overview
TRANSCRIPT
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Oveview of Wireless Sensor Networks
KD Kang
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Overview
• What is a sensor network?– Sensing– Microsensors– Constraints, Problems, and Design Goals
• Overview of Research Issues and Challenges
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Applications
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Applications
• Interface between Physical and Digital Worlds – Many applications
• Military– Target tracking/Reconnaissance– Weather prediction for operational planning– Battlefield monitoring
• Industry: industrial monitoring, fault-detection…• Civilian: traffic, medical…• Scientific: eco-monitoring, seismic sensors, plume
tracking…
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Microsensors for in-situ sensing
• Small• Limited resources
– Battery powered– Embedded processor, e.g., 8bit processor– Memory: KB—MB range– Radio: Kbps – Mbps, tens of meters– Storage (none to a few Mbits)
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Redwood trees: An application
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Mica2 Mote
128KB Instruction EEPROM
4KB Data RAM
Atmega 128Lmicroprocessor7.3827MHz
ChipcornCC1000Radio TranscieverMax 38Kbps- Lossy transmission
FlashMemory
128KB – 512KB
UART
51 pin expansionconnector
UART, ADC
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Objective• Large-scale, fine-grained,
heterogeneous sensing– 100s to 1000s of nodes providing high
resolution– Spaced a few feet to 10s of meters apart– In-situ sensing– Hetegerogeneous sensors
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Properties• Wireless
– Easy to deploy: ad hoc deployment– Most power-consuming: transmiting 1 bit ≈ executing 1000
instructions• Distributed, multi-hop
– Closer to phenomena– Improved opportunity for LOS– radio signal is proportional to 1/r4
– Centralized apporach do not scale– Spatial multiplexing
• Collaborative– Each sensor has a limited view in terms of location and sensor
type– Sensors are battery powered– In-network processing to reduce power consumption and data
redundancy
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Basic Terminology and Concepts
• Phenomenon: the physical entity being monitored
• Sink or base station: a collection point to which the sensor data is disseminated– Relatively resource rich node
• Sensor network periodically samples phenomena in space and time
• Sink floods a query
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Typical Scenario
DeployWake/Diagnosis
Self-Organize Disseminate
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Other variations
• Sensors mobile or not?• Phenomena discrete or continuous?• Monitoring in real-time or for replay
analysis?• Ad hoc queries vs. long-running
queries
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Protocol Stack
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Alternative, more data-centric model
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Protocol Stack: Physical Layer
• Frequency selection• Carrier frequency generation• Signal detection• Modulation
• Not the focus of this class– We will focus on the link layer and above
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Protocol Stack: Physical Layer
• Issues– Hardware cost
• How do we get down to $1/node?
– Radio• Ultrawideband?
– Very low powered, short pulse radio spread over several GHz
– 40Mbps ~ 600Mbps
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Protocol Stack: Physical Layer
• Radio (Cont.)– Zigbee/IEEE 802.15.4
• 2.4GHz radio band (= 802.11.b & Bluetooth)• 250Kbps• Up to 30 meters
– Pico radio• 100Kbps• Limit power consumption to 100 uW
– Other? (infra red, passive elements …)
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Protocol Stack: Data Link Layer
• Multiplexing of data streams• Data frame detection• Medium access • Error control
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Data Link Layer
• Goals:– Creation of the network infrastructure– Fair and efficient sharing of of communication resources
between sensor nodes
• Existing solutions?– Cellular: single hop network is impractical for sensor
networks– Ad hoc MACs, e.g., 802.11 or Bluetooth: Power
consuming– Scale– Data centric operation– Security
• WEP for 802.11 is broken
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Data Link Layer: Medium Access Control
• Basic strategy: turn off radio transceiver as much as possible, while receiving and transmitting data
• Techniques: TDMA, application-layer transmission scheduling, SMAC, ZMAC, BMAC, ...
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Protocol Stack: Network Layer
• Design principles – Power efficiency– Data-centric– Data aggregation when desired and
possible– Attribute-based addressing and location
awareness: no IP address
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Minimum Energy Routing
• Maximum power abailable route
• Minimum energy route
• Minimum hop (MH) route
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Directed Diffusion
• Route based on attributes and interests
• To be covered later in the semester
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Network Layer
• Data-centric routing– Directed Diffusion– Data Aggregation
• Flooding• Gossiping/non-uniform dissemination• Geographic routing
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Transport Layer
• End-to-end Reliability– Multi-hop retransmission: worth it?– Congestion: relatively little related work
• End-to-end security– Like SSL: authentication, encryption,
data integrity– Good? What about data aggregation?
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Protocol Stack: Application Layer
• Actual WSN applciations• Sensor database
– TinyDB– Cougar– SINA
• Virtual machines/middleware
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Other Important Issues
• Operating system – TinyOS– MANTIS OS
• Localization, Time Synchronization, and Calibration
• Aggregation/Data Fusion• Security
– Encryption– Authentication– Data integrity– Availability: DOS attacks
• Privacy
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Time and Space Problems
• Timing synchronization • Node Localization• Sensor Coverage
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Time Synchronization• Time sync is critical at many layers in sensor
nets– Aggregation, localization, power control
Ref: based on slides by J. Elson
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Sources of time synchronization error
• Send time– Kernel processing– Context switches– Transfer from host to NIC
• Access time– Specific to MAC protocol
• E.g. in 802.11, sender must wait for CTS (Clear To Send)
• Propagation time– Dominant factor in WANs
• Router-induced delays– Very small in LANs
• Receive time
• Common denominator: non-determinism
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Conventional Approaches• GPS at every node (around 10ns accuracy)
– doesn’t work indoo– cost, size, and energy issues
• NTP– Primary time servers are synchronized via atomic clock– Pre-defined server hierarchy– Nodes synchronize with one of a pre-specified time
servers– Can support coarse-grain time synchronization
• Inefficient when fine-grain sync is required– Sensor net applications, e.g., localization, beamforming,
TDMA – Discovery of time servers– Potentially long and varying paths to time-servers – Delay and jitter due to MAC and store-and-forward
relaying
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Localization
• Why each node should find its location? – Data meaningless without context– Geographical forwarding/addressing
• Why not just GPS at every node?– Large size and expensive– High power consumption– Works only outdoors with LOS to satellites– Overkill: Often only relative position is
needed
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What is Location?• Absolute position on geoid• Location relative to fixed beacons• Location relative to a starting point
– e.g. inertial platforms
• Most applications:– location relative to other people or objects,
whether moving or stationary, or the location within a building or an area
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Techniques for Localization
• Measure proximity to beacons– Near a basestation in a room
• Active Badge for indoor localization– Infrared basestations in every room– Localizes to a room as room walls act as barriers
• Most commercial RF ID Tag systems– Strategically located tag readers
– Beacon grid for outdoor localization• Estrin’s system for outdoor sensor networks
– Grid of outdoor beaconing nodes with know position– Position = centroid of nodes that can be heard
– Problem• Not location sensing but proximity sensing• Accuracy of location is a function of the density of
beacons
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Localization• Measure direction of landmarks
– Simple geometric relationships can be used to determine the location by finding the intersections of the lines-of-position
– e.g. Radiolocation based on angle of arrival (AoA)• can be done using directional antennas or
antenna arrays• need at least two measurements
BS
BS
BS
MS
12
3
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Localization: Range-based• Measure distance to beacons
– Measure signal-strength or time-of-flight– Estimate distance via received signal strength
• Mathematical model that describes the path loss attenuation with distance
• Use pre-measured signal strength contours around fixed beacon nodes
– Distance via Time-of-arrival (ToA)• Distance measured by the propagation delay
– Distance = time * c• Active vs. passive
– Active: receiver sends a signal that is bounced back so that the receiver know the round-trip time
– Passive: receiver and transmitter are separate» time of signal transmission needs to be known
– N+1 BSs give N+1 distance measurements to locate in N dimensions
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Radiolocation via ToA and RSSI
x1
x2
x3
d1
d3
d2
SENSOR
BS
BS
BS
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Many other issues
• What about errors? Collisions? No LOS?
• If sensors are mobile, when should we localize?
• Multi-hop localization?
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Sensor Network Coverage
• The Problem:– Given:
• Ad hoc sensor field with some number of nodes with known location• Start and end positions of an agent
– Want:• How well can the field be observed?
• Example usage– Commander
• Weakest path: what path is the enemy likely to take?
– Network manager• Weakest path: where to deploy additional nodes for optimum coverage?
– Soldier in the battlefield• Strongest path: what path to take for maximum coverage by my command?• Weakest path: how to walk through enemy sensor net or through minefield?
GATEWAYMAIN SERVER
CONTROLCENTER
Ref: based on slides by Seapahn Megerian
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Exposure Model of Sensors• Likelihood of detection by sensors is a function of
time interval and distance from sensors.
• Minimal exposure paths is worst case scenarios in a field:
Ref: based on slides by Seapahn Megerian
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Other Issues
• Coverage for continuous phenomena
• Role: Sensor as a source of information and as a router– What if we route through a critical node
and drain its energy compromising future coverage?
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Data Management Problems
phenomena
sensors
observer
• Observer interested in phenomena with certain tolerance– Accuracy, freshness, delay
• Sensors sample the phenomena• Sensor Data Management
– Determining spatio-temporal sampling schedule
• Difficult to determine locally– Data aggregation and fusion
• Interaction with routing– Network/Resource limitations
• Congestion management• Load balancing• QoS/Realtime scheduling
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Spatio-Temporal Sampling
• How to express interests and translate into actions
• How often should a given sensor report?– Collected data should meet application goals at
reasonable load to the network– Data/event driven– Locally difficult to determine appropriate
sampling/reporting rate– Collaboration needed to improve local estimate
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Data Aggregation and Fusion
• Aggregate related data from multiple sensors– Reduce data size and overall load– Provide more comprehensive estimate of
data importance to manage sensors better• Support for effective data aggregation
– Routing and MAC support– Sampling schedules should be coordinated– Tradeoff between data quality and resource
demand should be exposed to the application
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Summary: Key Design Challenges
• Energy efficiency– Sensor nodes should run for several
years without battery replacement– Energy efficient protocols are required– More efficient batteries
• But, efficient battery development is always slower than processor/memory development
– Energy harvesting
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Key Design Challenges
• Responsiveness– Periodic sleep & wake-up can reduce the
responsiveness of sensors• Important events could be missed
– In real-time applications, the latency induced by sleep schedules should be kept within bounds even when the network is congested
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Key Design Challenges
• Robustness– Inexpensive sensors deployed in a harsh
physical environment could be unreliable• Some sensor could be faulty or broken
– Global performance should not be sensitive to individual sensor failures
– Graceful performance degradation when there are faulty sensors
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Key Design Challenges
• Synergy– Moore’s law apply differently
• Sensors may not become more powerful in terms of computation and communication capability
• Cost reduction is the key to a large number of sensor deployment
– A WSN as a whole needs to be much more capable than a simple sum of the capabilities of the sensors
• Extract information rather than raw data
– Also support efficient collaborative use of computation, communication, and storage resources
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Key Design Challenges
• Scalability– 10,000 or more nodes for fine-granularity
sensing & large coverage area– Distributed, localized communication– Utilize hierarchical structure– Address fundamental problems first
• Failure handling• In-situ reprogramming, e.g., Deluge• Network throughput & capacity limits?
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Key Design Challenges
• Heterogenity– Heterogeneous sensing, computation, and
communication capabilities– e.g., a small number of devices of higher
computational capabilities & a large number of low capability nodes -> two-tier WSN architecture
– Best architecture exist for all application?– How to determine a right combination of
heterogeneous devices for a given application?
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Key Design Challenges
• Self-configuration– WSNs are unattended distributed
systems– Nodes have to configure their own
network topology;• Localize, synchronize & calibrate; and• Coordinate communications for themselves
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Key Design Challenges
• Self-optimization & adaptation– WSNs cannot be optimized a priori– Environment is unpredictble, and may
change drastically– WSN protocols should be adaptive &
adapt themseleves online
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Key Design Challenges
• Systematic design– Tradeoff btwn two alternatives
• (1) Fine-tuning to exploit application specific characteristics to improve performance
• (2) More flexible, easy-to-generalize design approaches sacrificing some performance
– Systematic design methodologies for reuse, modularity & run-time adaptation are required
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Key Design Challenges
• Security & Privacy– Security support for critical applications,
e.g., battlefield monitoring– Avoid sabotage in, e.g., structural
monitoring– Support privacy of medical sensor data– Severe resource limitations, but
challenging security & privacy issues
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Questions?