Lecture 8Wireless Sensor Networks:

Overview

Reading: • “Wireless Sensor Networks,” in Ad Hoc Wireless Networks: Architectures and

Protocols, Chapter 12, sections 12.1-12.2.• I. Akyildiz, W. Su, Y. Sankarasubramaniam and E. Cayirci, “A Survey on

Sensor Networks,” IEEE Communications Magazine, August 2002.• D. Estrin, R. Govindan, J. Heidemann, and S. Kumar, "Next Century

Challenges: Scalable Coordination in Sensor Networks," Proc. Mobicom '99, August 1999.

• “Wireless Sensor Network Protocols” by M. Perillo and W. Heinzelman. To appear in: Fundamental Algorithms and Protocols for Wireless and Mobile Networks, CRC Hall, 2005.

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Wireless Sensor Networks (WSNs)Microsensors

Low power, cheap sensorsSensor module (e.g., acoustic, seismic, image)A digital processor for signal processing and network protocol functionsRadio for communicationBattery-operated

Sensors monitor environmentCameras, microphones, physiological sensors, etc.Gather data for some purpose

Hundreds or thousands of nodes scattered throughout an environmentEach sensor can collect dataData routed via other sensors to a sink or base station node

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WSNs (cont.)Sensor data limited in range and accuracy

Each node can only gather data from a limited physical area of the environmentData may be noisyData aggregation enables higher quality (less noisy) data to be obtained that gives information about a larger physical area than any individual data signal

Networking sensors enablesExtended range of sensing improved quality Fault tolerance due to redundancy in data from different sensorsDistributed processing of large amounts of sensor dataScalability: quality can be traded for system lifetime“Team-work”: nodes can help each perform a larger sensing task

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WSN ApplicationsNew wireless networking paradigm

Requires autonomous operationHighly dynamic environments

Sensor nodes added/failEvents in the environment

Distributed computation and communication protocols required

ApplicationsHome securityMachine failure diagnosisChemical/biological detectionMedical monitoringSurveillance and reconnaissanceAnimal/plant monitoring (e.g., for research)

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Example Application: Environmental Monitoring

Traffic patterns many-to-oneRaw sensor data or high level descriptions about environmental phenomenaExample projects

ZebraNetEcology of rare plants in Hawaii

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Example Application: Health Monitoring

Sensors monitoring vital signsBlood pressure, heart rate, EKG, blood O2

One or more sensors indicate abnormalityInform personTake corrective measures

Control pacemaker functionsAutomatically dispense medication

Alert emergency teamSensing, processing, understanding, feedback/controlRequires protocols that are

ReliableFlexibleScalableSecure

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Sensor Platforms

Example platformsSmart Dust (UC Berkeley)Berkeley MotesTelos Motes (MoteIV)iBadge (UCLA)WINS (UCLA)

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Sensor Platforms

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WSN LimitationsSensor energy

Each sensor node has limited energy supplyNodes may not be rechargeableEventually nodes may be self-poweredEnergy consumption in sensing, data processing, and communication

Communication the most energy-intensiveMust use energy-conserving communication

SENSORS

Power consumption of node subsystems

0

5

10

15

20

Pow

er (m

W)

CPU TX RX IDLE SLEEP

RADIO

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WSN Limitations (cont.)Communication

The bandwidth is limited and must be shared among all the nodes in the sensor networkSpatial reuse essentialEfficient local use of bandwidth needed

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WSNs vs. MANETs

Many-to-one traffic patternOne-to-one traffic pattern

Data-centricAddress-centric

Very constrained energy and bandwidthConstrained energy and bandwidth

CooperativeCompetitive

Application-specific QoSQoS: delay, etc

Typically immobileTypically mobile

Large-scaleSmall-scale

Require self-configurationRequire self-configuration

Unreliable communicationUnreliable communication

Sensor NetworksGeneral Ad Hoc Networks

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Design FactorsWhat are the important features of WSNs?Fault tolerance/reliability

Network should be robust to individual node failuresFailures due to running out of energy, hardware failures, malicious intercept of sensor, etc.

ScalabilityProtocols must scale to thousands or millions of sensor nodesRequires intelligent management of high density nodes

CostMust have cheap sensors

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Design Factors (cont.)Topology

Deployment: random or deliberate placement of nodesChanges in topology during network operation

New nodes added to the systemNodes failingEnvironmental changes

Energy consumptionSensor functions: sensing, communication, data processingAll require energy

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Evaluating WSNsWhat are the performance metrics for WSNs?System lifetime

E.g., time until network partitionE.g., time until probability of missed detection exceeds a threshold

Quality of result of sensor networkApplication-specific measureLatency of data transferSNR of aggregate data signalProbability of missed detection or false alarm

Tradeoffs can be made among network parametersE.g., can reduce quality of result of sensor network to increasesystem lifetime

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Taxonomy of WSN ArchitecturesIn what ways do sensor networks for various applications differ?Data sink(s)

Embedded within networkNetwork edgesMobile access pointOne or several

Sensor mobilityTypically stationary sensorsSome projects use mobile sensors

ZebraNetMilitary operationsSelf-propelled sensorsRobots

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Taxonomy (cont.)Sensor resources

MemoryProcessingTransmit power (fixed vs. variable)Locations/density

Traffic patternsEvent-driven applicationsContinuous data generationQuery-driven applications

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Design IssuesNew protocols neededMAC

Cooperative nature of sensor networks (fairness not an issue)Exploit traffic patternsEnergy efficiency extremely important

Reduce idle listeningReduce unnecessary reception

RoutingDifferent traffic modelsData dissemination rather than point-to-point routingData-centric rather than address-centricLocation-aware sensorsResource-aware routing neededExploit local aggregation

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Design Issues (cont.)Topology control

Reduce idle power consumption nodes sleepCreate fully-connected dominating set from active routers

Transmission power controlHow to avoid “hot spot” problem?Provide connected network

QoS ManagementQoS determined by content of data rather than amountTransport layer

Intelligent congestion managementThrottle back irrelevant data rather than each node’s sending rate

CoverageEnsure enough sensors provide dataK-coverage: each location monitored by at least K sensors

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Design Issues (cont.)Time synchronization

Very important in sensor networksNeeded to determine if event sensed by two sensors is in fact the same eventNeeded to determine object speedApproaches

GPS – expensive, not energy-efficientNTP (used in computer networks) – not enough precisionNewer approaches being researched

Romer’s AlgorithmReference-Broadcast Synchronization (RBS)

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Design Issues (cont.)Localization

Important for same reasons as time synchronizationOften times, only relative position is necessary

GPS is overkill and unattractive for energy reasonsRSSI used to infer distancesTime of Arrival (ToA)Time Difference of Arrival (TDoA)Angle of Arrival (AoA)

Sensor can find its own location using received beaconsSensor can have other nodes measure its location

Sensor sends beacon message and neighbors use trilaterationbased on signal strength measurements

Problem – small scale fading

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Research Issues (1)Appropriate QoS model

Traditional networks: delay, packet delivery ratio, jitter Sensor networks: probability of missed detection of an event, signal-to-noise ratio and network sensing coverageDifficult to translate these data-specific QoS parameters into meaningful protocol parameters

Cross-layer ArchitecturesEntire protocol stack tailored to specific needs of WSN applicationProtocols should be integrated with hardwareTrade-off: generality and ease of network design to achieve lifetime increases

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Research Issues (2)

ReliabilityLinks and sensors may fail, temporarily or permanentlyMust design protocols to provide reliable service with these failures

Heterogeneous ApplicationsSensor nodes may be shared by multiple applications with differing goalsProtocols must efficiently serve multiple applications simultaneously

Heterogeneous SensorsHow to make best use of resources in heterogeneous sensor networks

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Research Issues (3)Security

How much and what type of security is really needed? How can data be authenticated? How can misbehaving nodes be prevented from providing false data? Can energy and security be traded-off such that the level of network security can be easily adapted?

ActuationEventually sensor networks will “close the loop”Data do not need to reach base station Current models for sensor networks may not be valid

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Research Issues (4)Distributed and Collaborative Data Processing

How to best process heterogeneous data?How much data and what type of data should be processed to meet application QoS goals while minimizing energy drain?

Integration with Other NetworksSensor networks may interface with other networks, such as a WiFi network, a cellular network, or the InternetWhat is the best way to interface these networks? Should the sensor network protocols support (or at least not compete with) the protocols of the other networks? Or should the sensors have dual network interface capabilities?

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Discussion