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 Statements • Increase the lifetime of the system while meeting functional requirements. • Including communication coverage • How to provide sensing coverage for a sensor network in a power-efficient way? • MANET – how to maintain communication coverage

4. Questions? • Will solar cells solve the problem? • Will energy scavenging solve the problem? • Will batteries just get much better?

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. Aging of System (cont.) Ideally, one node becomes activewhen only one node stops working. Possible (more than likely?) thatmultiple nodes become active.

8. Outline • Hardware layer • MAC layer - review • Routing layer – review • Localization and Clock Sync - review • Overarching power management schemes • Sentry service • Tripwire service • Duty cycle • Differentiated surveillance

9. 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

10. Power Costs - Examples • EYES nodes (with TI microprocessor) • CPU active 2.1 mA • CPU sleep 1.6 microA • Radio transmit 10 mA • Radio receive 4 mA • Radio sleep 20 microA

11. 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

12. Instructions and Modules Experimental Results

13. 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

14. TinyOS Cost Results

15. MAC Layer • Use overhearing (of wireless transmissions) and scheduling to reduce energy use • When a node hears another sending RTS (not for me) it can go to sleep • Synchronized schedules (TDMA) – when it is not my turn, listen then go to sleep • Good algorithm can also reduce collisions!

16. MAC Layer • 802.11 DCF doze mode • S-MAC (pack all messages into awake period) • B-MAC (duty cycle and CCA)

17. Routing Layer • Use multiple routes to balance energy consumption • E.g., SPEED protocol • Use data caching to minimize messages /energy • E.g., SEAD protocol (covered later) • Adjust communication range to lowest possible to just reach neighbor • Many papers on this, but is this a good idea? Not really, consider robustness

18. Routing Layer • Use only good neighbors (high link quality) • Avoids need for re-transmissions • Use data aggregation protocols • Directed diffusion • AIDA (not covered in this class) • … • Piggyback state table updates

19. Localization • Localization • How often? • Minimize messages • Long range beacons costly • Example: • APIT costs long range beacons (higher power) but few messages • Amorphous Localization costs - flooding • Walking GPS costs - receiving several messages

20. Clock Sync • Clock Sync • Minimize messages to the number needed for a particular sync accuracy • Example:RBS – one time message and then all neighbors must exchange messages • Example:TPSN – exchange 2 sync messages with parent node in spanning tree • Example: Internet NTS protocol sends many messages to the time server

21. 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 • Differential Surveillance

22. A Second Look

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

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

25. Power Management - Communication Coverage Virtual Grid – one node awake per grid Rotate

26. Power Management- Sensing Coverage • 100% coverage • > 100% coverage • FT • Higher quality/confidence sensing • < 100% coverage (aggressive) • Use movement (temporal) of object to compensate for less spatial coverage • Increased lifetime

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

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

29. Wake-Up Solution (1) • Periodic wake-up • Each (non-awake) node has a sleep/wakeup duty cycle based on local timer • Listen for stay awake message • Most current systems use this technique • Application dependent, often complicated, wastes energy • E.g., correct duty cycle depends on speed of targets, sensing ranges, types of sensors, … • May miss wakeup call Awake Sleep

30. Wake-Up Solution (2) • Special low power hardware stand-by component that (always) listens for a wakeup beacon (not the full radio component) • PicoRadio • Uses extra energy (but not as much as full radio component)

31. Wake-Up Solution (3) • Just-In-Time Wake-up Capability • A node does not wake up until it is needed • It uses no active listening energy • Uses radio-triggered hardware that extracts energy from the electromagnetic energy in the wakeup signal itself • Proposed – not built for WSN • Not RFID – they employ powerful readers to send strong radio signals

32. Just-in-Time Solution – Is it worth? • Is there much energy to save? • How is energy consumed on a network node? • Examine the distribution of consumed energy on a node in a sensor network for vehicle tracking • 10 events per day, each event lasts 2 minutes • Work/sleep current intensity: 20mA / 100uA • Note: Actual computation is more involved

33. Solution – Is it worth? • Scheme I: Always-on (No power management) • The node is on and actively sensing until it is out of power • 1% of the energy is used to track vehicles, 99% is used in a peeking state (uselessly sensing for potential passing vehicles) • Lifetime 3.3 days Energy wasted!!!

34. Solution – Is it worth? • Scheme II: Rotation-based (Periodic wake-up) • Nodes are awakened wirelessly by wake-up messages • Duty cycle 4.7% • 21% of the energy is used to track vehicles, 7% used in sleeping mode, and 72% is used in peeking state • Lifetime 50 days Energy wasted!!!

35. Solution – Is it worth it? • Scheme III: Radio-triggered • Nodes keep sleeping until events of interest happen • Nodes are awakened wirelessly by wake-up messages • 74% of the energy is used to track vehicles, 26% used in sleeping mode (minimal cpu energy) • Lifetime – 178 days

36. 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

37. 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

38. SBPM - Illustration All nodes are awake. Base node declares itself first as a sentry and sends SENTRY_DECLARE message. Communication at sensing range (ensure sensing coverage).

39. SBPM - Illustration Other nodes send SENTRY_DECLARE message as backoff expires (function of remaining energy).

40. SBPM - Illustration Other nodes send SENTRY_DECLARE message as backoff expires. If backoff expires and heard from a sentry then just join one sentry (first, closest) All sentries or attached to a sentry

41. SBPM - Illustration Increase communication range – at least 2X New communication Range All sentries or attached to a sentry

42. 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

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

44. Network Lifetime • Lifetime is determined by • Individual Mica 2 mote consumption • Energy plot for a sentry node • Energy plot for a sleep node

45. 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)

46. Lifetime Analysis

47. 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

48. Sentry Duty Cycle

49. Sentry Duty-Cycle Scheduling • 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

50. Lifetime Analysis