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Introduction to Wireless Sensor Networks and its H/W Design Experiences

Introduction to Wireless Sensor Networks and its H/W Design Experiences. Paper from: P. Zhang, C. Sadler, S. Lyon, and M. Martonosi, “ Hardware Design Experiences in ZebraNet, ” Proceedings of SenSys 2004, November 2004. Yueh-yi Wang 2005.11.17. Outline. Introduction to WSN

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Introduction to Wireless Sensor Networks and its H/W Design Experiences

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  1. Introduction to Wireless Sensor Networks and its H/W Design Experiences Paper from: P. Zhang, C. Sadler, S. Lyon, and M. Martonosi, “Hardware Design Experiences in ZebraNet,” Proceedings of SenSys 2004, November 2004 Yueh-yi Wang 2005.11.17

  2. Outline • Introduction to WSN • Hardware and system architecture of WSN • Case study: ZebraNet • Summary & conclusions

  3. Introduction to WSN – Why WSN? • Personal & institutional security • National defense • Radiology, medicine • Chemical plants • Toxic urban locations • Agriculture • Natural hazards • Many others …

  4. BS Pollutants monitored by sensors in the river bed Sensors report to the base monitoring station Application of Sensors in - Environment Monitoring • Measuring pollutant concentration • Pass on information to monitoring station • Predict current location of pollutant contour based on various parameters • Take corrective action

  5. Sensors in Unknown Terrain

  6. Composition of a sensor(-actuator) node • Portable and self-sustained (power, communication, intelligence) • Capable of embedded complex data processing • Note: Power consumed in transmitting 1Kb data over 100m is equivalent to 30M Instructions on 10MIPS processor • Technology trends predict small memory footprint may not be a limitation in future sensor nodes • Equipped with multiple sensing, programmable computing and communication capability Sensing + CPU + Radio = Thousands of potential applications

  7. Can be placed within implants on spining machinery and within composite materials No batteries - big advantage Uses an inductive link to receive power from external coil Can be used in monitoring temperatures in Jet turbine engines www.microstrain.com EmbedSense™ Wireless SensorA Wireless sensor and data acquisition system

  8. Different from traditional networks • Sensor networks are “data-centric” networks • Unique ID not effective in sensor networks • large number of nodes imply large id, thus, data sent may be less than the address • Adjacent nodes may have similar data

  9. Hardware architecture of WSN- Parameters • Cost • Lifetime • Performance • Speed (in ops/sec, in ops/joule) • Comms range (in m, in joules/bit/m) • Memory (size, latency) • Capable of concurrent operation • Reliability, security, size, packaging

  10. Hardware issue on WSN - A Generic Sensor Network Architecture SENSING SUB-SYSTEM PROCESSING SUB-SYSTEM COMMUNICATION SUB-SYSTEM ACTUATION SUB-SYSTEM POWER MGMT. SUB-SYSTEM SECURITY SUB-SYSTEM

  11. Processing subsystem - Illustration

  12. Processing subsystem- Microcontroller • von Neumann architecture (same address and data bus for I/D) • typical 4 bit, 8 bit, 16 bit or 32 bit architectures • speed 4 MHz-400MHz with 10-300 or more MIPS • operate at various power levels: • fully active: 1 to 50 mW • sleep (memory standby, interrupts active, clocks active, cpu off) • sleep (memory retained, interrupts active, clocks active, cpu off) • sleep (memory retained, interrupts active, clocks off, cpu off) 5uW • latency of wakeup is an issue • fixed point / floating point operations • multiple processors may be used (potentially on same core) • could be DSP, FPGA

  13. Processing subsystem- Memory • Considerations • Speed, capacity, price, power consumption, memory protection • Types: • SRAM: typical 0.5KB-64MB • Typical power consumption • retained: ~100ua; read/write: ~10ma if separate chip • retained: 2ua-100ua, read/write:~5ma if in core • DRAM: high power consumption in retained mode • Flash: 256KB-1GB or beyond • Typical power consumption • retained: negligible; read/write: ~7/20ma • erase operation is expensive • Large flashes are outside of core • EEPROM:4KB-512KB, often used as program store

  14. Processing subsystem- Peripherals • Clock generators / Dividers • Hardware Timers • Peripheral interfaces (for sensors, actuators, I/O, power)(analog and digital)(multiple buses with bridges between them) • SPI: Serial Peripheral Interface • I2C • UART: Serial communication • General Purpose Input Output pins (GPIO)

  15. Processing subsystem- Peripherals (contd.) • Interrupts: • Asynchronous breaks in program execution • Press of a button; expiration of a timer; completion of sensing data collection, of DMA transfer, of transmission event, … • When interrupt occurs, processor transitions to the corresponding interrupt handler to service interrupt and then resumes execution • Can have multiple priority levels • Interrupts are enabled and disabled through registers for each peripheral

  16. Processing subsystem- Timers Holds the value that initializes the timer at startup Holds value to compare against Controls the mode (interval or one-shot) Starts and stops the timer Enables/disables the interrupts for this timer

  17. Sensor Subsystem • Multiple types of sensors may be used: • Environmental: pressure, gas composition, humidity, light… • Motion or force: accelerometers, rotation, microphone, piezoresistive strain, position… • Electromagnetic: magnetometers, antenna, cameras… • Chemical/biochemical • Digital or analog output • MEMS enabling

  18. Power Management Subsystem • Voltage regulator • typical ranges: 1.8V, 3.3V, 5V • multiple voltages for various subsystem/power levels • Gauges for voltage or current • battery monitor (allows software to adapt computation) • Control of subsystems wakeup/sleep • Control of platform clock rate, processor voltage

  19. Mote Bluetooth IEEE 802.11 Communication Subsystem Idle current Startup time Energy per bit

  20. Design Principles • Key to Low Duty Cycle Operation: • Sleep – majority of the time • Wakeup – quickly start processing • Active – minimize work & return to sleep • Wtotal=Rsleep*Wsleep + Rwake*Wwake + Ractive*Wactive W: Power Dissipation R: Ratio of the time period • Dynamic Power Consumption • Pdynamic = Cswitched * VDD2 * fclk

  21. Case Study: Hardware Design Experiences in ZebraNet • Biologists Wishlist • Lightweight • Detailed 24/7 archival position logs • Mobile • No fixed base station (no cellular service) • Restricted human access to systems • ZebraNet: Wireless ad hoc network on zebras • Intelligent tracking collars placed on sampled set of zebras • Sensor network: data collected includes • GPS position info, temperature, … ➨ Energy-efficient ➨ GPS-enabled ➨ Wireless ➨ Peer-to-peer routing and data storage ➨ Plan 1 year of autonomous operation

  22. ZebraNet vs. Many Other Sensor Networks… • All nodes mobile: Even “base station” is mobile; • intermittent drive-bys upload data • Large spatial extent • 100s-1000s of sq. kilometers • “Coarse-Grained” nodes: Storage and processing capability >> many other sensor systems • Long-running and autonomous • Reliability and energy-efficiency are key

  23. Hardware Evolution

  24. Other Evolution • Change of -controller • Main reason is the variable clock frequency. • Lower power usage (switching clocks) • TI MSP430F149 allows multiple clocks • 32 KHz in sleep mode • 8 MHz in normal mode • 32 KHz clock consumes 0.05 mA more than sleep

  25. Important Features • Nodes obtain GPS reading every 8 minutes • GPS can sync to global clock • Nodes attempt to send information over radio every 2 hours • All data logged to onboard flash (local as well as received) • ~256 bytes per hour

  26. ZebraNet Protocols • Two peer-to-peer protocols evaluated here • Flooding: Send to everyone found in peer discovery. • History-Based: After peer discovery, choose at most one peer to send to per discovery period: the one with best past history of delivering data to base.

  27. Zebra show time Solar Power with loosely rotated => efficiency dropped

  28. GPS Data for 1 Zebra Over 24 Hours

  29. Power Consumption • Radio Tx consumes the most critical power. • The 2nd one is GPS. • Radio Rx takes the longest time while working. • Not much difference on u-C under 8MHz and 32KHz (odd?) • Dynamic Power Consumption • Pdynamic = Cswitched * VDD2 * fclk

  30. Summary and Conclusions • New design approach derived from the experience with resource constrained wireless sensor networks • Active mode needs to run quickly to completion • Wakeup time is crucial for low power operation • Wakeup time and sleep current set the minimal energy consumption for an application • Sleep most of the time • Tradeoffs between complexity/robustness and low power radios • Careful integration of hardware and peripherals

  31. Summary and Conclusions • Hardware choice worked very well for sparse node-to-node communication • Simplicity of software environment dictated -controller choice • Details matter in WSN power management • Future work of ZebraNet

  32. Thank you

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