docsensor node architecture - Distributed Embedded Systems
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sensor node architecture - Distributed Embedded Systems
SENSOR NODE ARCHITECTURE [NES] Winter 2014/2015 Sensor Node Architecture 1 System Aspects ² MicaZ ² iPhone ª 8 MHz ª 1-‐2 GHz ª 250 kbit/s (ZigBee) ª 1 Gbit/s (WiFi) ª 4 kB RAM ª 128-‐1024 MB RAM ª 128 kB Flash ª 16-‐128 GB Flash [NES] Winter 2014/2015 Sensor Node Architecture 2 Sensor Node Architecture ² ² ² ² Sensors Radio transceiver Processor (and memory) BaQery Sensor 1 … Radio transceiver Microcontroller Sensor n Memory Storage Battery [NES] Winter 2014/2015 Sensor Node Architecture 3 “TelosB” Mote PlaCorm GPIO Blocks Internal Antenna Temp / RH Sensor Reset Button IR / Light Sensors User Button CC2420 USB TX LED Antenna connector JTAG connector R/G/B LEDs USB RX LED [NES] Winter 2014/2015 Sensor Node Architecture 4 Ad hoc node architecture ² ² Core: essenTally the same But: Much more addiTonal equipment ª ² Hard disk, display, keyboard, voice interface, camera, … EssenTally: a laptop-‐class device [NES] Winter 2014/2015 Sensor Node Architecture 5 Controller ² Main opTons Microcontroller – general purpose processor, opTmized for embedded applicaTons, low power consumpTon ª DSPs – opTmized for signal processing tasks, not suitable here ª FPGAs – may be good for tesTng ª ASICs – only when peak performance is needed, no flexibility ª ² Tasks ª ª ª ª ExecuTon of (Tme sensiTve) tasks Control of communicaTon protocols ExecuTon and control of the primary applicaTon program Energy management § MulTple operaTon modes (acTve, idle, listen, sleep, …) [NES] Winter 2014/2015 Sensor Node Architecture 6 Microcontroller Controller Atmega128 TI MSP430 PIC 18F720 ARM920T RAM 4 KB 10 KB 4 KB 512 KB Flash 128 KB Program 512 KB Serial 48 KB 128 KB Program 512 KB Filesystem 1KB EEPROM 4 MB Clock rate 8 MHz 8 MHz 20 MHz 180 MHz Energy consumption Active: 8 mA Sleep: <15 uA Active: 1.8 mA Sleep: 5.1 uA Active: 8 mA Active: 25-90 mA Sleep: 32 uA Plattform MicaZ TelosB Particle SunSPOT Costs ~ 12 Euro [NES] Winter 2014/2015 ~ 5 Euro Sensor Node Architecture 7 Microcontroller MSP430 F1611 (Texas Instruments) Ultra-‐Low-‐Power MCU 200 μA / MIPS; standby mode available Highly integrated device Frequency Flash SRAM GPIO DAC Resolution Timers 16-bit Watchdog Brown Out Reset SVS Voltage Check USART DMA Multiplier Comparators Temp Sensor ADC 12-bit ADC Channels [WSN-‐PS] Winter 2013/2014 Hardware 8 MHz 48 KB 10240 B 48 12 Bit 2 Yes Yes Yes Yes Yes 16x16 Yes Yes SAR 8 8 CommunicaDon device ² Which transmission medium? ª ª ª ² ElectromagneTc at radio frequencies? ElectromagneTc, light? Ultrasound? ü Radio transceivers transmit a bit-‐ or byte stream in form of a radio wave ª Receive it, convert it back into bit-‐/byte stream [NES] Winter 2014/2015 Sensor Node Architecture 9 Transceiver characterisDcs ² CapabiliTes ª ª ² Interface: bit, byte, packet level? Supported frequency range? § Typically, somewhere in 433 MHz – 2.4 GHz, ISM band ª ª ª ² MulTple channels? Data rates? Range? ª ª ª ª ª ª ª Power consumpTon to send/ receive data? Time and energy consumpTon to change between different states? Transmission power control? Power efficiency (which percentage of consumed power is radiated?) [NES] Winter 2014/2015 ª ª Energy characterisTcs ª Radio performance Sensor Node Architecture ª ª ª ModulaTon? (ASK, FSK, …?) Noise figure? NF = SNRI/SNRO Gain? (signal amplificaTon) Receiver sensiTvity? (minimum S to achieve a given Eb/N0) Blocking performance (achieved BER in presence of frequency-‐ offset interferer) Out of band emissions Carrier sensing & RSSI characterisTcs Frequency stability (e.g., towards temperature changes) Voltage range 10 Transceiver states ² Transceivers can be put into different operaTonal states, typically: ª ª ª Transmit Receive Idle – ready to receive, but not doing so § Some funcTons in hardware can be switched off, reducing energy consumpTon a liQle ª Sleep – significant parts of the transceiver are switched off § Not able to immediately receive something § Recovery 5me and startup energy to leave sleep state can be significant ² Research issue: Wakeup receivers – can be woken via radio when in sleep state (seeming contradicTon!) [NES] Winter 2014/2015 Sensor Node Architecture 11 Example radio transceivers ² ² Almost boundless variety available Some examples ª 916 or 868 MHz 400 kHz bandwidth Up to 115,2 kbps On/off keying or ASK Dynamically tuneable output power Maximum power about 1.4 mW Low power consumpTon Chipcon CC1000 § Range 300 to 1000 MHz, programmable in 250 Hz steps § FSK modulaTon § Provides RSSI [NES] Winter 2014/2015 Sensor Node Architecture Chipcon CC 2400 § § § § RFM TR1000 family § § § § § § § ª ª ª Implements 802.15.4 2.4 GHz, DSSS modem 250 kbps Higher power consumpTon than above transceivers Infineon TDA 525x family § E.g., 5250: 868 MHz § ASK or FSK modulaTon § RSSI, highly efficient power amplifier § Intelligent power down, “self-‐ polling” mechanism § Excellent blocking performance 12 Radio ² CC2420 (Chipcon, TI) ª ª ª ª ª single-‐chip 2.4 GHz IEEE 802.15.4 compliant RF transceiver Data rate: 250 kbps Digital RSSI / LQI support Programmable output power “low power”: 18.8 / 17.4 mA (RX/TX) [NES] Winter 2014/2015 Sensor Node Architecture 13 Example radio transceivers for ad hoc networks ² ² Ad hoc networks: Usually, higher data rates are required Typical: IEEE 802.11 a/b/g/n is considered Up to 450 MBit/s ª RelaTvely long distance (100s of meters possible, typical 10s of meters at higher data rates) ª Works reasonably well (but certainly not perfect) in mobile environments ª Problem: expensive equipment, quite power hungry ª [NES] Winter 2014/2015 Sensor Node Architecture 14 Wakeup receivers ² Major energy problem: RECEIVING ª ² When to switch on a receiver is not clear ª ª ² Idling and being ready to receive consumes considerable amounts of power ContenTon-‐based MAC protocols: Receiver is always on TDMA-‐based MAC protocols: SynchronizaTon overhead, inflexible Desirable: Receiver that can (only) check for incoming messages ª ª ª When signal detected, wake up main receiver for actual recepTon Ideally: Wakeup receiver can already process simple addresses Not clear whether they can be actually built, however [NES] Winter 2014/2015 Sensor Node Architecture 15 OpDcal communicaDon ² ² OpTcal communicaTon can consume less energy Example: passive readout via corner cube reflector Laser is reflected back directly to source if mirrors are at right angles ª Mirrors can be “Ttled” to stop reflecTng à Allows data to be sent back to laser source ª [NES] Winter 2014/2015 Sensor Node Architecture 16 Ultra-‐wideband communicaDon ² Standard radio transceivers: Modulate a signal onto a carrier wave ª ² AlternaTve approach: Use a large bandwidth, do not modulate, simply emit a “burst” of power ª ª ª ª ª ² Requires relaTvely small amount of bandwidth Forms almost rectangular pulses Pulses are very short InformaTon is encoded in the presence/absence of pulses Requires Tght Tme synchronizaTon of receiver RelaTvely short range (typically) Advantages ª ª ª PreQy resilient to mulT-‐path propagaTon Very good ranging capabiliTes Good wall penetraTon [NES] Winter 2014/2015 Sensor Node Architecture 17 Sensors as such ² Main categories ª Any energy radiated? Passive vs. acTve sensors Sense of direcTon? OmidirecTonal? ª Passive, omnidirecTonal ª § Examples: light, thermometer, microphones, hygrometer, … ª Passive, narrow-‐beam § Example: Camera ª AcTve sensors § Example: Radar ² Important parameter: Area of coverage ª Which region is adequately covered by a given sensor? [NES] Winter 2014/2015 Sensor Node Architecture 18 Sensors ² Digital Temperature/Humidity Sensor ª ª ª ª ² Sensirion SHT11 Low power: 80 μW (typ.) -‐40 … +125°C 0 … 100% RH Visible Light / IR Sensor ª ª ª ª Hamamatsu S1087 & S1087-‐01 Low dark current (10 pA) 320 … 730 nm 320 … 1100 nm [NES] Winter 2014/2015 Sensor Node Architecture 19 ENERGY SUPPLY [NES] Winter 2014/2015 Sensor Node Architecture 20 Energy supply of mobile/sensor nodes ² Goal: provide as much energy as possible at smallest cost/ volume/weight/recharge Tme/longevity ª ² In WSN, recharging may or may not be an opTon OpTons Primary baQeries – not rechargeable ª Secondary baQeries – rechargeable, only makes sense in combinaTon with some form of energy harvesTng ª ² Requirements include ª ª ª ª ª ª Low self-‐discharge Long shelf life Capacity under load Efficient recharging at low current Good relaxaTon properTes (seeming self-‐recharging) Voltage stability (to avoid DC-‐DC conversion) [NES] Winter 2014/2015 Sensor Node Architecture 21 BaRery examples ² Energy per volume (Joule per cubic cenTmeter): Primary batteries Chemistry Zinc-air Lithium Alkaline Energy (J/cm3) 3780 2880 1200 Secondary batteries Chemistry Lithium NiMHd NiCd Energy (J/cm3) 1080 860 650 [NES] Winter 2014/2015 Sensor Node Architecture 22 Energy scavenging ² How to recharge a baQery? ª ª ² A laptop: easy, plug into wall socket in the evening A sensor node? – Try to scavenge energy from environment Ambient energy sources Light à solar cells – between 10 µW/cm2 and 15 mW/cm2 ª Temperature gradients – 80 µ W/cm2 @ 1 V from 5K difference ª VibraTons – between 0.1 and 10000 µ W/cm3 ª Pressure variaTon (piezo-‐electric) – 330 µ W/cm2 from the heel of a shoe ª Air/liquid flow (MEMS gas turbines) ª [NES] Winter 2014/2015 Sensor Node Architecture 23 Energy scavenging – overview [NES] Winter 2014/2015 Sensor Node Architecture 24 Energy consumpDon ² A “back of the envelope” esTmaTon ² Number of instrucTons ª ª ª ² LifeTme ª ª ² Energy per instrucTon: 1 nJ Small baQery (“smart dust”): 1 J = 1 Ws Corresponds: 109 instrucTons! Require a single day operaTonal lifeTme = 24 x 60 x 60 = 86400 s 1 Ws / 86400s ~ 11.5 µW as max. sustained power consumpTon! Not feasible! [NES] Winter 2014/2015 Sensor Node Architecture 25 MulDple power consumpDon modes ² Way out: Do not run sensor node at full operaTon all the Tme ª ª ² Typical modes ª ª ² If nothing to do, switch to power safe mode QuesTon: When to throQle down? How to wake up again? Controller: AcTve, idle, sleep Radio mode: Turn on/off transmiQer/receiver, both MulTple modes possible, “deeper” sleep modes ª ª ª Strongly depends on hardware TI MSP 430, e.g.: four different sleep modes Atmel ATMega: six different modes [NES] Winter 2014/2015 Sensor Node Architecture 26 Some energy consumpDon figures ² Microcontroller ª TI MSP 430 (@ 1 MHz, 3V): § Fully operaTon 1.2 mW § Deepest sleep mode 0.3 µW – only woken up by external interrupts (not even Tmer is running any more) ª Atmel ATMega § OperaTonal mode: 15 mW acTve, 6 mW idle § Sleep mode: 75 µW [NES] Winter 2014/2015 Sensor Node Architecture 27 Switching between modes ² ² Simplest idea: Greedily switch to lower mode whenever possible Problem: Time and power consumpTon required to reach higher modes not negligible ª ª ² ² Introduces overhead Switching only pays off if Esaved > Eoverhead Example: Event-‐triggered wake up from sleep mode Scheduling problem with uncertainty Pactive Psleep t1 [NES] Winter 2014/2015 Eoverhead Esaved Sensor Node Architecture τdown tevent time τup 28 AlternaDve: Dynamic voltage scaling ² ² Switching modes complicated by uncertainty how long a sleep Tme is available AlternaTve: Low supply voltage & clock ª ² Dynamic voltage scaling (DVS) RaTonale: ª Power consumpTon P depends on § Clock frequency § Square of supply voltage § P / f V2 ª ª ª Lower clock allows lower supply voltage Easy to switch to higher clock But: execuTon takes longer [NES] Winter 2014/2015 Sensor Node Architecture 29 Memory power consumpDon ² Crucial part: FLASH memory ª ² Power for RAM almost negligible FLASH wriTng/erasing is expensive ª ª ª Example: FLASH on Mica motes Reading: ~ 1.1 nAh per byte WriTng: ~ 83.3 nAh per byte [NES] Winter 2014/2015 Sensor Node Architecture 30 TransmiRer power/energy consumpDon for n bits ² Amplifier power: Pamp = αamp + βamp Ptx ª ª ª ² ² In addiTon: transmiQer electronics needs power PtxElec Time to transmit n bits: n / (R * Rcode) ª ² Ptx radiated power αamp, βamp constants depending on model Highest efficiency (η = Ptx / Pamp ) at maximum output power R nomial data rate, R coding rate code To leave sleep mode ª Time Tstart, average power Pstart à Etx = Tstart Pstart + n / (R * Rcode) (PtxElec + αamp + βamp Ptx) ² SimplificaTon: ModulaTon not considered [NES] Winter 2014/2015 Sensor Node Architecture 31 Receiver power/energy consumpDon for n bits ² Receiver also has startup costs ª ² ² ² Time Tstart, average power Pstart Time for n bits is the same n / (R * Rcode) Receiver electronics needs PrxElec Plus: energy to decode n bits EdecBits à Erx = Tstart Pstart + n / (R * Rcode) PrxElec + EdecBits ( R ) [NES] Winter 2014/2015 Sensor Node Architecture 32 Some transceiver numbers [NES] Winter 2014/2015 Sensor Node Architecture 33 Controlling transceivers ² Similar to controller, low duty cycle is necessary Easy to do for transmiQer – similar problem to controller: when is it worthwhile to switch off ª Difficult for receiver: Not only Tme when to wake up not known, it also depends on remote partners ! Dependence between MAC protocols and power consumpTon is strong! ª ² Only limited applicability of techniques analogue to DVS Dynamic ModulaTon Scaling (DSM): Switch to modulaTon best suited to communicaTon – depends on channel gain ª Dynamic Coding Scaling – vary coding rate according to channel gain ª CombinaTons ª [NES] Winter 2014/2015 Sensor Node Architecture 34 ComputaDon vs. communicaDon energy cost ² Tradeoff? Directly comparing computaTon/communicaTon energy cost not possible ª But: put them into perspecTve! ª Energy raTo of “sending one bit” vs. “compuTng one instrucTon”: Anything between 220 and 2900 in the literature ª To communicate (send & receive) one kilobyte = compuTng three million instrucTons! ª ² ² Hence: try to compute instead of communicate whenever possible Key technique in WSN – in-‐network processing! ª Exploit compression schemes, intelligent coding schemes, … [NES] Winter 2014/2015 Sensor Node Architecture 35
