Topology ManagementTopology Management----Power Efficient Spatial QueryPower Efficient Spatial Query

Presented byPresented by Weihang jiangWeihang jiang

Today’s planToday’s plan Introduction: 10-15 minsIntroduction: 10-15 mins Details : Details :

• heuristic algorithm 15 minsheuristic algorithm 15 mins• GreedyGreedy

Centralized 10minsCentralized 10mins Decentralized 5minsDecentralized 5mins

Questions Questions

Problem definitionProblem definition Select a Select a smallsmall number of sensors number of sensors

that are sufficient to answer the that are sufficient to answer the query accurately. Also these selected query accurately. Also these selected sensors should form a sensors should form a connectedconnected communication path, so that they communication path, so that they form a logical routing topologyform a logical routing topology

Context of researchContext of researchSensor Database [4]

Query operation

Power efficient organization[25]

Decentralized, cost of communication

Connected sensor CoverBroadcastMDCS [14][9,18,26,1,17]

Nodes cover

VS area cover

Art Gallery Problem [20,10]

Optimal placement VSOptimal selections

Geometric set cover problems[16 ,17 ,5]

Notion of connectivity

Sensor databaseSensor database P. Bonnet, J. Gehrke, and P. Seshadri. Towards sensor database systems. In Proc. of Intl. Conf. on Mobile Data Management, 2001. Example

• Factory Warehouse Sensor Database

• stored data: the set of sensors and environment • sensor data: produced by signal processing functions.

Query • Monitoring queries are long running.• The desired result of a query is typically a series of notifications of system activity• Queries need to correlate data produced simultaneously by different sensors.• Queries need to aggregate sensor data over time windows.• Most queries contain some condition restricting the set of sensors that are involved (usually geographical conditions).

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Art Gallery Problem DEMO:DEMO: http://valis.cs.uiuc.edu/~sariel/http://valis.cs.uiuc.edu/~sariel/

research/CG/applets/art_gallery/research/CG/applets/art_gallery/artgal.htmlartgal.html

http://www.cs.mcgill.ca/~thierry/http://www.cs.mcgill.ca/~thierry/507applet/triangulize.html507applet/triangulize.html

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Art Gallery Problem

Broadcast -- MDCS

The idea is to suppress redundant The idea is to suppress redundant broadcast by using only a small broadcast by using only a small number of nodes to broadcast, but number of nodes to broadcast, but ensuring that ensuring that all the nodesall the nodes in the in the network receive the broadcast network receive the broadcast messagemessage

Coverage: all the areaCoverage: all the areaBack

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Power efficient organizationPower Efficient Organization of Wireless Sensor NetworksSasa Slijepcevic, Miodrag Potkonjak Choose nodes rather than deploy noChoose nodes rather than deploy nodesdes Divide sensors into mutually exclusiDivide sensors into mutually exclusive sets, each of those sets completelve sets, each of those sets completely covers query areay covers query area

•Power savingPower saving Divide to as many groups as possibleDivide to as many groups as possible

AlgorithmAlgorithm Definition of fieldDefinition of field

• A set of points. Two points belong to A set of points. Two points belong to the same field iff they are covered by the same field iff they are covered by the same set of sensors the same set of sensors Critical ElementCritical Element

• A field covered by the minimal number of sensors A field covered by the minimal number of sensors • 2,3,6,8 are critical elements2,3,6,8 are critical elements

Find as many as possible exclusive covering setsFind as many as possible exclusive covering sets• 1) Start with a critical element1) Start with a critical element• 2) Then use objective function to choose one sensor which covers this cri2) Then use objective function to choose one sensor which covers this critical elementtical element• 3) If now all the chosen sensor cover the query area3) If now all the chosen sensor cover the query area

• we got one exclusive covering set Goto 1)we got one exclusive covering set Goto 1)Else Else • Goto 1)Goto 1)

objective functionobjective function

(1) favor sets that cover a high number of uncovered elements (less sensors) (2) favor sets that cover more sparsely covered elements (3) favor sets that do not cover the area redundantly (more exclusive sets) (4) favor sets that redundantly cover the elements that do not belong to sparsely covered areas

The heuristicThe heuristic

Power efficient organization(cont)

DrawbacksDrawbacks•CentralizedCentralized•Communication costCommunication cost

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Connected sensor Cover

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Compared with breath first flooding D+2qm VS 2qn (n>>m)

Important definitionsImportant definitions Subelement; Valid SubelementSubelement; Valid Subelement

• = Field ; a field in query area= Field ; a field in query area Candidate Sensor; Candidate pathCandidate Sensor; Candidate path

• A sensor contains a Subelement which has not beeA sensor contains a Subelement which has not been chosenn chosen• A path connects a candidate sensor with previously A path connects a candidate sensor with previously chosen sensorschosen sensors

Uncovered Valid Subelement; Benefit of a CanUncovered Valid Subelement; Benefit of a Candidate pathdidate path• Benefit = # of uncovered Valid Subelement / # of seBenefit = # of uncovered Valid Subelement / # of sensor on the path but not chosennsor on the path but not chosen

Greedy algorithm (centralized)Greedy algorithm (centralized) Start with chosen sensor set M (the origiStart with chosen sensor set M (the original sensor)nal sensor) Find out SC (set of candidate sensors)Find out SC (set of candidate sensors) Basing on Benefit of a Candidate path, cBasing on Benefit of a Candidate path, choose one candidate sensor, add it and thoose one candidate sensor, add it and the path into Mhe path into M Goto beginningGoto beginning

DecentralizedDecentralized Instead send Candidate Path Search to tInstead send Candidate Path Search to the SC (set of candidate sensors, which is he SC (set of candidate sensors, which is hard to find out locally), send CPS to the hard to find out locally), send CPS to the Candidate sensors around newest addeCandidate sensors around newest added sensord sensor Seems no much impact on # of selected Seems no much impact on # of selected sesorssesors

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Question???Question???

MotivationMotivation Sensor DatabaseSensor Database

Limited Battery PowerLimited Battery Power

OverviewOverview MotivationMotivation