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Tutorial, Wireless sensor networks, mobicom 2002
1. Introduction – Deborah2. Sensor Node Platforms & Energy Issues – Mani3. Time & Space Problems in Sensor Networks – M
ani4. Sensor Network Protocols – Deborah5. Collaborative Signal Processing – Akbar6. Discussion - All
Contents
IntroductionFeatures of sensor networkSensor hardware platformEnergy issueTime synchronizationNode localizationSensor coverageConclusion
What is the sensor?
Sensor: a transducer that converts a physical, chemical, or biological parameter into an electrical signal
Actuator: a transducer that accepts an electrical signal and converts it into a physical, chemical, or biological action
Transducer: a device converting energy from one domain into another. The device may either be a sensor or an actuator.
Sensor Network Architecture
Tens of thousand nodes Densely deployed
Internet, Internet, Satellite, Satellite, etcetc
Sink
Sink
TaskManager
Sensor Network Applications
Seismic monitoringContaminant transportEcosystem monitoringTransportation and urban monitoringInfant monitoringPersonalized adv.Etc.
Sensor Network Features
Densely deployed and prone to failureThe topology changes very frequently May leverage broadcasting than point-to-
point communicationsMay operate in aggregate fashionSensor nodes are limited in power,
computational capacities, and memoryMay not have global ID like IP addressNeed tight integration with sensing tasks
Sensor Node HW PlatformC
apabili
tie
s
Size, Power Consumption, Cost
MICA Mote
iBadge
MK - II
StarGate
Sensor Node HW Platform
COTS dust prototypes (Kris Pister et al.) weC Mote (~30 produced, 1998) Rene Mote (850+ produced, 1999-2000) Dot (1000 produced, 2000) Mica node ( 5000+ produced, 2001) Mica2 (2002) MicaZ, Telos (2004)
Sensor Node SW Platform
TinyOS Programming concepts for resource constrained networ
ked embedded devicesSOS
Enabling dynamic embedded software.
Contents
IntroductionFeatures of sensor networkSensor hardware platformEnergy issueTime synchronizationNode localizationSensor coverageConclusion
Where does the energy go?
ADC : Analog to Digital Converter
Power UnitPower Unit
Sensor ADCProcessorProcessor
MemoryMemoryTransceiverTransceiver
Location Finding SystemLocation Finding System MobilizerMobilizer
Energy Observation
Communication >> computation (at short range)
Radio RX power May dominate (at short range)
Energy Spent in idle RX dominates lifetime energy consumption
Radio Energy Management
Short range links Shutdown based Turn off sender and receiver Topology management schemes exploit this
Long range links Scaling based Slow down transmissions Energy-aware packet schedulers exploit this
Motivation for Time Synchronization
Most applications require some synchronization accuracy Fire and flood tracking Animal movement Vehicle movement Gunshot detection
Synchronization in Sensor Network
Network time protocol (NTP) for Internet clock synchronization
Difference: for sensor networks Time synchronization requirements more
stringent (µs instead of ms) Power limitations contain resources May not have easy access to synchronized
global clocks NTP assumes that pairs of nodes are constantly
connected and experience consistent communication delays
Often, local synchronization sufficient
Network Time Protocol (NTP)
Primary servers (S1) synchronize to national time standards
Satellite, radio, modem
Secondary servers (S2, …) synchronize to primary servers and other secondary servers
Hierarchical subnet S3 S3 S3
S4
S2 S2 S2 S2
S3 S3
S1 S1 S1 S1
S2
S1 S1
S2 S2
Primary
}Secondary
RBS (Reference Broadcast Sync.)
NICSender
Receiver
Critical Path
NICSender
Receiver 1
Receiver 2
Critical Path
Time
Traditional critical path:From the time the sender
reads its clock, to when the receiver reads its clock
RBS: Only sensitive to the differences in receive time
and propagation delay
RBS (cont.)
Receiver to receiver synchronizationTwo stage
Transmitter broadcast clock time Receivers exchange observations
Assumptions Propagation delay is zero No clock skew
RBS outperforms NTP 11usec precision over 19.2K radios
Why is Localization Important?
Very fundamental component for many other services GPS does not work everywhere Smart Systems – devices need to know where
they are Geographic routing & coverage problems People and asset tracking Need spatial reference when monitoring spatial
phenomena
Techniques for Location Sensing
Measure proximity to “landmarks”Dead reckoning: position relative to an
initialization pointMeasure direction of landmarksMeasure distance to landmarksMeasure difference in distances to two
landmarks
Solving over multiple hops
Interative Multilateration
Beacon node(known position)
Unknown node(known position)
ProblemsError accumulationMay get stuck!!!
Solving over multiple hops
Collaborative Multilateration
1
2
3
45
2
1
3
45
1
2
3
45
Sensor coverage
How well can the field be observed? As the measure of QoS of a sensor network.Example usage
Commander Weakest path : what path is the enemy likely to tak
e?
Worst-case Coverage
Voronoi Diagram
Path of Maximal Breach of Surveillance in the sensor field lies on the Voronoi diagram lines.
When adding node, the next node is deployed along the edge closest to the original nodes.
Conclusion
In the future, this wide range of application areas will make sensor networks an integral part of our lives.
Sensor network has various constraints.Briefly, introduce energy issue, timing
synchronization, node localization, and sensing coverage.
