Wireless Sensor Networks Power Management

1. Wireless Sensor NetworksPower Management Professor Jack Stankovic Department of Computer Science University of Virginia

2. Critical Issue – Cross Cuts

3. Problem Statement • Increase the lifetime of the system while meeting functional requirements. • Maintain (high quality) communication coverage • Provide sensing coverage

4. Questions? • Will solar cells solve the problem? • Will energy scavenging (in general) solve the problem? • Will batteries just get much better? • Will devices require less and less power?

5. Questions • How do you define system lifetime? • Can we solve the lifetime problem with high density? Ideal: 7 x 9 = 63 nodes Per area Rotate 63 times increase In lifetime

6. Aging of System(with sleeping nodes) After a certain amount of time, active nodes eventually die off. Neighboring active nodes must detect this loss and issue help message (via wakeup) OR neighboring passive nodes must periodically wake up and detect loss and switch to active state.

7. Outline • Hardware layer • MAC layer • Routing layer • Overarching power management schemes • Sentry service • Tripwire service • Duty cycle • Adaptive PM using control theory

8. Power Management- Hardware layer • Turn off/on • CPU • Memory • Sensors • Radio (most expensive) • Fully awake ………… Deep Sleep • Dynamic voltage scaling also possible • SW ensures a node/components are awake when needed

9. Power Costs - Examples • Motes • ATmega 128 – six working modes with different energy saving features • Most aggressive sleep can be very small % of active working mode • Working – 8 mA • Sleep – 100 microA • Radio • 10 microA sleeping • 7.5 mA Rcv • 12 mA Tx

10. Instructions and Modules Experimental Results

11. Power Cost Tradeoff • Communication versus calculation • Energy consumed for 1,000 basic calculations is the same as for transmitting a single bit! • Means: sending a 50 byte packet same energy cost as 400,000 instructions • Implies: trade off calculation for messages

12. MAC Layer • 802.11 DCF doze mode • S-MAC (pack all messages into awake period) • B-MAC (duty cycle and CCA) Active Passive/Sleep 115 ms 885 ms

13. Routing Layer • Use multiple routes to balance energy consumption • E.g., SPEED protocol • Adjust communication range to lowest possible to just reach neighbor • Many papers on this, but is this a good idea? Not really, consider robustness

14. Two Viewpoints • Power Management in the Small • Individual protocols • Power Management in the Large • Overarching protocols for additional power savings • Sentry Service • Tripwire Management Service • Duty Cycle

15. Power Management

16. Power Management – Communication Coverage Minimum awake - still communicate

17. Sensing Coverage (r) FirstThen 2r for Communication

18. Application Scenario • A small number of nodes stay awake • Most of the network sleeps • Rare events

19. Application Scenario • Awakened nodes detect an event • Messages are sent to wake up other nodes

20. 3 4 2 1 Sentry-Based Power Management (SBPM) • Two classes of nodes: sentries and non-sentries • Sentries are awake • Non-sentries can sleep • Sentries • Provide coarse monitoring & backbone communication network • Sentries “wake up” non-sentries for finer sensing • Sentry rotation • Even energy distribution • Prolong system lifetime • Decentralized Algorithm • See photo

21. SBPM • Basic Algorithm • Each node sets timer inversely proportional to the amount of energy it has remaining • Implies: node with most energy will declare itself a sentry FIRST • Other nodes hearing sentry declare themselves as non-sentries

22. Tripwire Service – Scaling to 1000s Network partitioning • 2 tripwire sections • 8 dormant sections • 100 motes, 1 relay per section • Size and number of sections reconfigurable • Rotate sections Sentries • N% in tripwire section • Rotate sentries

23. Creating Sections • How many sections? • How to create sections? • How (or do) base stations communicate? • What if base station fails?

24. Summary -Power Management • Sentry Service – x% in a region are awake • Tripwire – many regions to handle scale • Within a Region - Area only wakeup (each region may be large)

25. Lifetime Analysis

26. Sentry Duty Cycle • Sentry can also sleep based on • Sensing range • Speed of targets • Lifetime of events (static/moving) • Required probability of detection • Use spatial properties to detect moving target/event • If first sentry is asleep what is the probability that the second one will be too

27. Sentry Duty Cycle

28. Sentry Duty-Cycle • A common period p and duty-cycle βis chosen for all sentries, while starting times Tstart are randomly selected Non-sentries Sentries A t B t Target Trace C A D t E D C t B E t 0 p 2p Sleeping Awake

29. Lifetime Analysis

30. Adaptive Power Control TP1 TP2 TP2 Function of Location Time of day Weather Obstacles Interferences

31. Choose Low Power Level(to achieve good quality comm.) T2 TP1 T1 TP2 T2 TP2 The minimum transmission power level to save energy and maintain specified link quality

32. Model for ATPC • Use a linear model to approximate a non-linear correlation • rssi(tp) = a · tp + b • Dynamic model • a and b vary from time to time

33. Pairwise Feedback Control

34. FC • Competing loops • Different neighbors • Adjacent hops • Competing streams • End-to-end control

35. CPS Physical World Issues • ATPC accurately adjusts the transmission power • Adapting to spatial and temporal physical world factors • Location, time, weather, congestion, interference, etc. • Assumes low traffic system

36. Summary • Power Management in the Large and Small • Models can be used to estimate lifetime tradeoffs • Aside: Fault tolerance solutions might use heartbeat beacons -> energy cost • Metrics • Total energy consumed per period X • Lifetime • Half-life • Energy consumption balance