Chapter 3: Factors Influencing Sensor Network Design

1Wireless Sensor Networks

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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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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

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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SensingSensing




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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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SensingSensing

Data Processing (Computation)Data Processing (Computation)



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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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SensingSensing




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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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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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69

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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)(~)(

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

Documents

Chapter 3: Factors Influencing Sensor Network Design