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CS 712 | Fall 2007 Using Mobile Relays to Prolong the Lifetime of Wireless Sensor Networks Wei Wang, Vikram Srinivasan, Kee-Chaing Chua. National University of Singapore. MobiCom’05 27th September ‘07 JunSup Lee Dependable Software LAB. KAIST 1
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CS 712 | Fall 2007 Agenda Problem Statement Motivation Overview Related work Contents Experiment & Conclusion 2
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CS 712 | Fall 2007 Problem statement WSN in inaccessible and hostile environments Nodes are randomly scattered Nodes are expected to self-organize to form a multi-hop network Sink : gateway to a backbone network Relay : relaying information from other sensors to the sink Existing approaches to improving WSN lifetime Device research Design low power hardware and efficient energy sources Network research Design efficient algorithms at the various networking layers 3 Limitation : They assumes entire network is composed of static nodes
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CS 712 | Fall 2007 Motivation A large sensor field composed of Static nodes : simple, cheap, large numbers Mobile nodes : rich, expensive, a few Question Should a mobile node act as a sink or a relay? What is the lifetime of the network for each of these alternatives? What are the trade-offs associated with each of these alternatives? 4 Only static nodes bottleneck
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CS 712 | Fall 2007 Overview Investigate the benefits of a heterogeneous architecture for WSN composed of Rich mobile nodes (Small #) Mobile relays Mobile sinks Static nodes (Lange #) Mobile nodes act as Relays or Sinks ? Investigate the performance(lifetime) in different routing algorithm Case 1 : when the network is all static Case 2 : when there is one mobile sink Case 3 : when there is one mobile relay 5 Because : distributed the bottleneck nodes all around the network Using mobile node as a sink results in the maximum improvement
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CS 712 | Fall 2007 Overview However, in hostile terrains, it is not always possible for the sink to be mobile Mobile nodes as Relays (and gives a joint mobility and routing algorithm) To ensure performance, mobile nodes need to stay only within a two hop radius of the sink It requires all the nodes to be aware of the location of the mobile node. Proposed Algorithm Mobile nodes as Relays (and gives a joint mobility and routing algorithm) Achieves same performance as the mobile sink Requires only a limited number of nodes to be aware of location of the mobile Only nodes within a certain distance of the sink need to be aware of the location of the mobile relay Experiment Compare the performance of the mobile relay and mobile sink For a densely deployed sensor field of radius R hops Proposed algorithm requires O(R ) mobile relays to achieve the same performance as the mobile sink 6
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CS 712 | Fall 2007 Related Work Mobile nodes as mobile sinks to save energy Randomly moving Predictable and controllable moving Disadvantage Delays : Static nodes need to wait for the sink to approach it Sink monitoring : Static nodes have to be aware of the location of the mobile sink to route information Maintain connection : It will be difficult to engineer a system whereby a mobile sink is connected at all times to the backbone network Approaches in this paper Static sink Mobile nodes act as relays In a large and dense network Mobile nodes only need to patrol in a two hop radius around the sink 7
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CS 712 | Fall 2007 Comparison of Energy Conserving Methods Mobile relay Static routing The mobile relay decides a schedule, which id given by a sequence li : location of node i ui : duration of time the mobile relay spends at that location Dynamic routing Sensors need to know where the mobile relays are and redirect their packets according to the position of the mobile relays The mobile can attract more traffic to pass through it and the traffic through bottle neck nodes can be decreased. 8
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CS 712 | Fall 2007 Mobile Relays In Dense Network | Assumption and Network Model Assumptions Sensors are densely deployed Poisson point process Density : λ Circular area : Radius R >> 1 Dense? The packet can travel as far as the transmission range in any direction Data logging application Sensors send their sensing data at certain rate Date generation rate, Transmission range is same Energy consumptions Energy of the sink is unlimited Sensor nodes have the same initial energy : E Total energy consumed by sensors in sending out one packet : e Average number of packets flowing out from the sensor k per time unit : f k 9 Sink n0 N nodes R >> 1
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CS 712 | Fall 2007 Mobile Relays In Dense Network | Upper Bounds on Lifetime Static sensor set According to distance to the sink Set of nodes which can reach the sink with minimal hop count i : P i P k = Set of nodes which can reach the sink within j hops Q j = Upper bound Lifetime of static network : Lifetime of dense network with one mobile node (upper bound) The least upper bound on lifetime is when i = 2 Mobile Relay needs to stay only within a two hop radius of the sink in order to maximize the lifetime 10
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CS 712 | Fall 2007 Mobile Relays In Dense Network | Joint Mobility and Routing Algorithm Aggregation Routing Algorithm (ARA) Joint mobility and routing algorithm In a dense static network with one mobile relay Life time is close to the upper bound The mobile relay needs to only stay within a two hop radius Aggregation node in P3 : #a3 > #a1 + #a2; ; Process P1 : Directly send to the sink P2 : Send to none aggregation nodes in P3 (P3/G) P3 : Send to P3/G, P3/G relay data to aggregation nodes Else : Relay the packets to line OM, OM nodes relay until the packets reach the aggregation point 11
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CS 712 | Fall 2007 Mobile Relays In Dense Network | Routing With Few Nodes Aware of Mobile Location Aggregation Routing Algorithm with Limited Nodes (ARALN) ARA problem All nodes need to know mobile node location ARALN Only limited nodes have to aware the location of mobile node : inside nodes of Qs Aggregation Ring : After packet reach Ps Process : Switch(k:the index of Pk where n is element of Pk) Case S+1, ……., R : Shortest path to the sink Case S : Send to current Aggregation ring (k-1+r) or next ring (K-2+r) Case 4, ….. S-1 : Send to OM line until it travel in Pk or reach to OM and send to next ring (K-2+r) Case 1,2,3 Call method ARA; Lifetime 12
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CS 712 | Fall 2007 Mobile Sink VS Mobile Relays | Performance Compare The lifetime improvement of two mobile approach Mobile Sink times better than the all static network Mobile Relays With R mobile nodes, we can achieve improvement as the mobile sink solution Network with m mobile relays M mobile nodes in the network will stay within and get nearly 4m times lifetime when R is large 13
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CS 712 | Fall 2007 Experiment Results & Conclusion | Network Lifetime Static network compare with one mobile node 200 nodes and moderate node density Simulation is based on simplified energy model Static network uses linear optimization algorithm (because of this R<= 4) 14
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CS 712 | Fall 2007 Experiment Results & Conclusion | Lifetime Improvement with One Mobile Relay Simulation result 15
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CS 712 | Fall 2007 Q&A Thank you ^^ More information? jslee@dependable.kaist.ac.kr 16
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