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

CS 712 | Fall 2007 Agenda  Problem Statement  Motivation  Overview  Related work  Contents  Experiment & Conclusion 2

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

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

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

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

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

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

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

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

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

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

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

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

CS 712 | Fall 2007 Experiment Results & Conclusion | Lifetime Improvement with One Mobile Relay  Simulation result 15

CS 712 | Fall 2007 Q&A  Thank you ^^  More information?  16