chapter 3: factors influencing sensor network design
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 3:Chapter 3:Factors Influencing Sensor Network Factors Influencing Sensor Network DesignDesign
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Factors Influencing Sensor Network Factors Influencing Sensor Network DesignDesign
A. Hardware ConstraintsA. Hardware Constraints
B. Fault Tolerance (Reliability)B. Fault Tolerance (Reliability)
C. ScalabilityC. Scalability
D. Production CostsD. Production Costs
E. Sensor Network TopologyE. Sensor Network Topology
F. Operating Environment (Applications)F. Operating Environment (Applications)
G. Transmission Media G. Transmission Media
H. Power Consumption (Lifetime)H. Power Consumption (Lifetime)
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Sensor Node HardwareSensor Node Hardware
Power UnitPower Unit AntennaAntenna
Sensor ADCSensor ADCProcessorProcessor
MemoryMemoryTransceiverTransceiver
Location Finding SystemLocation Finding System MobilizerMobilizer
SENSING UNIT PROCESSING UNIT
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Fault ToleranceFault Tolerance(Reliability)(Reliability)
Sensor nodes may fail due to lack of power, Sensor nodes may fail due to lack of power, physical damage or environmental interferencephysical damage or environmental interference
The failure of sensor nodes should not affect the The failure of sensor nodes should not affect the overall operation of the sensor networkoverall operation of the sensor network
This is called This is called RELIABILITY or FAULT TOLERANCE, RELIABILITY or FAULT TOLERANCE, i.e., ability to sustain sensor network functionality i.e., ability to sustain sensor network functionality without any interruptionwithout any interruption
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Fault Tolerance (Reliability)Fault Tolerance (Reliability) Reliability R (Fault Tolerance) of a sensor node k is Reliability R (Fault Tolerance) of a sensor node k is
modeled: modeled:
i.e., by Poisson distribution, to capture the probability of i.e., by Poisson distribution, to capture the probability of not having a failure within the time interval (0,t) with lnot having a failure within the time interval (0,t) with lkk is is the failure rate of the sensor node k and t is the time period.the failure rate of the sensor node k and t is the time period.
)()( tk
ketR
G. Hoblos, M. Staroswiecki, and A. Aitouche,G. Hoblos, M. Staroswiecki, and A. Aitouche, “Optimal Design of Fault Tolerant Sensor “Optimal Design of Fault Tolerant Sensor Networks,” Networks,” IEEE Int. Conf. on Control ApplicationsIEEE Int. Conf. on Control Applications, pp. 467-472, Sept. 2000., pp. 467-472, Sept. 2000.
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Fault Tolerance (Reliability) Fault Tolerance (Reliability)
Reliability (Fault Tolerance) of a broadcast range Reliability (Fault Tolerance) of a broadcast range with N sensor nodes is calculated fromwith N sensor nodes is calculated from
])(1[1)(1
N
kk tRtR
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Fault Tolerance (Reliability)Fault Tolerance (Reliability)
EXAMPLE:EXAMPLE:
How many sensor nodes are needed within a How many sensor nodes are needed within a broadcast radius (range) to have 99% fault tolerated broadcast radius (range) to have 99% fault tolerated network?network?
Assuming all sensors within the radio range have Assuming all sensors within the radio range have same reliability, previous equation becomes:same reliability, previous equation becomes:
Drop t and substitute f = (1-R) 0.99 = (1 – fN) N=2
NtRtR )](1[1)(
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Fault Tolerance (Reliability)Fault Tolerance (Reliability)
REMARKREMARK::
1. Protocols and algorithms may be designed 1. Protocols and algorithms may be designed to to
address the level of fault tolerance address the level of fault tolerance required by required by
sensor networks.sensor networks.
2. If the environment has little interference, 2. If the environment has little interference, then then
the requirements can be more relaxed.the requirements can be more relaxed.
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Fault Tolerance (Reliability)Fault Tolerance (Reliability) Examples:Examples:
1.1. HouseHouse to keep track of humidity and temperature to keep track of humidity and temperature levels levels the sensors cannot be damaged easily or the sensors cannot be damaged easily or interfered by environment interfered by environment lowlow fault tolerance fault tolerance (reliability) requirement!!!!(reliability) requirement!!!!
2.2. BattlefieldBattlefield for surveillance the sensed data are critical for surveillance the sensed data are critical and sensors can be destroyed by enemies and sensors can be destroyed by enemies highhigh fault fault tolerance (reliability) requirement!!! tolerance (reliability) requirement!!!
Bottom line:Bottom line: Fault Tolerance (Reliability) Fault Tolerance (Reliability) depends heavily on applications!!!depends heavily on applications!!!
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ScalabilityScalability
The number of sensor nodes may reach thousands The number of sensor nodes may reach thousands in some applicationsin some applications
The density of sensor nodes can range from few to The density of sensor nodes can range from few to several hundreds in a region (cluster) which can be several hundreds in a region (cluster) which can be less than 10m in diameterless than 10m in diameter
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Node DensityNode Density: : The number of expected nodes per unit areaThe number of expected nodes per unit area::
N is the number of scattered sensor nodes in region A N is the number of scattered sensor nodes in region A Node DegreeNode Degree: The number of expected nodes in the transmission range of a : The number of expected nodes in the transmission range of a nodenode
R is the radio transmission rangeR is the radio transmission range
Basically: Basically: mm(R(R) ) is the number of sensor nodes within the transmission is the number of sensor nodes within the transmission
radius R of each sensor node in region A.radius R of each sensor node in region A.
Scalability Scalability
AN /
2)( RR
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Scalability Scalability
EXAMPLE:EXAMPLE:Assume sensor nodes are evenly distributed in the sensor Assume sensor nodes are evenly distributed in the sensor field. Determine the node density and node degree if 200 sensor field. Determine the node density and node degree if 200 sensor nodes are deployed in a 50x50 mnodes are deployed in a 50x50 m22 region where each sensor region where each sensor node has a broadcast radius of 5m.node has a broadcast radius of 5m.
Use the eq. Use the eq.
6508.0)( 2 R
08.0)5050/(200
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ScalabilityScalabilityExamples: Examples:
1.1. Machine Diagnosis Application: Machine Diagnosis Application: less than 50 sensor nodes in a 5 m x 5 m region.less than 50 sensor nodes in a 5 m x 5 m region.
2.2. Vehicle Tracking Application:Vehicle Tracking Application:Around 10 sensor nodes per cluster/region.Around 10 sensor nodes per cluster/region.
3.3. Home Application: Home Application: tens depending on the size of the house.tens depending on the size of the house.
4.4. Habitat Monitoring Application:Habitat Monitoring Application: Range from 25 to 100 nodes/clusterRange from 25 to 100 nodes/cluster
5.5. Personal Applications:Personal Applications:Ranges from tens to hundreds, e.g., clothing, eye glasses, shoes, watch, Ranges from tens to hundreds, e.g., clothing, eye glasses, shoes, watch, jewelry.jewelry.
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Production CostsProduction Costs
Cost of sensors must be low so that sensor Cost of sensors must be low so that sensor networks can be justified!networks can be justified!
PicoNode: less than $1PicoNode: less than $1
Bluetooth system: around $10,- Bluetooth system: around $10,-
THE OBJECTIVE FOR SENSOR COSTS THE OBJECTIVE FOR SENSOR COSTS
must be lower than $1!!!!!!!must be lower than $1!!!!!!!
Currently Currently ranges from $25 to $180 ranges from $25 to $180
(STILL VERY EXPENSIVE!!!!)(STILL VERY EXPENSIVE!!!!)
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Sensor Network TopologySensor Network Topology
Internet, Internet, Satellite, UAVSatellite, UAV
Sink
Sink
TaskManager
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Sensor Network Topology Sensor Network Topology
Topology maintenance and change:Topology maintenance and change:
Pre-deployment and Deployment Phase Pre-deployment and Deployment Phase
Post Deployment PhasePost Deployment Phase
Re-Deployment of Additional NodesRe-Deployment of Additional Nodes
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Sensor Network TopologySensor Network TopologyPre-deployment and Deployment PhasePre-deployment 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 “interesting” areas
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Sensor Network TopologySensor Network TopologyInitial Deployment SchemesInitial Deployment Schemes
Reduce installation cost Reduce installation cost
Eliminate the need for any pre-organization and Eliminate the need for any pre-organization and pre-planningpre-planning
Increase the flexibility of arrangement Increase the flexibility of arrangement
Promote self-organization and fault-tolerancePromote self-organization and fault-tolerance
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Sensor Network TopologySensor Network TopologyPOST-DEPLOYMENT PHASEPOST-DEPLOYMENT PHASE
Topology changes may occur: Topology changes may occur:
PositionPosition
Reachability (due to jamming, noise, moving Reachability (due to jamming, noise, moving obstacles, etc.)obstacles, etc.)
Available energyAvailable energy
MalfunctioningMalfunctioning
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Operating EnvironmentOperating Environment
* SEE ALL THE APPLICATIONS discussed before* SEE ALL THE APPLICATIONS discussed before
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TRANSMISSION MEDIATRANSMISSION MEDIA
Radio, Infrared, Optical, Acoustic, Magnetic Media Radio, Infrared, Optical, Acoustic, Magnetic Media
ISM ISM (Industrial, Scientific and Medical) (Industrial, Scientific and Medical) Bands (433 Bands (433 MHz ISM Band in Europe and 915 MHz as well as MHz ISM Band in Europe and 915 MHz as well as 2.4 GHz ISM Bands in North America)2.4 GHz ISM Bands in North America)
REASONS:REASONS: Free radio, huge spectrum allocation Free radio, huge spectrum allocation and global availability.and global availability.
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POWER CONSUMPTIONPOWER CONSUMPTION
Sensor node has limited power sourceSensor node has limited power source
Sensor node LIFETIME depends on BATTERY lifetime Sensor node LIFETIME depends on BATTERY lifetime
Goal: Provide as much energy as possible at smallest Goal: Provide as much energy as possible at smallest cost/volume/weight/rechargecost/volume/weight/recharge
Recharging may or may not be an optionRecharging may or may not be an option
OptionsOptions
Primary batteries – not rechargeable Primary batteries – not rechargeable
Secondary batteries – rechargeable, only makes Secondary batteries – rechargeable, only makes sense in combination with some form of energy sense in combination with some form of energy harvestingharvesting
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Battery ExamplesBattery Examples
Energy per volume (Joule per cubic centimeter): Energy per volume (Joule per cubic centimeter): 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
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Energy Scavenging Energy Scavenging (Harvesting)(Harvesting)Ambient Energy Sources (their power density)Ambient Energy Sources (their power density)
Solar (Outdoors) Solar (Outdoors) – 15 mW/cm– 15 mW/cm22 (direct sun)(direct sun)Solar (Indoors)Solar (Indoors) – 0.006 mW/cm – 0.006 mW/cm22 (office desk)(office desk) 0.57 mW/cm0.57 mW/cm2 2 (<60 W desk lamp)(<60 W desk lamp) Temperature GradientsTemperature Gradients – 80 – 80 W/cmW/cm22 at about 1V from a at about 1V from a 5Kelvin temp. difference5Kelvin temp. differenceVibrationsVibrations – 0.01 and 0.1 mW/cm – 0.01 and 0.1 mW/cm33 Acoustic NoisesAcoustic Noises – 3*10 – 3*10{-6} {-6} mW/cmmW/cm2 2 at 75dBat 75dB - 9.6*10- 9.6*10{-4}{-4} mW/cm mW/cm2 2 at 100dBat 100dBNuclear Reaction – Nuclear Reaction – 80 mW/cm80 mW/cm33
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POWER CONSUMPTIONPOWER CONSUMPTION
Sensors can be a Sensors can be a DATA ORIGINATORDATA ORIGINATOR or a or a DATA DATA ROUTER.ROUTER.
Power conservation and power management are Power conservation and power management are importantimportant
POWER AWARE COMMUNICATION PROTOCOLSPOWER AWARE COMMUNICATION PROTOCOLSmust be developed.must be developed.
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POWER CONSUMPTIONPOWER CONSUMPTION
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Power ConsumptionPower Consumption
Power consumption in a sensor network can be Power consumption in a sensor network can be divided into three domains divided into three domains
SensingSensing
Data Processing (Computation) Data Processing (Computation)
CommunicationCommunication
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Power ConsumptionPower Consumption
Power consumption in a sensor network can be Power consumption in a sensor network can be divided into three domains divided into three domains
SensingSensing
Data Processing (Computation) Data Processing (Computation)
CommunicationCommunication
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Power Consumption Power Consumption SensingSensing
Depends onDepends on ApplicationApplication Nature of sensing: Sporadic or ConstantNature of sensing: Sporadic or Constant Detection complexity Detection complexity Ambient noise levelsAmbient noise levels
Rule of thumb (ADC power consumption)Rule of thumb (ADC power consumption)
FFss - sensing frequency, ENOB - effective number of bits - sensing frequency, ENOB - effective number of bits
Ps FS 2ENOB
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Power ConsumptionPower Consumption
Power consumption in a sensor network can be Power consumption in a sensor network can be divided into three domains divided into three domains
SensingSensing
Data Processing (Computation)Data Processing (Computation)
CommunicationCommunication
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Power Consumption in Power Consumption in Data Processing (Computation)Data Processing (Computation) (Wang/Chandrakarasan: Energy Efficient DSPs for Wireless Sensor (Wang/Chandrakarasan: Energy Efficient DSPs for Wireless Sensor Networks. IEEE Signal Proc. Magazine, July 2002. also from Shih paper)Networks. IEEE Signal Proc. Magazine, July 2002. also from Shih paper)
)(** */2 TVndddd
VOddP eIVVCfP
The power consumption in data processing (PThe power consumption in data processing (Ppp) is) 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 (n~21.26; Io ~ 1.196 mA)
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Power Consumption in Power Consumption in Data ProcessingData Processing (Computation) (Computation)
The second term indicates the power loss due to The second term indicates the power loss due to leakage currentsleakage currents
In general, leakage energy accounts for about 10% In general, leakage energy accounts for about 10% of the total energy dissipationof the total energy dissipation
In low duty cycles, leakage energy can become In low duty cycles, leakage energy can become large (up to 50%)large (up to 50%)
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Power Consumption in Power Consumption in Data Processing Data Processing
This is much less than in communication.This is much less than in communication.
EXAMPLE: EXAMPLE: (Assuming: Rayleigh Fading wireless (Assuming: Rayleigh Fading wireless channel; fourth power distance loss)channel; fourth power distance loss)
Energy cost of transmitting Energy cost of transmitting 1 KB1 KB over a distance of over a distance of 100 m is approx. equal to executing 100 m is approx. equal to executing 0.25 Million 0.25 Million instructionsinstructions by a 8 million instructions per second by a 8 million instructions per second processor (MicaZ).processor (MicaZ).
Local data processing is crucial in minimizing Local data processing is crucial in minimizing power consumption in a multi-hop networkpower consumption in a multi-hop network
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Memory Power ConsumptionMemory Power Consumption
Crucial part: FLASH memoryCrucial part: FLASH memory
Power for RAM almost negligiblePower for RAM almost negligible
FLASH writing/erasing is expensiveFLASH writing/erasing is expensive
Example: FLASH on Mica motesExample: FLASH on Mica motes
Reading: ¼ 1.1 nAh per byteReading: ¼ 1.1 nAh per byte
Writing: ¼ 83.3 nAh per byteWriting: ¼ 83.3 nAh per byte
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Power ConsumptionPower Consumption
Power consumption in a sensor network can be Power consumption in a sensor network can be divided into three domains divided into three domains
SensingSensing
Data Processing (Computation) Data Processing (Computation)
CommunicationCommunication
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Power Consumption for Power Consumption for CommunicationCommunication
A sensor spends maximum energy in data A sensor spends maximum energy in data communication (both for transmission and reception).communication (both for transmission and reception).
NOTE:NOTE: For short range communication with low radiation For short range communication with low radiation
power (~0 dbm), transmission and reception power power (~0 dbm), transmission and reception power costs are approximately the same, costs are approximately the same, e.g., modern low power short range transceivers e.g., modern low power short range transceivers
consume between consume between 15 and 300 mW 15 and 300 mW of power when of power when sending and receivingsending and receiving
Transceiver circuitry has both active and start-up Transceiver circuitry has both active and start-up power consumptionpower consumption
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Power Consumption forPower Consumption forCommunicationCommunication
Power consumption for Power consumption for data communicationdata communication (P(Pcc))
PPcc = P = P0 0 + P+ Ptx tx + P + Prxrx
PPte/rete/re is the power consumed in the transmitter/receiver is the power consumed in the transmitter/receiver
electronics (including the start-up power)electronics (including the start-up power) PP0 0 is the output transmit power is the output transmit power
TX RXTX RX
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Power Consumption for Power Consumption for CommunicationCommunication
START-UP POWER/ START-UP TIMESTART-UP POWER/ START-UP TIME A transceiver spends upon waking up from sleep mode, A transceiver spends upon waking up from sleep mode,
e.g., to ramp up e.g., to ramp up phase locked loops or voltage phase locked loops or voltage controlled oscillatorscontrolled oscillators..
During start-up time, no transmission or reception of During start-up time, no transmission or reception of data is possible. data is possible.
Sensors communicate in short data packetsSensors communicate in short data packets Start-up power starts dominating as packet size is Start-up power starts dominating as packet size is
reduced reduced It is inefficient to turn the transceiver ON and OFF It is inefficient to turn the transceiver ON and OFF
because a large amount of power is spent in turning the because a large amount of power is spent in turning the transceiver back ON each time.transceiver back ON each time.
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Wasted EnergyWasted Energy
Fixed cost of communication: Fixed cost of communication: Startup TimeStartup Time High energy per bit for small packets High energy per bit for small packets (from Shih paper)(from Shih paper)
Parameters: R=1 Mbps; TParameters: R=1 Mbps; Tstst ~ 450 msec, P ~ 450 msec, Ptete~81mW; P~81mW; Poutout = 0 dBm = 0 dBm
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Energy vs Packet SizeEnergy vs Packet Size
TR 1000 (115kbps)
0
10
20
30
40
50
60
10 100 1000 10000
Packet Size (bits)
Eb
it ( pJ )
Energy per Bit(pJ)
As packet size is reduced the energy 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.
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Start-Up and SwitchingStart-Up and Switching
Startup energy consumptionStartup energy consumption
EEstst = P = PLOLO x t x tstst
PPLOLO, power consumption of the circuitry , power consumption of the circuitry
(synthesizer and VCO); t(synthesizer and VCO); tstst, time required to start up , time required to start up
all componentsall components
Energy is consumed when transceiver switches Energy is consumed when transceiver switches from transmit to receive modefrom transmit to receive mode
Switching energy consumptionSwitching energy consumption
EEswsw = P = PLOLO x t x tswsw
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Start-Up Time and Sleep ModeStart-Up Time and Sleep Mode The effect of the transceiver startup time will The effect of the transceiver startup time will
greatly depend on the type of MAC protocol used. greatly depend on the type of MAC protocol used.
To minimize power consumption, it is desirable to To minimize power consumption, it is desirable to have the transceiver in a have the transceiver in a sleep modesleep mode as much as as much as possiblepossible
Energy savings up to 99.99% (59.1mW Energy savings up to 99.99% (59.1mW 3 3mmW)W) BUT…BUT… Constantly turning on and off the transceiver also Constantly turning on and off the transceiver also
consumes energy to bring it to readiness for consumes energy to bring it to readiness for transmission or reception.transmission or reception.
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Receiving and Transmitting Energy Receiving and Transmitting Energy ConsumptionConsumption
Receiving energy consumptionReceiving energy consumption
EErxrx = (P = (PLOLO + P + PRXRX ) t ) trxrx
PPRXRX, power consumption of active components, e.g., , power consumption of active components, e.g.,
decoder, tdecoder, trxrx, time it takes to receive a packet, time it takes to receive a packet
Transmitting energy consumptionTransmitting energy consumption
EEtxtx = (P = (PLOLO + P + PPAPA ) t ) ttxtx
PPPAPA, power consumption of power amplifier, power consumption of power amplifier
PPPAPA = 1/ = 1/ P Poutout
power efficiency of power amplifier, Ppower efficiency of power amplifier, Poutout, desired , desired
RF output power levelRF output power level
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RF output powerRF output power
http://memsic.com/support/documentation/wireless-sensor-networks/category/7-datasheets.html?download=148%3Amicazhttp://memsic.com/support/documentation/wireless-sensor-networks/category/7-datasheets.html?download=148%3Amicaz
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Power Amplifier Power ConsumptionPower Amplifier Power Consumption Receiving energy consumptionReceiving energy consumption
PPPAPA = 1/ = 1/∙ ∙ PA PA ∙ ∙ r r ∙ ∙ ddnn
PAPA, amplifier constant (antenna gain, wavelength, , amplifier constant (antenna gain, wavelength, thermal noise power spectral density, desired thermal noise power spectral density, desired signal to noise ratio (SNR) at distance d), signal to noise ratio (SNR) at distance d),
r, data rate, r, data rate, n, path loss exponent of the channel (n=2-4)n, path loss exponent of the channel (n=2-4) d, distance between nodesd, distance between nodes
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Let’s put it together…Let’s put it together…
Energy consumption for communicationEnergy consumption for communication
EEcc = E = Estst + E + Erxrx + E + Eswsw + E + Etxtx
= P= PLOLO t tstst + (P + (PLOLO + P + PRXRX)t)trxrx + P + PLOLO t tswsw + (P + (PLOLO+P+PPAPA)t)ttxtx
Let tLet trxrx = t = ttxtx = l = lPKTPKT/r /r
EEcc = P = PLOLO (t (tstst+t+tswsw)+(2P)+(2PLOLO + P + PRXRX)l)lPKTPKT/r + 1//r + 1/∙ ∙ PA PA ∙ ∙ llPKTPKT ∙ ∙ ddnn
Distance-independentDistance-independent Distance-dependentDistance-dependent
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A SIMPLE ENERGY MODELA SIMPLE ENERGY MODEL
Operation Energy Dissipated
Transmitter Electronics ( ETx-elec)
Receiver Electronics ( ERx-elec)
( ETx-elec = ERx-elec = Eelec )
50 nJ/bit
Transmit Amplifier {eamp} 100 pJ/bit/m2
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)
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Power ConsumptionPower Consumption(A Simple Energy Model)(A Simple Energy Model)
Assuming a sensor node is only operating in transmit and receive modes with the following assumptions: Energy to run circuitry:
Eelec = 50 nJ/bit Energy for radio transmission:
eamp = 100 pJ/bit/m2
Energy for sending k bits over distance D
ETx (k,D) = Eelec * k + eamp * k * D2
Energy for receiving k bits:
ERx (k,D) = Eelec * k
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Example using the Simple Energy ModelExample using the Simple Energy Model
What is the energy consumption if 1 Mbit of information is transferred from the source to the sink where the source and sink are separated by 100 meters and the broadcast radius of each node is 5 meters?
Assume the neighbor nodes are overhearing each other’s broadcast.
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EXAMPLEEXAMPLE
100 meters / 5 meters = 20 pairs of transmitting and receiving nodes (one node transmits and one node receives)
ETx (k,D) = Eelec * k + eamp * k * D2
ETx = 50 nJ/bit . 106 + 100 pJ/bit/m2 . 106 . 52 = = 0.05J + 0.0025 J = 0.0525 J
ERx (k,D) = Eelec * kERx = 0.05 J
Epair = ETx + ERx = 0.1025J
ET = 20 . Epair = 20. 0.1025J = 2.050 J
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VERY DETAILED ENERGY MODEL
sleepsleeponon TPTPE Simple Energy Consumption Model
A More Realistic ENERGY MODEL*
LTPTPBTGP
BTL
NE trsynoncond
bon
BT
L
BT
L
f
on
on /2
214
ln123
41
2
2
* S. Cui, et.al., “Energy-Constrained Modulation * S. Cui, et.al., “Energy-Constrained Modulation Optimization,” Optimization,” IEEE Trans. on Wireless CommunicationsIEEE Trans. on Wireless Communications, , September 2005.September 2005.
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Details of the Realistic Model Details of the Realistic Model
onTB
L
M
M
M
2
1
13
1
L – packet lengthL – packet lengthB – channel bandwidthB – channel bandwidth
NNff – receiver noise figure – receiver noise figure
22 – power spectrum energy – power spectrum energy
PPbb – probability of bit error – probability of bit error
GGdd – power gain factor – power gain factor
PPcc – circuit power consumption – circuit power consumption
PPsynsyn – frequency synthesizer power – frequency synthesizer power
consumptionconsumption
TTtrtr – frequency synthesizer settling time (duration of – frequency synthesizer settling time (duration of transient mode)transient mode)
TTonon – transceiver on time – transceiver on time
M – Modulation parameterM – Modulation parameter
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Enery Consumption: Important Variables: Enery Consumption: Important Variables:
PPre re 4.5 mA 4.5 mA (energy consumption at receiver)(energy consumption at receiver)
PPtete 12.0 mA 12.0 mA (energy consumption at transmitter)(energy consumption at transmitter)
PPclcl 12.0 mA 12.0 mA (basic consumption without radio)(basic consumption without radio)
PPslsl 8mA (0.008 mA) 8mA (0.008 mA) (energy needed to sleep)(energy needed to sleep)
ANOTHER EXAMPLE
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Capacity (Watt) = Current (Ampere) * Voltage (Volt)Capacity (Watt) = Current (Ampere) * Voltage (Volt) Rough estimation for energy consumption and sensor lifetime:Rough estimation for energy consumption and sensor lifetime:
Let us assume that each sensor should wake up once a Let us assume that each sensor should wake up once a second, measure a value and transmit it over the network.second, measure a value and transmit it over the network.
a) Calculations needed: 5K instructions (for measurement anda) Calculations needed: 5K instructions (for measurement and preparation for sending)preparation for sending)
b) Time to send information: 50 bytes for sensor data, b) Time to send information: 50 bytes for sensor data, (another 250 byte for forwarding external data)(another 250 byte for forwarding external data)
c) Energy needed to sleep for the rest of the time (sleep c) Energy needed to sleep for the rest of the time (sleep mode)mode)
EXAMPLE
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Time for Calculations and Energy Consumption:Time for Calculations and Energy Consumption:
MSP430 running at 8 MHz clock rate MSP430 running at 8 MHz clock rate one cycle one cycle takes 1/(8*10takes 1/(8*1066) seconds) seconds
1 instruction needs an average of 3 cycles 1 instruction needs an average of 3 cycles 3/ 3/ (8* 10(8* 1066) sec, 5K instructions, 15/(8*10) sec, 5K instructions, 15/(8*1033) sec) sec
15/(8*1015/(8*1033) * 12mA = 180/8000 = 0.0225 mAs) * 12mA = 180/8000 = 0.0225 mAs
EXAMPLE
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Time for Sending Data and Energy Consumption:Time for Sending Data and Energy Consumption:
Radio sends with 19.200 baud (approx. 19.200 bits/sec)Radio sends with 19.200 baud (approx. 19.200 bits/sec)
1 bit takes 1/19200 seconds1 bit takes 1/19200 seconds
We have to send 50 bytes (own measurement) We have to send 50 bytes (own measurement)
and we have to forward 250 bytes (external and we have to forward 250 bytes (external
data): 250+50=300 which takes data): 250+50=300 which takes
300*8/19200s*24mA (energy basic + sending) = 3mAs300*8/19200s*24mA (energy basic + sending) = 3mAs
EXAMPLE
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Energy consumed while sleeping:Energy consumed while sleeping:
Time for calculation 15/8000 + time for transmissionTime for calculation 15/8000 + time for transmission
300*8/19200 ~ 0.127 sec300*8/19200 ~ 0.127 sec
Time for sleep mode = 1 sec – 0.127 = 0.873 sTime for sleep mode = 1 sec – 0.127 = 0.873 s
Energy consumed while sleeping Energy consumed while sleeping
0.008mA * 0.873 s = 0.0007 mAs0.008mA * 0.873 s = 0.0007 mAs
EXAMPLE
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Total Amount of energy and resulting lifetimeTotal Amount of energy and resulting lifetime::
The ESB needs to be supplied with 4.5 V so we need The ESB needs to be supplied with 4.5 V so we need 3 * 1.5V AA batteries.3 * 1.5V AA batteries.
3*(0.0225 + 3 + 0.007) ~ 3 * 3.03 mWs 3*(0.0225 + 3 + 0.007) ~ 3 * 3.03 mWs
Energy of 3AA battery ~ 3 * 2300 mAh = 3*2300*60*60 mWsEnergy of 3AA battery ~ 3 * 2300 mAh = 3*2300*60*60 mWs
Total lifetime Total lifetime 3*2300*60*60/3*3.03 ~ 32 days. 3*2300*60*60/3*3.03 ~ 32 days.
EXAMPLE
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NOTES:NOTES:
Battery suffers from large current (losing about 10% energy/year)Battery suffers from large current (losing about 10% energy/year)
Small network (forwarding takes only 250 bytes)Small network (forwarding takes only 250 bytes)
Most important:Most important: Only sending was taken into account, not receivingOnly sending was taken into account, not receiving
If we listen into the channel rather than sleeping 0.007 mA has to be If we listen into the channel rather than sleeping 0.007 mA has to be replaced by (12+4.5)mAreplaced by (12+4.5)mA
which results in a lifetime of ~ 5 days.which results in a lifetime of ~ 5 days.
EXAMPLE
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Power Consumption for Power Consumption for Communication Communication (Detailed Formula)(Detailed Formula)
)]([)]()([ stonreRonOstonteTc RRPNTPTTPNP wherewhere
PPtete is power consumed by transmitter is power consumed by transmitterPPre re is power consumed by receiveris power consumed by receiverPPOO is output power of transmitter is output power of transmitterTTonon is transmitter “on” time is transmitter “on” time
RRonon is receiver “on” time is receiver “on” time
TTstst is start-up time for transmitter is start-up time for transmitter
RRstst is start-up time for receiver is start-up time for receiver
NNTT is the number of times transmitter is the number of times transmitter
is switched “on” per unit of timeis switched “on” per unit of time
NNRR is the number of times receiver is the number of times receiver
is switched “on” per unit of timeis switched “on” per unit of time
E. Shih et al.,”Physical Layer Driven Protocols and Algorithm Design for E. Shih et al.,”Physical Layer Driven Protocols and Algorithm Design for Energy-Efficient Wireless Sensor Networks”, ACM MobiCom, Rome, July Energy-Efficient Wireless Sensor Networks”, ACM MobiCom, Rome, July 2001.2001.
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Power Consumption forPower Consumption forCommunicationCommunication
TTonon = L / R = L / R where L is the packet size in bits and R is the where L is the packet size in bits and R is the
data rate.data rate. NNTT and N and NRR depend on MAC and applications!!! depend on MAC and applications!!!
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What can we do to Reduce Energy ConsumptionWhat can we do to Reduce Energy Consumption
Multiple Power Consumption Modes Multiple Power Consumption Modes
Way out:Way out: Do not run sensor node at full operation all the Do not run sensor node at full operation all the timetime If nothing to do, switch to If nothing to do, switch to power safe modepower safe modeQuestion: When to throttle down? How to wake up Question: When to throttle down? How to wake up
again? again? Typical modesTypical modes
Controller: Active, idle, sleepController: Active, idle, sleepRadio mode: Turn on/off Radio mode: Turn on/off transmitter/receiver, bothtransmitter/receiver, both
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Multiple Power Consumption ModesMultiple Power Consumption Modes
Multiple modes possible Multiple modes possible “ “Deeper” sleep modesDeeper” sleep modesStrongly depends on hardwareStrongly depends on hardware
TI MSP 430, e.g.: four different sleep modesTI MSP 430, e.g.: four different sleep modes
Atmel ATMega: six different modesAtmel ATMega: six different modes
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Multiple Power Consumption ModesMultiple Power Consumption Modes
MicrocontrollerMicrocontrollerTI MSP 430 TI MSP 430
Fully operation 1.2 mW Fully operation 1.2 mW Deepest sleep mode 0.3 Deepest sleep mode 0.3 W – only woken up by W – only woken up by
external interrupts (not even timer is running any external interrupts (not even timer is running any more)more)
Atmel ATMegaAtmel ATMegaOperational mode: 15 mW active, 6 mW idleOperational mode: 15 mW active, 6 mW idleSleep mode: 75 Sleep mode: 75 W W
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Switching between ModesSwitching between Modes
Simplest idea: Greedily switch to lower mode whenever Simplest idea: Greedily switch to lower mode whenever possiblepossible
Problem: Time and power consumption required to reach Problem: Time and power consumption required to reach higher modes not negligible higher modes not negligible
Introduces overhead Introduces overhead
Switching only pays off if ESwitching only pays off if Esavedsaved > E > Eoverheadoverhead
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Switching between ModesSwitching between Modes
Example: Event-triggered wake up from sleep modeExample: Event-triggered wake up from sleep mode
Scheduling problem with uncertainty Scheduling problem with uncertainty
Pactive
Psleeptimeteventt1
Esaved
tdown tup
Eoverhead
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Alternative: Dynamic Voltage ScalingAlternative: Dynamic Voltage Scaling
Switching modes complicated by uncertainty on Switching modes complicated by uncertainty on how long a sleep time is availablehow long a sleep time is available
Alternative: Low supply voltage & clock Alternative: Low supply voltage & clock
Dynamic Voltage Scaling (DVS)Dynamic Voltage Scaling (DVS)
A controller running at a lower speed, i.e., lower A controller running at a lower speed, i.e., lower clock rates, consumes less powerclock rates, consumes less power
Reason: Supply voltage can be reduced at lower Reason: Supply voltage can be reduced at lower clock rates while still guaranteeing correct clock rates while still guaranteeing correct operationoperation
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Alternative: Dynamic Voltage ScalingAlternative: Dynamic Voltage Scaling
Reducing the voltage is a very efficient way to reduce Reducing the voltage is a very efficient way to reduce power consumption.power consumption.
Actual power consumption P depends quadratically Actual power consumption P depends quadratically on the supply voltage Von the supply voltage VDDDD, thus, , thus,
P ~ VP ~ VDDDD22
Reduce supply voltage to decrease energy Reduce supply voltage to decrease energy consumption !consumption !
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Alternative: Dynamic Voltage ScalingAlternative: Dynamic Voltage Scaling Gate delay also depends on supply voltageGate delay also depends on supply voltage
K and a are processor dependent (a ~ 2)K and a are processor dependent (a ~ 2)
Gate switch period Gate switch period TT00=1/f=1/f
For efficient operationFor efficient operation
TTgg <= T <= Too
athdd
ddg VVK
VT
)(
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Alternative: Dynamic Voltage ScalingAlternative: Dynamic Voltage Scaling
)(~)(
cVKVdd
VVKf dd
athdd
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 processor should operate at the lowest voltage for a given clock frequency
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Computation vs. Communication Energy Computation vs. Communication Energy costcost
Tradeoff?Tradeoff?
Directly comparing computation/communication Directly comparing computation/communication energy cost not possibleenergy cost not possible
But: put them into perspective!But: put them into perspective!
Energy ratio of “sending one bit” vs. “computing Energy ratio of “sending one bit” vs. “computing one instruction”: Anything between 220 and 2900 one instruction”: Anything between 220 and 2900 in the literaturein the literature
To communicate (send & receive) To communicate (send & receive) one kilobyteone kilobyte = = computing computing three million instructions!three million instructions!
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Computation vs. Communication Energy Computation vs. Communication Energy CostCost
BOTTOMLINEBOTTOMLINE
Try to compute instead of communicate Try to compute instead of communicate whenever possiblewhenever possible
Key technique in WSN – Key technique in WSN – in-network processingin-network processing!!
Exploit compression schemes, intelligent coding Exploit compression schemes, intelligent coding schemes, aggregation, filtering, … schemes, aggregation, filtering, …
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BOTTOMLINE:BOTTOMLINE:Many Ways to Optimize Power ConsumptionMany Ways to Optimize Power Consumption
Power aware computingPower aware computing Ultra-low power microcontrollersUltra-low power microcontrollers Dynamic power management HWDynamic power management HW
Dynamic voltage scaling (e.g Intel’s PXA, Transmeta’s Dynamic voltage scaling (e.g Intel’s PXA, Transmeta’s Crusoe)Crusoe)
Components that switch off after some idle timeComponents that switch off after some idle time Energy aware softwareEnergy aware software
Power aware OS: dim displays, sleep on idle times, power Power aware OS: dim displays, sleep on idle times, power aware schedulingaware scheduling
Power management of radiosPower management of radios Sometimes listen overhead larger than transmit overheadSometimes listen overhead larger than transmit overhead
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BOTTOMLINE:BOTTOMLINE:Many Ways to Optimize Power ConsumptionMany Ways to Optimize Power Consumption
Energy aware packet forwardingEnergy aware packet forwarding
Radio automatically forwards packets at a lower Radio automatically forwards packets at a lower power level, while the rest of the node is asleeppower level, while the rest of the node is asleep
Energy aware wireless communicationEnergy aware wireless communication
Exploit performance energy tradeoffs of the Exploit performance energy tradeoffs of the communication subsystem, better neighbor communication subsystem, better neighbor coordination, choice of modulation schemescoordination, choice of modulation schemes
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COMPARISONCOMPARISON
Technology Data RateTx
CurrentEnergy per
bitIdle
CurrentStartup
time
Mote 76.8 Kbps 10 mA 430 nJ/bit 7 mA Low
Bluetooth 1 Mbps 45 mA 149 nJ/bit 22 mA Medium
802.11 11 Mbps 300 mA 90 nJ/bit 160 mA High
IEEE 802.11
Bluetooth
Mote
Energy per bit
Startup time
Idle current