Power Issues in Wireless Sensor NetsOutlineModel of operationPassive VigilanceMote Power ParametersBatteriesDesign of a Low Power NodeKey Design ElementsMote Platform Evolution802.15.4 PlatformsTinyOS-driven architectureMicrocontrollersRadioFlash TechnologyPower States at Node LevelTiny OS ConceptsCooperative InterfacesPower-supply SubsystemImportance or primary bufferIt’s all about leakageRechargingSlide 22BasicsThe Basic PrimitiveRouting MechanismCommunication and PowerTDMA variantsComplexity of ConnectivityS-MAC Ye, Heidemann, and Estrin, INFOCOM 2002Low Power Listening (LPL)Communication Trade-offsThe Common Case: Data GatheringRadio CellsContinuous Network DiscoveryLocal Operations => Global BehaviorPower-aware RoutingCommunication SchedulingIn-network ProcessingExceptional Event DetectionExample: Detection & TrackingKey Areas to ImproveWhere to read for moreDiscussion3/31/05Defense Science Board 1Power Issues in Wireless Sensor NetsDavid CullerUniversity of California, Berkeleyhttp://www.cs.berkeley.edu/~culler3/31/05Defense Science Board 2Outline•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•Discussion3/31/05Defense Science Board 3Model of operation•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 )SleepWakeUP WorkSleepWakeUP WorkActiveActiveDuty Cycle3/31/05Defense Science Board 4Passive 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 triggers3/31/05Defense Science Board 5Mote Power Parameters•1s Microwatts sleep•10s of milliwatts active (wakeup or work)•Wakeup substantial–Milliseconds (1000s of instructions)•1% Duty Cycle is common–Wakeup matters3/31/05Defense Science Board 6Batteries•Still the best energy store•Issues–Voltage–Source current–Leakage–Voltage profile–Recharge3/31/05Defense Science Board 7Design of a Low Power Node3/31/05Defense Science Board 8Key Design Elements•Efficient wireless protocol primitives•Flexible sensor interface•Ultra-low power standby•Very Fast wakeup•Watchdog and Monitoring•Data SRAM is critical limiting resourceprocDataSRAMpgmEPROMtimersSensor Interfacedigital sensorsanalog sensorsADCWireless NetInterfaceWired NetInterfaceRFtransceiverantennaserial linkUSB,EN,…Low-powerStandby & WakeupFlash Storagepgm imagesdata logsWD3/31/05Defense Science Board 9Mote Platform Evolution33/31/05Defense Science Board 10802.15.4 Platforms•Focused on low power •Sleep - Majority of the time–Telos: 2.4A–MicaZ: 30A•Wakeup–As quickly as possible to process and return to sleep–Telos: 290ns typical, 6s max–MicaZ: 60s 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»1.6A sleep»460A 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 devUCB TelosXbow MicaZ3/31/05Defense Science Board 11TinyOS-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, 20033/31/05Defense Science Board 12Microcontrollers•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 restoreDMA Support: permits ADC sampling while processor is sleeping3/31/05Defense Science Board 13Radio•Trade-offs: –resilience / performance => slow wake up–Wakeup vs interface level–Ability to optimize vs dedicated support3/31/05Defense Science Board 14Flash TechnologyNOR AT45DB NANDErase Slow (seconds) Fast (ms) Fast (ms)Erase Unit Large (64K-128K) Small (256) Medium (8K-32K)Writes Slow (100 kB/s) Slow (60 kB/s) Fast (MB/s)Write Unit 1 bit 256 bytes 100’s bytesBit-Errors Low Low High (requires ECC)Read Fast Slow + I/O Bus Fast + I/O BusErase Cycles 10^4 – 10^5 10^4 10^5 – 10^7Intended Use Code storage Data storage Data storage•One write per bit per erase cycle•Flash characteristics:Not used in current motes3/31/05Defense Science Board 15Power States at Node LevelSleepWakeUP WorkSleepWakeUP WorkActiveActive3/31/05Defense Science Board 16Tiny 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 Layeringstructured event-driven executionNever wait or spinMessaging ComponentinitPower(mode)TX_packet(buf)TX_packet_done (success)RX_packet_done (buffer)Internal Stateinitpower(mode)send_msg(addr, type, data)msg_rec(type, data)msg_send_done)internal threadCommandsEvents3/31/05Defense Science Board 17Cooperative 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
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