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Introduction to Wireless Sensor Networks Energy Considerations in WSNs I 3 February 2005 The University of Iowa. Copyright© 2005 1 A. Kruger Organizational Class Website www.engineering.uiowa.edu/~ece195/2005/ Class Time Monday 4:30-5:20 Room 4511 SC Thursday 12:30-1:20 Room 3220 SC Please note that the room numbers are different for Mondays and Thursdays. Office Hours Monday 5:20-6:20 Room 1126 SC Thursday 1:30-2:30 Room 1126 SC Other By appointment Room 523C SHL The University of Iowa. Copyright© 2005 2 A. Kruger Organizational • Class Topics (3 students/per topic) • Students read and help prepare a lecture on one of: – ZigBee Protocol • Brief overview • Example application – What is RFID? • Brief overview • Explanation of terminology – Sample Energy Budget • Worked example of energy considerations in WSNs – Directed Diffusion • Explanation of the algorithm – Overview of TinyDB – Sensor Characteristics • Soil moisture, temperature, humidity, wind speed, vibration – Analog/Digital Conversion • Number of bits, linearity, sampling rate, power, etc. – Smart Dust • Will post a signup sheet The University of Iowa. Copyright© 2005 3 A. Kruger Organizational • Update Lecture 3 with two slides – Path loss • Added supplementary material to website • Lab demonstration next Thursday The University of Iowa. Copyright© 2005 4 A. Kruger Review Questions • • • • • • • • • • What is “LOS”? What is WSN? True or false – Visual LOS implies RF LOS A 1-km 2.4 GHz link has two antennas that are 2 m above the ground. Do we have LOS? True of false: In free space RF power loss ~1/R2 but when the transmitter and receiver is close to the ground, the loss can be ~1/R4 What is RSSI? How is it used in WSNs? What are the ISM bands True or false: ISM bands are unregulated Explain with a simple sketch and paragraph how multipath propagation can diminish or enhance radio propagation What is BER? The University of Iowa. Copyright© 2005 5 A. Kruger Review Questions • A manufacturer claims its radio can make reliable reception if the received power is -105 dBm. How many mW is this? Are you impressed? • Explain what TDMA is. • Explain what FDMA is. • Explain (to grandma) what Spread Spectrum communication is. • What is S/N and SNR? • What are the common units of SNR? The University of Iowa. Copyright© 2005 6 A. Kruger Review Questions • True or false – everything else being equal, RF path loss are higher at 2.5 GHz than at 900 MHz • Estimate the path loss in dB at 900 MHz in an indoor environment. There are two floors are six walls between transmitter and receiver. • The antennas of a 2.4 GHz RF link are 1 m above the ground, and are 100 m apart. Is the path loss ~ 1/R2 or ~ 1/R4?. Is the communication LOS? The University of Iowa. Copyright© 2005 7 A. Kruger Energy/Power Considerations • Terms – Cell, Battery – Energy (Joule) – Power (J/s or Watt) – Ampere-hour (AH) – Deep-cycle – MCU – Sleep Modes – ADC – BPS The University of Iowa. Copyright© 2005 8 A. Kruger Where Does The Power Go? The University of Iowa. Copyright© 2005 9 A. Kruger Microcontroller Unit (MCU) • Intel’s StrongARM, Atmel AVR (PIC?) • Low power modes – Active, Idle, nap, shutdown, sleep modes – For some MCUs, in deep sleep modes, the power consumption can almost be negligible – Takes longer to wake from a deep sleep than just a nap – Wakeup time also takes power – Wakeup impact processing The University of Iowa. Copyright© 2005 10 A. Kruger Radio • Radio typically contain embedded controller that provides many functions – Uses RSSI to adjust transmit power – Error detection and correction in hardware • Several modes – Receive only, transmit + receive, idle, etc. • Transmit in general requires most power • Careful consider radio spec and modes • Mode change can consume a lot of power – May be better to shutdown completely rather than go into idle mode The University of Iowa. Copyright© 2005 11 A. Kruger Bandwidth vs. Power Consumption • Higher bandwidth (BPS) generally requires – Better S/N • => more signal power – More sophisticated modulation encoding/decoding algorithms • => more powerful CPU, more power • Receive power normally much lower than transmit power The University of Iowa. Copyright© 2005 12 A. Kruger Radio Power Consumption The University of Iowa. Copyright© 2005 13 A. Kruger Conventional vs WSN Power Management • Conventional – Well developed techniques – Objective is to minimize power consumption of individual device: sleep modes, low voltage, low clock speed, etc. • WSNs – Objective is to maximize lifetime of individual node, but more importantly – The network as a whole • For example – It may be better for a node to deplete its power source for the greater good of the network The University of Iowa. Copyright© 2005 14 A. Kruger Sensors Passive & low power (~mW and smaller) – Soil moisture, temperature, light, humidity • Active & high power – Anemometers, disdrometers, cameras • Many sensors are inherently analog, but some sensors have digital interfaces (provided by embedded controllers) • Conditioning/wakeup times need to be considered • Analog-Digital Converters (ADC) – Can be a major power consumer – More bits and high conversion rate requires more power – Don’t over specify The University of Iowa. Copyright© 2005 15 A. Kruger Battery • Uses chemical reaction to provide electrical energy – Temperature depended • Batteries are often the most bulky part of a mote • Capacity measured in Ampere-hours or Ah. Note that the capacity does consider voltage… – The capacity is the nominal number of hours it can supply a given current The University of Iowa. Copyright© 2005 16 A. Kruger Capacity 1.25 Ah The University of Iowa. Copyright© 2005 17 A. Kruger May be possible to use curve to gauge battery state. Must be under load conditions. The University of Iowa. Copyright© 2005 18 A. Kruger