Lecture 11WSNs: Sensor Management

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

and Protocols, Chapter 12, section 12.7.• “Sensor Management” by M. Perillo and W. Heinzelman. In Wireless

Sensor Networks, Kluwer Academic Publishers, 2004.

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Sensor ManagementWhat is sensor management and why is it needed?Sensors often deployed with added redundancy

Fault toleranceTime-varying application requirementsTime-varying environmental phenomenaExtend network lifetime

Sensor management goalSelect sensor roles to provide application-specific QoS

SensorsRouters

Turn other sensors off to save energyRotate active sensor sets

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Example Quality MetricsCoverage

Determines how well network can observe eventDepends on range and location of sensorsWorst-case coverage: areas where coverage is poorest

Can be used to determine where to deploy additional sensorsMaximal breach path through field

Path intruder can take such that path is maximum distance from all sensors

Best-case coverage: areas where coverage is bestMaximum support/exposure pathBest-case coverage path

Path that is minimum distance at all points to sensors

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Example Quality MetricsCoverage (cont.)

K-coverageEntire area must be within sensing range of at least Ksensors

ExposureAbility of sensor network to observe target in fieldBased on

Sensing model for particular point in sensing rangeSensor locations

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Sensor Management (cont.)Different application types require different sensor management protocols

Differing QoS requirementsDiffering time-varying behavior

Possible to find optimal schedules for sensor rolesComputationally intensiveRequires global knowledgeNot robust to changes in network state/application state

Distributed techniquesTopology control select active routersSensor mode selection select active sensors

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Topology ControlGoal

Ensure enough nodes activated to provide connected network so all sensors can route data to sink(s)Reduce energy consumption by allowing non-selected nodes to sleep

Rotate active routers to balance energyEnsure robustness so one/few sensor losses does not disconnect networkExample protocols

GAFSpanASCENTSTEM

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Geographic Adaptive Fidelity (GAF)

Idea: neighboring nodes equivalent from routing perspectiveOverlay virtual grid on network

Each node assigned a cell in gridOnly one node per cell assigned to be activeGrid size chosen so that any node in network can reach node in neighboring grid:

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tx range

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GAF (cont.)Different states

DiscoveryActive stateSleep stateNodes periodically enter discovery state to determine if they should become active

GAF extends network lifetime proportionally to node density

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SpanGoal: create connected backbone of router nodesNodes assign themselves “coordinators”.

Selected set of coordinators chosen so capacity of backbone network approaches potential capacity of complete network

Nodes rotate coordinator positionBalance energyEnsure network remains connected and high capacity

Becoming a coordinatorMinimum distance between two of node’s neighbors exceeds three hopsBackoff delays before coordinator announcement

Node with higher energy and more connectivity more likely to become coordinators

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Adaptive Self-Configuring sEnsorNetworks Topologies (ASCENT)

Goal: select active routers to retain connected network while other nodes sleepBecoming active based on

ConnectivityObserved data loss ratesProvides ability to trade energy consumption for communication reliability

StatesTest state: route, probe channel, learn loss ratesActive: remain active permanentlyPassive: gathers same information as in Test state but does not route dataSleep state: turn off radioMust periodically re-enter passive state from sleep state

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ASCENT (cont.)

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Sparse Topology and Energy Management (STEM)

Previous approaches proactively create connected backboneSensor networks may not continuously need active routers

Only require routers when sensors send dataMay be infrequent for some sensor network applications

STEM goal: reactively turn on routers only when data to sendPaging channel used to awaken downstream neighborsSTEM-T: use a tone as wake-up messageSTEM-B: use a beacon as wake-up message

STEM can be combined with proactive topology control protocolsOnly have “active” routers listen to paging channel

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Sensor Mode SelectionGoal

Select sensing modes to ensure data provides application-specified QoSReduce energy consumption by allowing non-selected nodes to sleep

Mode selectionDetermine which sensors to activate/deactivateDetermine sensing features

Sensing frequencyData resolution

Influence what traffic generated on networkGreatly reduces energy dissipationMay be necessary to avoid network congestion

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Sensing Mode Selection (cont.)

Sensing mode selection application-specificDifferent application have different QoS requirementsExamples

Coverage-preserving applications: require K-coverage of some areaTracking applications: require minimum tracking accuracyDetection applications: require maximum missed detection probability and/or false alarm probability

Coverage-preserving applicationsIntruder detectionBiological/chemical agent detectionEnvironmental monitoring

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Probing Environment and Adaptive Sleeping (PEAS)

Goal: provide consistent environmental coverage and robustness to node failuresNodes send “probe” messages to neighbors

Neighbors reply after backoffIf no replies node becomes activeIf replies node sleeps

Probing range chosen to meet transmission and sensing coverageProbing rate adaptive

Tradeoff between energy savings and robustnessLong delay in recovering from node failures if probing rate long

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Node Self-Scheduling Scheme (NSSS)

Goal: select active sensors to cover full sensing areaNode measures sectors/central angles covered by neighboring sensors

If coverage is full 360°, node sleepsSome redundancy not accounted for by this model

Backoff and double checks used to ensure simultaneous deactivation of nodes does not leave areas uncovered

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Gur Game ModelGoal: nodes set sending state so sink receives predetermined number of packetsNodes operate as single chain finite state machinesAfter each round, sink sends information r that tells nodes how to move in their FSMs

Network settles at desired resolutionRobust to sensor failures or new sensors

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Reference Time-based Scheduling Scheme

Goal: maintain coverage at every grid point while minimizing number of sensorsNodes broadcast random reference time in [0,T)

T is round lengthBroadcast to all sensors in 2x sensing range

For each grid location of a sensorSensor sorts reference times of all sensors that can monitor pointSensor schedules itself to be active halfway between its reference time and the reference time of sensor immediately preceding it in listSame for sensor immediately after it in list

Sensor remains active for union of scheduled slots calculated for each grid point in sensing range

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Reference Time-based Scheduling Scheme

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Coverage Configuration Protocol (CCP)

Goal: maintain K-coverage of areaNodes find intersection points between borders of neighbors’ sensing radii and edges of area

Node eligible for deactivation if these intersection points all K-covered

Ex: S4 deciding whether to activate, K=1Knows that S1-S3 activeIntersection points 1-5S1 covers points 1 and 3, S2 covers points 2 and 4, S3 covers point 5

S4 can deactivate

In second case, point 6 not covered so S4 must remain active

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Integration of TC and SMWhy might it be beneficial to integrate topology control and sensor management?Selecting sensors selecting routers

Higher performance via integration of role selectionMay have more sensors activated as routers than needed if do not take into account traffic patterns

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Connected Sensor CoverJoint sensing mode selection and topology controlGoal: find minimum set of sensors and routers to efficiently process query over given region

Sensors added in greedy fashionSensors calculate added coverage and required routers if they would be added to setSet with most coverage and least additional routers needed added to setContinues until entire region covered

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Application-based Routing Cost

Sensors whose data are “important” to the application should not be used as routers

Sensors must determine “application value”E.g., “Redundant” sensorsless important

How to determine a good application cost?

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Application CostEach subregion characterized by unique sensor set Application cost:

apptotal

1Cost (S ) max ( , ) C(S )E ( , )i ix y

x y= ∈

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Example Routes

0 5 10 15 20 25 30 35 40 45 500

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50Fewest Hops PathSmallest App. Cost Path

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Distributed Activation with Pre-determined Routes (DAPR)

Integrates coverage preservation with route discoveryPre-calculate shortest cost routes Activate sensors incrementally until environment is fully covered

RoundN-1

RouteDisc

Role Discovery

Opt In Opt Out

Normal Operation

RoundN+1

Round N

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DAPR ProtocolBase station sends Round Start messageSensors forward messages, adding routing costs as

Propagate these messages incorporating delays proportional to cost

rjapptiappjilink ESCESCSSC *)(*)(),( +=

RoundN-1

RouteDisc

Role Discovery

Opt In Opt Out

Normal Operation

RoundN+1

Round N

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DAPR ProtocolDuring Opt In phase, nodes set backoffs proportional to cumulative path costDuring Opt Out phase, nodes backoff in reverse order and deactivate if possibleBeacons sent over distance of 2 x sensing range

RoundN-1

RouteDisc

Role Discovery

Opt In Opt Out

Normal Operation

RoundN+1

Round N

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DAPR Protocol

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Simulation Scenario

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Life

time

(hou

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Constant CostEnergy CostApplication Cost200 nodes

Most placed in densely covered regionsFew placed in sparsely covered region

Application cost: 56% improvement over energy cost

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Choosing ProtocolsBased on

Application requirementsMAC protocol – why?Bandwidth resourcesAvailability of network services

LocalizationSynchronization

Radio characteristicsTrade-offs

Energy vs. robustnessLocalization vs. guaranteed coverageDelay vs. energy

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Discussion