1、Chapter 3: Factors Influencing Sensor Network Design,Factors Influencing Sensor Network Design,A. Hardware Constraints B. Fault Tolerance (Reliability) C. Scalability D. Production Costs E. Sensor Network Topology F. Operating Environment (Applications) G. Transmission Media H. Power Consumption (Li

2、fetime),Sensor Node Hardware,Fault Tolerance (Reliability),Sensor nodes may fail due to lack of power, physical damage or environmental interference The failure of sensor nodes should not affect the overall operation of the sensor network This is called RELIABILITY or FAULT TOLERANCE, i.e., ability

3、to sustain sensor network functionality without any interruption,Fault Tolerance (Reliability),Reliability R (Fault Tolerance) of a sensor node k is modeled: i.e., by Poisson distribution, to capture the probability of not having a failure within the time interval (0,t) with k is the failure rate of

4、 the sensor node k and t is the time period.,G. Hoblos, M. Staroswiecki, and A. Aitouche, “Optimal Design of Fault Tolerant Sensor Networks,” IEEE Int. Conf. on Control Applications, pp. 467-472, Sept. 2000.,Fault Tolerance (Reliability),Reliability (Fault Tolerance) of a broadcast range with N sens

5、or nodes is calculated from,Fault Tolerance (Reliability),Examples: House to keep track of humidity and temperature levels the sensors cannot be damaged easily or interfered by environment low fault tolerance (reliability) requirement! Battlefield for surveillance the sensed data are critical and se

6、nsors can be destroyed by enemies high fault tolerance (reliability) requirement! Bottom line: Fault Tolerance (Reliability) depends heavily on applications!,Scalability,The number of sensor nodes may reach thousands in some applications The density of sensor nodes can range from few to several hund

7、reds in a region (cluster) which can be less than 10m in diameter,Scalability,Examples: Machine Diagnosis Application: less than 50 sensor nodes in a 5 m x 5 m region. Vehicle Tracking Application:Around 10 sensor nodes per cluster/region. Home Application: tens depending on the size of the house. H

8、abitat Monitoring Application: Range from 25 to 100 nodes/cluster Personal Applications:Ranges from tens to hundreds, e.g., clothing, eye glasses, shoes, watch, jewelry.,Production Costs,Cost of sensors must be low so that sensor networks can be justified! PicoNode: less than $1 Bluetooth system: ar

9、ound $10,- THE OBJECTIVE FOR SENSOR COSTS must be lower than $1! Currently ranges from $25 to $180 (STILL VERY EXPENSIVE!),Sensor Network Topology,Topology maintenance and change: Pre-deployment and Deployment Phase Post Deployment Phase Re-Deployment of Additional Nodes,Sensor Network TopologyPre-d

10、eployment and Deployment Phase,Dropped from aircraft (Random deployment) Well Planned, Fixed (Regular deployment) Mobile Sensor Nodes Adaptive, dynamic Can move to compensate for deployment shortcomings Can be passively moved around by some external force (wind, water) Can actively seek out “interes

11、ting” areas,Operating Environment,* SEE ALL THE APPLICATIONS discussed before,TRANSMISSION MEDIA,Radio, 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 radi

12、o, huge spectrum allocation and global availability.,POWER CONSUMPTION,Sensor node has limited power source Sensor node LIFETIME depends on BATTERY lifetime Goal: Provide as much energy as possible at smallest cost/volume/weight/recharge Recharging may or may not be an option Options Primary batteri

13、es not rechargeable Secondary batteries rechargeable, only makes sense in combination with some form of energy harvesting,Battery Examples,Energy per volume (Joule per cubic centimeter):,Energy Scavenging (Harvesting)Ambient Energy Sources (their power density),Solar (Outdoors) 15 mW/cm2 (direct sun

14、) Solar (Indoors) 0.006 mW/cm2 (office desk) 0.57 mW/cm2 (60 W desk lamp) Temperature Gradients 80 W/cm2 at about 1V from a 5Kelvin temp. difference Vibrations 0.01 and 0.1 mW/cm3 Acoustic Noises 3*10-6 mW/cm2 at 75dB - 9.6*10-4 mW/cm2 at 100dB Nuclear Reaction 80 mW/cm3,POWER CONSUMPTION,Sensors ca

15、n be a DATA ORIGINATOR or a DATA ROUTER. Power conservation and power management are important POWER AWARE COMMUNICATION PROTOCOLS must be developed.,POWER CONSUMPTION,Power Consumption,Power consumption in a sensor network can be divided into three domains Sensing Data Processing (Computation) Comm

16、unication,Power Consumption Sensing,Depends on Application Nature of sensing: Sporadic or Constant Detection complexity Ambient noise levels Rule of thumb (ADC power consumption) Fs - sensing frequency, ENOB - effective number of bits,Power Consumption,Power consumption in a sensor network can be di

17、vided into three domains Sensing Data Processing (Computation) Communication,The power consumption in data processing (Pp) is f clock frequency C is the aver. capacitance switched per cycle (C 0.67nF); Vdd is the supply voltage VT is the thermal voltage (n21.26; Io 1.196 mA),Power Consumption in Dat

18、a Processing (Computation) (Wang/Chandrakarasan: Energy Efficient DSPs for Wireless Sensor Networks. IEEE Signal Proc. Magazine, July 2002. also from Shih paper),Power Consumption in Data Processing (Computation),The second term indicates the power loss due to leakage currents In general, leakage en

19、ergy accounts for about 10% of the total energy dissipation In low duty cycles, leakage energy can become large (up to 50%),Power Consumption,Power consumption in a sensor network can be divided into three domains Sensing Data Processing (Computation) Communication,Power Consumption for Communicatio

20、n,A sensor spends maximum energy in data communication (both for transmission and reception). NOTE: For short range communication with low radiation power (0 dbm), transmission and reception power costs are approximately the same, e.g., modern low power short range transceivers consume between 15 an

21、d 300 mW of power when sending and receiving Transceiver circuitry has both active and start-up power consumption,Ptx/rx is the power consumed in the transmitter /receiver electronics (including the start-up power) P0 is the output transmit power,Power Consumption forCommunication,Power consumption

22、for data communication (Pc),Pc = P0 + Ptx + Prx,TX RX,Wasted Energy,Fixed cost of communication: Startup Time High energy per bit for small packets (from Shih paper) Parameters: R=1 Mbps; Tst 450 msec, Pte81mW; Pout = 0 dBm,Energy vs Packet Size,Energy per Bit (pJ),As packet size is reduced the ener

23、gy consumption is dominated by the startup time on the order of hundreds of microseconds during which large amounts of power is wasted. NOTE: During start-up time NO DATA CAN BE SENT or RECEIVED by the transceiver.,Start-Up and Switching,Startup energy consumption Est = PLO x tst PLO, power consumpt

24、ion of the circuitry (synthesizer and VCO); tst, time required to start up all components Energy is consumed when transceiver switches from transmit to receive mode Switching energy consumption Esw = PLO x tsw,Start-Up Time and Sleep Mode,The effect of the transceiver startup time will greatly depen

25、d on the type of MAC protocol used. To minimize power consumption, it is desirable to have the transceiver in a sleep mode as much as possible Energy savings up to 99.99% (59.1mW 3mW) BUT Constantly turning on and off the transceiver also consumes energy to bring it to readiness for transmission or

26、reception.,RF output power,Lets put it together,Energy consumption for communication Ec = Est + Erx + Esw + Etx = PLO tst + (PLO + PRX)trx + PLO tsw +(PLO+PPA)ttx Let trx = ttx = lPKT/r Ec = PLO (tst+tsw)+(2PLO +PRX) lPKT/r + 1/h gPA lPKT dn,Distance-independent,Distance-dependent,A SIMPLE ENERGY MO

27、DEL,Transmit Electronics,Tx Amplifier,ETx (k,D),Eelec * k,eamp* k* D2,k bit packet,Receive Electronics,Eelec * k,k bit packet,D,Etx (k,D) = Etx-elec (k) + Etx-amp (k,D) Etx (k,D) = Eelec * k + eamp * k * D2,ERx (k) = Erx-elec (k) ERx (k) = Eelec * k,ERx (k),ETx-elec (k),ETx-amp (k,D),Power Consumpti

28、on forCommunication,Ton = L / R where L is the packet size in bits and R is the data rate. NT and NR depend on MAC and applications!,What can we do to Reduce Energy Consumption Multiple Power Consumption Modes,Way out: Do not run sensor node at full operation all the time If nothing to do, switch to

29、 power safe mode Question: When to throttle down(減慢)? How to wake up again? Typical modes Controller: Active, idle, sleep Radio mode: Turn on/off, transmitter/receiver, both,Multiple Power Consumption Modes,Multiple modes possible “Deeper” sleep modes Strongly depends on hardware TI MSP 430, e.g.: f

30、our different sleep modes Atmel ATMega: six different modes,Multiple Power Consumption Modes,Microcontroller TI MSP 430 Fully operation 1.2 mW Deepest sleep mode 0.3 W only woken up by external interrupts (not even timer is running any more) Atmel ATMega Operational mode: 15 mW active, 6 mW idle Sle

31、ep mode: 75 W,Switching between Modes,Simplest idea: Greedily switch to lower mode whenever possible Problem: Time and power consumption required to reach higher modes not negligible Introduces overhead Switching only pays off if Esaved Eoverhead,Switching between Modes,Example: Event-triggered wake

32、 up from sleep mode Scheduling problem with uncertainty,Pactive,Psleep,time,tevent,t1,tdown,tup,Alternative: Dynamic Voltage Scaling,Switching modes complicated by uncertainty on how long a sleep time is available Alternative: Low supply voltage & clock Dynamic Voltage Scaling (DVS) A controller run

33、ning at a lower speed, i.e., lower clock rates, consumes less power Reason: Supply voltage can be reduced at lower clock rates while still guaranteeing correct operation,Alternative: Dynamic Voltage Scaling,Reducing the voltage is a very efficient way to reduce power consumption. Actual power consum

34、ption P depends quadratically on the supply voltage VDD, thus, P VDD2 Reduce supply voltage to decrease energy consumption !,Alternative: Dynamic Voltage Scaling,Gate delay also depends on supply voltage K and a are processor dependent (a 2) Gate switch period T0=1/f For efficient operation Tg = To,

35、f is the switching 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 p

36、rocessor should operate at the lowest voltage for a given clock frequency,Alternative: Dynamic Voltage Scaling,Computation vs. Communication Energy Cost,Tradeoff? Directly comparing computation/communication energy cost not possible But: put them into perspective! Energy ratio of “sending one bit” vs. “computing one instruction”: Anything between 220 and 2900 in the literature