1

Lecture 12:Lecture 12:Factors Influencing Sensor Network Factors Influencing Sensor Network DesignDesign

(textbook slides)(textbook slides)Ian F. Akyildiz, Mehmet Can Vuran, “Wireless Sensor Ian F. Akyildiz, Mehmet Can Vuran, “Wireless Sensor

Networks” WILEY Publisher, ISBN: 978-0-470-03601-Networks” WILEY Publisher, ISBN: 978-0-470-03601-3, August 2010.3, August 2010.

2

InstructionsInstructions

Print course and section number Print course and section number 5910159101 (for ECE591) in (for ECE591) in the the first 5 positions of the STUDENT ID NUMBERfirst 5 positions of the STUDENT ID NUMBER box. box. There is NO need to fill in the corresponding ovals. There is NO need to fill in the corresponding ovals.

STUDENT NAME Box: STUDENT NAME Box: WANGWANG

Queries on the Questionnaire are matched to the Queries on the Questionnaire are matched to the numbers on the Answer Sheet.numbers on the Answer Sheet.

E: Strongly Agree; D: Agree; C: Neutral; B: Disagree; A: Strongly Disagree

3

Form InstructionForm Instruction

4

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)

5

Sensor Node HardwareSensor Node Hardware

Power UnitPower Unit AntennaAntenna

Sensor ADCSensor ADCProcessorProcessor

MemoryMemoryTransceiverTransceiver

Location Finding SystemLocation Finding System MobilizerMobilizer

SENSING UNIT PROCESSING UNIT

6

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

7

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

8

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.

9

POWER CONSUMPTIONPOWER CONSUMPTION

10

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

11

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

12

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 2 E NO B

13

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

14

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 TVnd dd d

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)

15

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%)

16

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

17

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

18

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

19

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

20

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

21

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 A transceiver spends upon waking up from sleep

mode.mode. During start-up time, no transmission or reception During start-up time, no transmission or reception

of data is possible. of 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 because a large amount of power is spent in turning the transceiver back ON each time.turning the transceiver back ON each time.

22

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.

23

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

24

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.

25

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

26

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

27

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)

28

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

29

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.

30

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

31

VERY DETAILED ENERGY MODEL

s leeps leepo no n 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.

32

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

33

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!

34

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, …

35

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

36

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