-by-hop transport reliability+Sel. ARQ. • E2e transport reliability+ Sel. ARQ. • Hbh transport reliability without ARQ breaks down at high error rates. • Routing has hard time establishing routes.

  16. CODA

  17. COngestion Detection and Avoidance • Importance of congestion control.

  18. What is CODA ? • Energy efficient congestion control. • Three mechanisms are involved: • Congestion detection • Open-loop hop-by-hop backpressure. • Closed-loop multi-source regulation.

  19. Congestion detection • Accurate and efficient congestion detection is important • Channel loading – sample channel at appropriate rate to detect congestion.

  20. 1 3 2 4 Congestion detected 5 6 Open-loop h-by-h backpressure Upstream node decides to propagate backpressure or not.

  21. 1 2 Regulate bit is set 1,2,3 ACK 4,5,6 Congestion detected 7,8 ACK Closed loop multi-source regulation

  22. Congestion detection schemes • Buffer occupancy. • Not reliable in CSMA networks. • Channel loading. • Good for the immediate neighborhood. • Energy considerations. • Report rate. • Report rate goes down, congestion. • Detection based on report rate needs to react on longer time scale.

  23. CODA overview • Combination of backpressure (fast time scale) with closed-loop congestion control. • Backpressure targets “local” congestion, whereas closed-loop regulation targets persistent congestion. • Backpressure is cheaper/simpler since it’s open loop. • Congestion control requires a feedback loop. • Uses ACK from sink to self-clock.

  24. CODA performance metrics • Average Energy Tax = Total packets dropped in network / Total packets received at sink • Average Fidelity Penalty = Difference between average number of packets delivered at sink using CODA and using ideal congestion scheme.

  25. Simulation Setup • Random network topologies with network size from 30 to 120 nodes. • 2Mbps IEEE 802.11 MAC (RTS/CTS are disabled). • Directed diffusion is used as routing core. • Fixed work load, 6 sources and 3 sinks. • Source generate data at different rates. • Event packet is 64 bytes and interest packet is 36 bytes.

  26. Simulation Results(Case 1: Dense Source , High Rate)

  27. Simulation Results(Case 2: Sparse Sources, Low Rate)

  28. Simulation ResultsCase 2: Sparse Source, Low Rate

  29. Simulation Results (Case 3: Sparse Sources, High Rate) Network Size (#no of nodes)

  30. Conclusion • CODA’s energy efficiency. • CODA’s ability to handle persistent and transient congestion.

  31. Transport protocols: summary

  32. Pump Slow Fetch Quickly PSFQ • For sink-to-source communication (e.g. network reprogramming) • Reliability via retransmissions • Sequence-driven loss detection C.Y. Wan, A.T. Campbell, and L. Krishnamurthy. PSFQ: A Reliable Transport Protocol for Wireless Sensor Networks. WSNA'02, September 28, 2002, Atlanta, Georgia, USA.

  33. RMST • End-to-end or hop-by-hop repair (the latter is generally better) • Suggests that repair could be done at either MAC layer (ARQ retransmissions) or Transport Layer (requests based on fragment numbers etc.) • Timer-driven loss detection and local data caches • Fits with the Directed Diffusion API F. Stann and J. Heidemann. RMST: Reliable Data Transport in Sensor Networks. IEEE SNPA'03.

  34. ESRT • Aim for overall quality of service rather than node-to-node reliability Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks ", In Proc. ACM MobiHoc`03

  35. CODA • Receiver based congestion detection • Open loop hop-by-hop backpressure • Closed-Loop multi-source regulation Sankarasubramaniam, Y., Akan, O.B., and Akyildiz, I.F., "ESRT: Event-to-Sink Reliable Transport in Wireless Sensor Networks ", In Proc. ACM MobiHoc`03

  36. Summarizing Transport Issues • Because of harsh conditions and severe constraints, it may be better to implement reliability in a hop-by-hop rather than end-to-end manner at either the MAC or transport layer • For energy efficiency, it is best to avoid congestion entirely, or have packet losses occur close to the source. Back pressure is a useful technique. • Where possible, scheduled solutions are preferable. s

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