Next Century Challenges: Next Century Challenges: Scalable Coordination in Scalable Coordination in

Sensor NetworksSensor Networks

Deborah Estrin, Ramesh Govindan, John Heidemann, Satish Kumar

(Some images and slides adopted from Santhosh R Thampuran - CMU)

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OutlineOutline

• Characteristics of sensor devices.• Motivating applications.• Key requirements of a sensor network and

differences with current networks.• Localized algorithms for coordination.• Directed Diffusion – a model for describing

localized algorithms.

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Characteristics of Sensor Characteristics of Sensor DevicesDevices

• Ability to monitor a wide variety of ambient conditions: – temperature, – pressure, – mechanical stress level on attached objects…

• Will be equipped with significant processing, memory, and wireless communication capabilities.

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Applications: Environmental Applications: Environmental AnalysisAnalysis

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Applications: Contaminant Flow Applications: Contaminant Flow MonitoringMonitoring

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Applications: Traffic ControlApplications: Traffic Control

• Sensor attached to every vehicle.

• Capable of detecting their location, vehicle sizes, speeds and densities; road conditions…

• Alternate routes, estimate trip times…

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Applications: Biological SystemsApplications: Biological Systems

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Key RequirementsKey Requirements

• These futuristic scenarios bring out two key requirements of sensor networks:

– support for very large numbers of unattended autonomous nodes.

– adaptivity to environment and task dynamics.

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Differences with Current Differences with Current NetworksNetworks

• Sensor Networks: ratio of communicating nodes to users is much greater.– extremely difficult to pay special attention to any

individual node.

• Sensors may be inaccessible:– embedded in physical structures.– thrown into inhospitable terrain.

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Differences with Current Differences with Current NetworksNetworks

• There are large scale unattended systems, today.

• Automated factories are deployed with very careful planning and react to very few external events.

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Differences with Current Differences with Current NetworksNetworks

• Sensor networks deployed in very ad hoc manner.

• They will suffer substantial changes as nodes fail: battery exhaustion, accidents; new nodes are added; nodes move.

• User and environmental demands also contribute to dynamics.

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Overall Design of Sensor Overall Design of Sensor NetworksNetworks

– Is it sufficient to design sensor network applications using Internet technologies coupled with ad-hoc routing mechanisms?

• Data-Centric; Application-Specific.

• Sensor network coordination applications are better realized using localized algorithms: distributed as opposed to centralized.

– scales with increase in network size, robust to network partitions and node failures.

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Localized Algorithms for Localized Algorithms for CoordinationCoordination

• Clustering: efficient coordination.

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Localized Clustering AlgorithmLocalized Clustering Algorithm

• For every sensor, level radius• Advertisement = {hierarchical level, parent ID,

remaining energy}

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DD

wait time

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Localized Clustering AlgorithmLocalized Clustering Algorithm

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promotion timer

• Start promotion timer if no parent.• Promotion timer: inv prop (remaining energy, number

of other sensors from whom level 0 adv was received)

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Localized Clustering AlgorithmLocalized Clustering Algorithm

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• Periodic advertisements at the level 1 radius.• Advertisement = {B,C,E}

level 1 sensor

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Localized Clustering AlgorithmLocalized Clustering Algorithm

• Two key design constraints:– asymmetric communication in the network.– limited energy of sensors.

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Application of Clustering Application of Clustering AlgorithmAlgorithm

• Aim: To pinpoint in an energy-efficient manner, the exact location of objects.

• Accuracy: widest possible measurement baseline.

• Energy efficiency: fewest number of sensors participating in the triangulationtriangulation.

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TriangulationTriangulation

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•Determine position in space.

•Can specify approx direction of object relative to its own location.

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Base-line EstimationBase-line Estimation

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Advantages of Cluster-based Advantages of Cluster-based ApproachApproach

• Sensor algorithms only use local information.– generally lower energy consumption in comparison

to global communication.

• Robust to link or node failures and network partitions– mechanisms for self-configuration can be simpler.

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Advantages of Cluster-based Advantages of Cluster-based ApproachApproach

• Local communication and per-hop data filtering– avoid transmitting large amounts of data over long

distances.– preserving node energy resources.

• Node energy resources are better utilized– cluster-heads adapt to changing energy levels.

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Disadvantage of Cluster-based Disadvantage of Cluster-based ApproachApproach

• Non-optimal under certain terrain conditions.

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Several Sensors Electing Several Sensors Electing ThemselvesThemselves

ObstacleObstacle

Allow a cluster-head to switch on some number of child sensors in its cluster to do object location.

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Adaptive Fidelity AlgorithmsAdaptive Fidelity Algorithms

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quality of the answer can be traded against battery lifetime, network bandwidth, or number of active sensors.

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TradeoffsTradeoffs

• Localized algorithms exhibit good robustness and scaling properties.

• May sacrifice resource utilization or sensing fidelity, responsiveness, or immunity to cascading failures.

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Directed DiffusionDirected Diffusion

• A novel data-centric, data disemmination paradigm for sensor networks.

• Data generated by sensor node is named using attribute-value pairs.

• A sensing task is disseminated throughout

the sensor network as an interest for named data.

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Directed DiffusionDirected Diffusion

• This dissemination sets up gradients within the network designed to "draw" data matching the interest.

• Events start flowing towards the originators of interests along multiple paths. The sensor network reinforces one, or a small number of these paths.

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Directed DiffusionDirected Diffusion

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Directed DiffusionDirected Diffusion

• Allows intermediate nodes to cache or locally transform data.– leverages the application-specificity that is

possible in sensor networks.

• The diffusion model’s data naming and local data transformation features capture the data-centricity and application-specificity inherent in sensor networks.

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Related Work

• Ad-hoc Networks– Proactive vs. reactive routing protocols

• Energy-efficiency issues

• Distributed Robotics– Robots cooperate to discover entire map

• Internet Multicast and web caching– Lightweight session

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Current Developments

• Smartdust project: – cubic millimeter sensors– Sensors float in air like dust

• WINS (wireless integrated wireless Sensors)

• WSN (Wireless Sensing Network)

• Odyssey

• Habitat monitoring

• The Cricket Indoor Location System