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SPAN: Energy Efficient MAC

SPAN: Energy Efficient MAC. Brian Blum October 28, 2002 Mobile Computing. Presentation Overview. Problem Prior Work SPAN: Design Simulation Results and Analysis Issues / Discussion Questions. Problem: Target Environment. Mobile Ad-Hoc Multi-Hop Networks

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SPAN: Energy Efficient MAC

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  1. SPAN: Energy Efficient MAC Brian Blum October 28, 2002 Mobile Computing

  2. Presentation Overview • Problem • Prior Work • SPAN: Design • Simulation Results and Analysis • Issues / Discussion • Questions

  3. Problem: Target Environment • Mobile Ad-Hoc Multi-Hop Networks • Power Restricted (2 AA batteries) • Location Aware (GPS?) • Time Synchronized • Single Channel • Scalable (100 -> 1,000,000 nodes) • Bandwidth (50 kbps -> 200 kbps)

  4. Related Work • Clustering • GAF: 1 leader / grid, location aware • AFECA: prob. Connectivity • Packet Delivery • Turn off overhearing • Poll BS for buffered Packets • Routing • Route Selection • Radio Radius Modulation • Sensor Networks (CMU – Sentry service)

  5. SPAN: Design Goals • Maintain Throughput and Connectivity • Network Layer solution integrated with Routing component • Link Layer Independent • Rotate Responsibilities

  6. SPAN: Basic Features • Elect Coordinators that: • Minimize total # of coordinators • Maintain connectivity between neighbors • Depend on Local Information • Announcement Delay Depends on: • Number of Neighbors • Remaining Energy • Message Delivery • Buffer Packets at upstream node (coord.)

  7. SPAN: HELLO Message • Periodically Broadcast status msg. • Current coordinators • Current neighbors • Node Maintains • Neighbor Table (route to coord. only) • Neighbor Neighbor’s Table

  8. SPAN: Coordinator Election • If nodes’ 2 neighbors cannot comm. via coordinator, then set delay • Delay Timer (contention prevention) • Number of Neighbors Connected • Round Trip Delay • Energy Remaining

  9. Ci: # Nodes Being Connected Ni: # Neighbors T: Time Delay (100 ms) R: NORM(0,1) Er: Energy Remaining Em: Maximum Energy SPAN:Calculating Broadcast Delay • Delay = ((1- Er/ Em) + (1- Ci/ NiC2) + R) X Ni X T Node Ci Ni C Er Em R Delay Ni 2 675ms 1 3 5 10 90J 100J .55 6 1 3 3 40J 100J .45 516ms

  10. SPAN: Coordinator Withdrawal • Basic • Every Pair of Neighbors can reach each other directly or via some other coordinator • Energy Distribution • If every pair can reach each other via some neighbor (non-coordinator) • Withdraw and Set Grace Period • Connectivity maintained by withdrawal delay

  11. SPAN - How it Works • Sample Network 4 1 5 2 2 6 7 1 3 3 7 5 6 4 Coordinator Node

  12. T0: 5 Announces Withdrawal T1: 1 and 6 Spot Problems 1: (6,3) (7,3) (7,5) 6: (5,7) Calculate Broadcast Delay 1: 675 ms 6: 516 ms T2: 6 Broadcasts HELLO T3: 1 Recognizes 6 T4: 5 Relinquishes Position T5: 1 spots (7,3) problem…. SPAN - Choosing Coordinators 4 2 7 1 3 5 6

  13. Simulation • Simulator: NS-2 • Location: GOD module • MAC: 802.11 w/ Power Saving • Routing: Geographic Forwarding • Via Coordinators w/ followers as backup routes

  14. Simulation Details • SPAN optimizations • Source, Destination: Coordinators • non-coordinator -> coordinator transition when used for routing • 802.11 Power Saving • ATIM window (40 ms) - advertise • Beacon Period (200 ms) - send • Expected Delay (1/2 beacon period)

  15. SPAN on 802.11 Power Saving No advertisements for c->c traffic 1:1 ratio of broadcasts advertised:sent to allow sleep once all are received Advertised traffic window outside of which, only c->c traffic (non-coord. can turn off) ATIM Window (20 ms) Beacon Period (300 ms) Advertised Traffic Window (100 ms) Simulation Details cont.

  16. Simulation Analysis • Capacity • Loss Rate

  17. Simulation Analysis cont. • SPAN vs. Ideal # Coord’s

  18. Simulation Analysis cont. • Power Conservation • System Lifetime

  19. Issues/Discussion • Symmetrical channels • Dec comm. radius w/ dec energy • Large HELLO messages • Neighbor’s neighbors table (large) • Parameter Tuning (how to set?) • Power saving is not function of density • Is full connectivity always necessary? • Buffering: are nodes sinks? • 802.11: appropriate?

  20. Conclusion • SPAN conserves energy in dense networks of mobile devices • Maintain throughput and connectivity • Distribute energy consumption • Handle failures • What type of network is SPAN appropriate for?

  21. Questions Any questions?

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