Power Issues in Wireless Sensor Nets

1. Power Issues in Wireless Sensor Nets David Culler University of California, Berkeley http://www.cs.berkeley.edu/~culler Defense Science Board

2. Outline • Basic model of operation • Node Design a for low power consumption • Operating System Issues • Design of the power-supply subsystem • MAC-level network design for power • Higher-level network design for power • Application level • Important areas of development • Discussion Defense Science Board

3. WakeUP WakeUP Work Work Sleep Sleep Duty Cycle Model of operation Active Active • Sleep – Active [Wakeup / Work] • Peak Power • Essentially sum of subsystem components • MW in supercomputer, kW in server, Watts in PDA • milliwatts in “mote” class device • Sleep power • Minimal running components + leakage • Microwatts in mote-class • Average power • Pave = = (1-factive)*Psleep + factive*Pactive • Pave=fsleep*Psleep + fwakeup*Pwakeup+ fwork*Pwork • Lifetime • EnergyStore / (Pave - Pgen ) Defense Science Board

4. Passive Vigilance • Sense only when there is something useful to detect • Listen only when there is something useful to hear • How do you know? • By arrangement • By cascade of lower power triggers Defense Science Board

5. Mote Power Parameters • 1s Microwatts sleep • 10s of milliwatts active (wakeup or work) • Wakeup substantial • Milliseconds (1000s of instructions) • 1% Duty Cycle is common • Wakeup matters Defense Science Board

6. Batteries • Still the best energy store • Issues • Voltage • Source current • Leakage • Voltage profile • Recharge Defense Science Board

7. Design of a Low Power Node Defense Science Board

8. Sensor Interface analog sensors ADC digital sensors Data SRAM pgm EPROM Key Design Elements • Efficient wireless protocol primitives • Flexible sensor interface • Ultra-low power standby • Very Fast wakeup • Watchdog and Monitoring • Data SRAM is critical limiting resource Flash Storage timers proc data logs Wireless Net Interface antenna RF transceiver pgm images WD Wired Net Interface serial link USB,EN,… Low-power Standby & Wakeup Defense Science Board

9. Mote Platform Evolution 3 Defense Science Board

10. Focused on low power Sleep - Majority of the time Telos: 2.4mA MicaZ: 30mA Wakeup As quickly as possible to process and return to sleep Telos: 290ns typical, 6ms max MicaZ: 60ms max internal oscillator, 4ms external Process Get your work done and get back to sleep Telos: 4MHz 16-bit MicaZ: 8MHz 8-bit TI MSP430 Ultra low power 1.6mA sleep 460mA active 1.8V operation Standards Based IEEE 802.15.4, USB IEEE 802.15.4 CC2420 radio 250kbps 2.4GHz ISM band TinyOS support New suite of radio stacks Pushing hardware abstraction Must conform to std link Ease of development and Test Program over USB Std connector header Interoperability Telos / MicaZ / ChipCon dev 802.15.4 Platforms UCB Telos Xbow MicaZ Defense Science Board

11. TinyOS-driven architecture • 3K RAM = 1.5 mm2 • CPU Core = 1mm2 • multithreaded • RF COMM stack = .5mm2 • HW assists for SW stack • Page mapping • SmartDust RADIO = .25 mm2 • SmartDust ADC 1/64 mm2 • I/O PADS • Expected sleep: 1 uW • 400+ years on AA • 150 uW per MHz • Radio: • .5mm2, -90dBm receive sensitivity • 1 mW power at 100Kbps • ADC: • 20 pJ/sample • 10 Ksamps/second = .2 uW. jhill mar 6, 2003 Defense Science Board

12. Microcontrollers • Memory starved • Far from Amdahl-Case 3M rule • Fairly uniform active inst per nJ • Faster MCUs generally a bit better • Improving with feature size • Min operating voltage • 1.8 volts => most of battery energy • 2.7 volts => lose half of battery energy • Standby power • Recently a substantial improvement • Probably due to design focus • Fundamentally SRAM leakage • Wake-up time is key • Trade sleep power for wake-up time • Memory restore DMA Support: permits ADC sampling while processor is sleeping Defense Science Board

13. Radio • Trade-offs: • resilience / performance => slow wake up • Wakeup vs interface level • Ability to optimize vs dedicated support Defense Science Board

14. Flash Technology • One write per bit per erase cycle • Flash characteristics: Not used in current motes Defense Science Board

15. WakeUP WakeUP Work Work Sleep Sleep Power States at Node Level Active Active Defense Science Board

16. msg_rec(type, data) msg_send_done) Tiny OS Concepts • Scheduler + Graph of Components • constrained two-level scheduling model: threads + events • Component: • Commands, • Event Handlers • Frame (storage) • Tasks (concurrency) • Constrained Storage Model • frame per component, shared stack, no heap • Very lean multithreading • Efficient Layering structured event-driven execution Never wait or spin Events Commands send_msg(addr, type, data) power(mode) init Messaging Component internal thread Internal State TX_packet(buf) Power(mode) TX_packet_done (success) init RX_packet_done (buffer) Defense Science Board

17. Application: query processing detection, reporting Sensor Drivers & Filters Comm Stack Timer Sensor 3 Sensor 2 Radio Clock Sensor1 Power Management Cooperative Interfaces • Power management extends std control • 1000-fold range of power draw • Components informed of intention to go to sleep • Take internal actions • Propagate control • Scoreboard determined permissible depth of sleep state • Scheduler drops to sleep on idle • Key interrupts drive wake-up • Rich communication interfaces • Signal strength • Post-MAC time-stamping • Sub-carrier signaling Defense Science Board

18. Power-supply Subsystem • Energy Store • Power Source • Consumer • Management & Control Defense Science Board

19. Importance or primary buffer • Node is able to operate from capacitors • Moderate period of time (~week) • Source active load (mAs !) • Absorb energy input • Perform frequent charge cycles (daily) • shallow • Source high voltage recharge of secondary • Power MCU during secondary recharge Defense Science Board

20. It’s all about leakage • Bigger isn’t better • More doesn’t help • We use two 22 F in series • operate in the flat under load (1%, 10 mA) Defense Science Board

21. Recharging • High density & Low leakage • Software on MCU manages recharge sequence • Include temperature compensation • Pulsing charge current (~1x battery capacity) to 80% • High level charge management and load control Defense Science Board

22. Outline • Basic model of operation • Node Design a for low-power consumption • Operating System Issues • Design of the power-supply subsystem • Communication Basics • MAC-level network design for power • Higher-level network design for power • Important areas of development • Discussion Defense Science Board

23. Basics • Power required to transmit a given distance grows like the r3 in free space with omnidirectional antenna • Can be as bad as r7 close to the ground • Slower growth rate with directional, but … • Power required to route data hop-by-hop a given distance grows only linearly • Connectivity determined by a host of factors • SNR • Transmission power, receiver sensitivity, distance • Interference • obstructions Defense Science Board

24. The Basic Primitive • Transmit a packet • Received by a set of nodes • Dynamically determined • Depends on physical environment at the time • What other communication is on-going • Each selects whether to retransmit • Potentially after modification • And if so, when Defense Science Board

25. Routing Mechanism • Upon each transmission, one of the recipients retransmit • determined by source, by receiver, by … • on the ‘edge of the cell’ Defense Science Board

26. TX RX TX Communication and Power • Costs power whenever radio is on • Transmitting, receiving, or just listening • Transmit is easy, Rcv is what’s tricky • Want to turn it on just when there is something to hear • Two approaches • Schedule transmission intervals • Statically, dynamically, globally, locally • Make listening cheap listen off off RX TX Defense Science Board

27. TDMA variants • Time Division Media Access • Each node has a schedule of awake times • Typically used in star around coordinator • Bluetooth, ZIGBEE • Coordinator hands out slots • Far more difficult with multihop (mesh) networks • Further complicated by network dynamics • Noise, overhearing, interference Defense Science Board

28. Complexity of Connectivity • Direct Reception • Non-isotropic • Large variation in affinity • Asymmetric links • Long, stable high quality links • Short bad ones • Varies with traffic load • Collisions • Distant nodes raise noise floor • Reduce SNR for nearer ones • Many poor “neighbors” • Good ones mostly near, some far Defense Science Board

29. Carrier Sense Media Access Synchronized protocol with periodic listen periods Integrates higher layer functionality into link protocol Hard to maintain set of schedules T-MAC [van Dam and Langendoen, Sensys 2003] Reduces power consumption by returning to sleep if no traffic is detected at the beginning of a listen period sleep sleep listen listen sync sync sync sync Schedule 1 Schedule 2 Wei Ye, USC/ISI S-MACYe, Heidemann, and Estrin, INFOCOM 2002 Node 1 sleep sleep listen listen Node 2 Defense Science Board

30. Energy Cost = RX + TX + Listen Scheduling tries to reduce listening Alnternative, reduce listen cost Example of a typical low level protocol mechanism Periodically wake up, sample channel, sleep Properties Wakeup time fixed “Check Time” between wakeups variable Preamble length matches wakeup interval Robust to variation Complementary to scheduling Overhear all data packets in cell Duty cycle depends on number of neighbors and cell traffic wakeup wakeup wakeup wakeup wakeup wakeup wakeup wakeup wakeup Low Power Listening (LPL) TX sleep sleep sleep Node 1 time RX sleep sleep sleep Node 2 time Defense Science Board

31. Communication Trade-offs • Connectivity graph is not static • Complicates explicit scheduling • Time Synchronization • Time of reference required for rendezvous • Low-power listening (preamble sampling) • Reduce the cost to listen • Allows coarser time synch and more flexible schedules Defense Science Board

32. The Common Case: Data Gathering • Collection of nodes take periodic samples • Stream data towards a root node • Root announces interest • depth = 0 • Nodes listen • When hear ‘neighbor’ with smaller depth • start transmitting data to “best” lower neighbor • set own depth to one greater (and include with data) • Data transmission continuously reinforces & adjusts routes • Aggregation within nodes or within the tree Defense Science Board

33. Radio Cells Defense Science Board

34. 2 2 2 2 1 1 2 Continuous Network Discovery 0 Defense Science Board

35. Local Operations => Global Behavior • Nodes continually ‘sense’ network environment • uncertain, partial information • Packets directed to a “parent” neighbor • all other neighbors “hear” too • carry additional organizational information • Each nodes builds estimate of neighborhood • adjusted with every packet and with time • Interactively selects parent • # trans := 1/ParentRate + #trans(Parent->root) • Routes traffic upward • Collectively they build and maintain a stable spanning tree • takes energy to maintain structure Defense Science Board

36. Power-aware Routing • Cost-based Routing • Minimize number of hops • Minimize loss rate along the path • Perform local retransmissions, minimize number along path • Energy balance • Utilize nodes with larger energy resources • Utilize redundancy • Nodes near the sink route more traffic, hence use more energy • Give them bigger batteries or provide more of them and spread the load • Randomize routes • Utilize heterogeneity • Route through nodes with abundant power sources Defense Science Board

37. Communication Scheduling • TDMA-like scheduling of listening slots • Node allocates • listen slots for each child • Transmission slots to parent • Hailing slot to hear joins • To join listen for full cycle • Pick parent and announce self • Get transmission slot • CSMA to manage media • Allows slot sharing • Little contention • Reduces loss & overhearing • Connectivity changes cause mgmt traffic Defense Science Board

38. In-network Processing • Best way to reduce communication cost is to not communicate • Compute at the sensor • Only communicate important events • Compute over localized regions of the network • Distributed detection • Validation Defense Science Board

39. Exceptional Event Detection Energy-Quality Hierarchy Low High • Challenge: Detecting Exceptional Events • Rare most time spent monitoring noise • Random nearly continuous sampling • Ephemeral low bound of duty-cycling off time • Approach: Passive Vigilance • Multi-modal, low-power sensors • Duty-cycled, where possible and arranged in • Energy-Quality hierarchy with low (E, Q) sensors • Triggering higher (E, Q) sensors, and so on… • How to Estimate Energy Consumption? • Power = idle power + energy/event x events/time • Estimate event rate probabilistically: p(tx) = from ROC curve and decision threshold for H0 & H1 • How to Optimize Energy-Quality? • Let x* = (x1*, x2*,..., xn*) be the n decision boundaries between H0 & H1. for n processes. Then, given a set of ROC curves, optimizing for energy-quality is a matter of minimizing the function f(x*) = E[power(x*)] subject to the power, probability of detection, and probability of false alarm constraints of the system. False Alarm Rate Energy Usage High Low • Trigger network includes hardware wakeup, passive infrared, microphone, magnetic, fusion, and radio, arranged hierarchically • Nodes: sensing, computing, and comm processes • Edges: <Energy, PFA>  <Energy, PFA> Defense Science Board sub. SPOTS 05

40. Example: Detection & Tracking Defense Science Board

41. Key Areas to Improve • Low leakage SRAM • 1T sram • Low leakage supercaps • Communication accelerators • Radio wake-up cascade • Very low power detection of signal triggers receiver • Robust communication protocols • Sensor detection cascades Defense Science Board

42. Where to read for more • Telos: Enabling Ultra-Low Power Wireless Research, Joseph Polastre, Robert Szewczyk, David CullerTo appear in The Fourth International Conference on Information Processing in Sensor Networks: Special track on Platform Tools and Design Methods for Network Embedded Sensors (IPSN/SPOTS), April 25-27, 2005 • Perpetual Environmentally Powered Sensor Networks, Xiaofan Jiang, Joseph Polastre, David CullerTo appear in The Fourth International Conference on Information Processing in Sensor Networks: Special track on Platform Tools and Design Methods for Network Embedded Sensors (IPSN/SPOTS), April 25-27, 2005 • Versatile Low Power Media Access for Wireless Sensor Networks, Joe Polastre and David Culler,   The Second ACM Conference on Embedded Networked Sensor Systems, Nov. 2004 • "Design of a Wireless Sensor Network Platform for Detecting Rare, Random, and Ephemeral Events",Prabal Dutta, Mike Grimmer, Anish Arora, Steve Bibyk, and David Culler, In The Fourth International Conference on Information Processing in Sensor Networks (IPSN'05), 2005. Defense Science Board

43. Discussion Defense Science Board