电子科大 无线传感器网络通用课件 wsn_chapter_3_传感器网络设计影响因素 (1)

1、0University of Electronic Science and Technology of ChinaWireless Sensor NetworksChapter 3:Factors Influencing Sensor Network Design1University of Electronic Science and Technology of ChinaWireless Sensor NetworksFactors Influencing Sensor Network DesignA. Hardware ConstraintsB. Fault Tolerance (Rel

2、iability)C. ScalabilityD. Production CostsE. Sensor Network TopologyF. Operating Environment (Applications)G. Transmission Media H. Power Consumption (Lifetime)2University of Electronic Science and Technology of ChinaWireless Sensor NetworksSensor Node HardwareSENSING UNITPROCESSING UNIT3University

3、of Electronic Science and Technology of ChinaWireless Sensor NetworksFault Tolerance (Reliability)Sensor nodes may fail due to lack of power, physical damage or environmental interferenceThe failure of sensor nodes should not affect the overall operation of the sensor networkThis is called RELIABILI

4、TY or FAULT TOLERANCE, i.e., ability to sustain sensor network functionality without any interruption4University of Electronic Science and Technology of ChinaWireless Sensor Networks Fault Tolerance (Reliability)Reliability R (Fault Tolerance) of a sensor node k is modeled: i.e., by Poisson distribu

5、tion, to capture the probability of not having a failure within the time interval (0,t) with k is the failure rate of the sensor node k and t is the time period.)()(tkketRG. Hoblos, M. Staroswiecki, and A. Aitouche, “Optimal Design of Fault Tolerant Sensor Networks,” IEEE Int. Conf. on Control Appli

6、cations, pp. 467-472, Sept. 2000.5University of Electronic Science and Technology of ChinaWireless Sensor Networks Fault Tolerance (Reliability) Reliability (Fault Tolerance) of a broadcast range with N sensor nodes is calculated from )(1 1)(1NkktRtR6University of Electronic Science and Technology o

7、f ChinaWireless Sensor Networks Fault Tolerance (Reliability)Examples:1. House to keep track of humidity and temperature levels the sensors cannot be damaged easily or interfered by environment low fault tolerance (reliability) requirement!2. Battlefield for surveillance the sensed data are critical

8、 and sensors can be destroyed by enemies high fault tolerance (reliability) requirement!Bottom line: Fault Tolerance (Reliability) depends heavily on applications!7University of Electronic Science and Technology of ChinaWireless Sensor NetworksScalabilityThe number of sensor nodes may reach thousand

9、s in some applicationsThe density of sensor nodes can range from few to several hundreds in a region (cluster) which can be less than 10m in diameter8University of Electronic Science and Technology of ChinaWireless Sensor NetworksScalabilityExamples: 1.Machine Diagnosis Application: less than 50 sen

10、sor nodes in a 5 m x 5 m region.2.Vehicle Tracking Application:Around 10 sensor nodes per cluster/region.3.Home Application: tens depending on the size of the house.4.Habitat Monitoring Application: Range from 25 to 100 nodes/cluster5.Personal Applications:Ranges from tens to hundreds, e.g., clothin

11、g, eye glasses, shoes, watch, jewelry.9University of Electronic Science and Technology of ChinaWireless Sensor NetworksProduction CostsCost of sensors must be low so that sensor networks can be justified!PicoNode: less than $1Bluetooth system: around $10,- THE OBJECTIVE FOR SENSOR COSTS must be lowe

12、r than $1!Currently ranges from $25 to $180 (STILL VERY EXPENSIVE!)10University of Electronic Science and Technology of ChinaWireless Sensor NetworksSensor Network Topology Topology maintenance and change: Pre-deployment and Deployment Phase Post Deployment Phase Re-Deployment of Additional Nodes11U

13、niversity of Electronic Science and Technology of ChinaWireless Sensor NetworksSensor Network TopologyPre-deployment and Deployment PhaseDropped from aircraft (Random deployment)Well Planned, Fixed (Regular deployment)Mobile Sensor Nodes Adaptive, dynamicCan move to compensate for deployment shortco

14、mingsCan be passively moved around by some external force (wind, water)Can actively seek out “interesting” areas12University of Electronic Science and Technology of ChinaWireless Sensor NetworksOperating Environment * SEE ALL THE APPLICATIONS discussed before13University of Electronic Science and Te

15、chnology of ChinaWireless Sensor NetworksTRANSMISSION MEDIARadio, Infrared, Optical, Acoustic, Magnetic Media ISM (Industrial, Scientific and Medical) Bands (433 MHz ISM Band in Europe and 915 MHz as well as 2.4 GHz ISM Bands in North America)REASONS: Free radio, huge spectrum allocation and global

16、availability.14University of Electronic Science and Technology of ChinaWireless Sensor NetworksPOWER CONSUMPTIONSensor node has limited power sourceSensor node LIFETIME depends on BATTERY lifetime Goal: Provide as much energy as possible at smallest cost/volume/weight/recharge Recharging may or may

17、not be an optionOptionsPrimary batteries not rechargeable Secondary batteries rechargeable, only makes sense in combination with some form of energy harvesting15University of Electronic Science and Technology of ChinaWireless Sensor NetworksBattery ExamplesEnergy per volume (Joule per cubic centimet

18、er): Primary batteriesChemistryZinc-airLithiumAlkalineEnergy (J/cm3)378028801200Secondary batteriesChemistryLithiumNiMHdNiCdEnergy (J/cm3)108086065016University of Electronic Science and Technology of ChinaWireless Sensor NetworksEnergy Scavenging (Harvesting)Ambient Energy Sources (their power dens

19、ity)Solar (Outdoors) 15 mW/cm2 (direct sun)Solar (Indoors) 0.006 mW/cm2 (office desk) 0.57 mW/cm2 ( Eoverhead 39University of Electronic Science and Technology of ChinaWireless Sensor NetworksSwitching between ModesExample: Event-triggered wake up from sleep modeScheduling problem with uncertainty P

20、activePsleeptimeteventt1EsavedtdowntupEoverhead40University of Electronic Science and Technology of ChinaWireless Sensor NetworksAlternative: Dynamic Voltage ScalingSwitching modes complicated by uncertainty on how long a sleep time is availableAlternative: Low supply voltage & clock Dynamic Vol

21、tage Scaling (DVS)A controller running at a lower speed, i.e., lower clock rates, consumes less powerReason: Supply voltage can be reduced at lower clock rates while still guaranteeing correct operation41University of Electronic Science and Technology of ChinaWireless Sensor NetworksAlternative: Dyn

22、amic Voltage ScalingReducing the voltage is a very efficient way to reduce power consumption.Actual power consumption P depends quadratically on the supply voltage VDD, thus, P VDD2 Reduce supply voltage to decrease energy consumption !42University of Electronic Science and Technology of ChinaWirele

23、ss Sensor NetworksAlternative: Dynamic Voltage ScalingGate delay also depends on supply voltageK and a are processor dependent (a 2)Gate switch period T0=1/fFor efficient operationTg = ToathddddgVVKVT)(43University of Electronic Science and Technology of ChinaWireless Sensor Networksf is the switchi

24、ng frequency where a, K, c and Vth are processor dependent variables (e.g., K=239.28 Mhz/V, a=2, and c=0.5)REMARK: For a given processor the maximum performance f of the processor is determined by the power supply voltage Vdd and vice versa.NOTE: For minimal energy dissipation, a processor should op

25、erate at the lowest voltage for a given clock frequencyAlternative: Dynamic Voltage Scaling)()(cVKVddVVKfddathdd44University of Electronic Science and Technology of ChinaWireless Sensor NetworksComputation vs. Communication Energy CostTradeoff?Directly comparing computation/communication energy cost not possibleBut: put them into perspective!Energy ratio of “sending one bit” vs. “computing one instruction”: Anything between 220 and 2900 in the literatureTo communicate (send & receive) on