FIND: Future Internet Network Design
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Transcript FIND: Future Internet Network Design
Wireless Sensor
Networks
Nov 1, 2006
Jeon Bokgyun ([email protected])
Reference
Tutorial, Wireless sensor networks,
mobicom 2002
1. Introduction – Deborah
2. Sensor Node Platforms & Energy Issues –
Mani
3. Time & Space Problems in Sensor
Networks – Mani
4. Sensor Network Protocols – Deborah
5. Collaborative Signal Processing – Akbar
6. Discussion - All
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Contents
Introduction
Features of sensor network
Sensor hardware platform
Energy issue
Time synchronization
Node localization
Sensor coverage
Conclusion
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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.
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Sensor Network Architecture
Internet,
Satellite, etc
Sink
Sink
Task
Manager
Tens of thousand nodes
Densely deployed
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Sensor Network Applications
Seismic monitoring
Contaminant transport
Ecosystem monitoring
Transportation and urban monitoring
Infant monitoring
Personalized adv.
Etc.
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Sensor Network Features
Densely deployed and prone to failure
The topology changes very frequently
May leverage broadcasting than point-topoint communications
May operate in aggregate fashion
Sensor nodes are limited in power,
computational capacities, and memory
May not have global ID like IP address
Need tight integration with sensing tasks
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Capabilities
Sensor Node HW Platform
StarGate
MK - II
iBadge
MICA Mote
Size, Power Consumption, Cost
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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)
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Sensor Node SW Platform
TinyOS
Programming concepts for resource constrained
networked embedded devices
SOS
Enabling dynamic embedded software.
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Contents
Introduction
Features of sensor network
Sensor hardware platform
Energy issue
Time synchronization
Node localization
Sensor coverage
Conclusion
11/26
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Where does the energy go?
Mobilizer
Location Finding System
Processor
Transceiver
Sensor ADC
Memory
Power Unit
ADC : Analog to Digital Converter
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Energy Observation
Communication >> computation (at short
range)
Radio RX power May dominate (at short
range)
Energy Spent in idle RX dominates lifetime
energy consumption
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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
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Motivation for Time Synchronization
Most applications require some synchronization
accuracy
Fire and flood tracking
Animal movement
Vehicle movement
Gunshot detection
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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
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Network Time Protocol (NTP)
Primary servers (S1)
synchronize to
national time
standards
S1
Satellite, radio, modem
Secondary servers (S2,
…) synchronize to
primary servers and
other secondary
servers
Hierarchical subnet
S1
S1
S2
S1
S2
S2
S2
S2
S2
S3
S3
S4
S3
}
S1
S2
S3
S3
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S1
Primary
Secondary
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RBS (Reference Broadcast Sync.)
NIC
NIC
Sender
Sender
Receiver
Receiver 1
Critical Path
Time
Receiver 2
Critical Path
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
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RBS (cont.)
Receiver to receiver synchronization
Two 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
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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
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Techniques for Location Sensing
Measure proximity to “landmarks”
Dead reckoning: position relative to an
initialization point
Measure direction of landmarks
Measure distance to landmarks
Measure difference in distances to two
landmarks
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Solving over multiple hops
Interative Multilateration
Problems
Error accumulation
May get stuck!!!
Unknown node
(known position)
Beacon node
(known position)
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Solving over multiple hops
Collaborative Multilateration
1
1
1
3
3
5
4
2
3
5
5
4
4
2
2
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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 take?
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Worst-case Coverage
Voronoi Diagram
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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.
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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.
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