IEEE MASS 2007, Pisa, Italy9 Oct An Adaptive Delay-Minimized Route Design for Wireless Sensor-Actuator Networks Edith C.-H. Ngai 1, Jiangchuan Liu 2, and Michael R. Lyu 1 1 Department of Computer Science and Engineering, Chinese University of Hong Kong, Hong Kong 2 School of Computing Science, Simon Fraser University, BC, Canada

2 Outline Introduction Related Work Route Design Problem (RDP) Probabilistic Route Design (PROUD) Algorithm Distributed Implementation Enhancements Simulations Conclusion

3 WSN Distributed and large-scale network like the Internet A group of static sensors resource constrained wireless communications Introduction

4 WSAN Collection of sensors and actuators Sensors numerous resource-limited and static devices monitor the physical world Actuators resource-rich devices equipped with more energy, stronger computation power, longer transmission range, and usually mobile make decisions and actuate adaptively in response to the sensor measurements Introduction

5 Mobile Elements Architecture using moving entities (Data Mules) to collect sensor data [Shah et. al. SNPA’03] Mobile sinks with predictable and controllable moving pattern [Chakrabarti et al. IPSN’03, Kansal et al. Mobisys’04] Mobile sinks can find the optimal time schedule to stay at appropriate sojourn points [Wang et al. HICC’05] Message ferry (MF) approach to address the network partition problem in sparse ad hoc network [Zhao et al. Mobihoc’04] Related Work

6 Mobile Elements (cont.) Joint mobility and routing algorithm with mobile relays to prolong the network lifetime [Luo et al. Infocom’05] Partitioning-based algorithm to schedule the movement of mobile element (ME) to avoid buffer overflow and reduce minimum required ME speed [Gu et al. Secon’05] Using of mobile sensors and controlled mobility to provide quality of coverage on the fraction of events captured [Bisnik et al. Mobicom’06] Vehicle routing problem (VRP) Considers scheduling vehicles stationed at a central facility to support customers with known demands Minimize the total distance traveled Related Work

7 Network Model We consider a WSAN with M mobile actuators and N static sensors Each of the sensors and actuators are equipped with a wireless transceiver Actuators move in the sensing field along independent routes, at constant or variable speeds Each static sensor has a limited buffer to accommodate locally sensed data When an actuator approaches, the sensor upload the data to actuator and free its buffer Sensors have different weights, according to their data generation rates or event frequencies Sensors with higher weights expect shorter average actuator inter- arrival times Route Design Problem (RDP)

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9 Adaptive Delay-Minimized Route Design To minimize the data collection time in a stochastic and dynamically changing sensing environment We propose a probabilistic route design algorithm (PROUD) for wireless sensor-actuator networks This is a departure from the previous static and deterministic mobile element scheduling problems PROUD offers delay-minimized routes for actuators and adapts well to network dynamics and sensors with non- uniform weights This is achieved through a probabilistic visiting scheme along pre-calculated routes Adaptive Delay-Minimized Route Design (PROUD)

10 Small-scale Network STEP 1: Form a Priori Route A priori route is formed by constructing a TSP path which contains all locations to be visited Many polynomial-time approximation algorithm has been proposed for the NP-hard TSP problem We adopt the well-known Approx-TSP-Tour algorithm here for its low cost and bounded performance Adaptive Delay-Minimized Route Design (PROUD)

11 Small-scale Network STEP 2: Visit sensors probabilistically Actuators visit the sensors on the priori route in sequence probabilistically We set the visiting probability p i of a location i to be w i, where w i is the (normalized) weight of the sensors Sensors with higher weights should be assigned with a higher probability Adaptive Delay-Minimized Route Design (PROUD)

12 Small-scale Network STEP 3: Allocate the actuators The expected route length with probabilistic visiting can be calculated as For a sensor i with a visiting probability p i, its average actuator inter-arrival time A i is thus Adaptive Delay-Minimized Route Design (PROUD)

13 Time complexity analysis: Bound analysis: Analysis Adaptive Delay-Minimized Route Design (PROUD)

14 Large-scale Network STEP 1: Form clusters It divides an MST into two sub-trees by removing its longest edge e Adaptive Delay-Minimized Route Design (PROUD)

15 Large-scale Network STEP 2: Form priori routes and visit sensors probabilistically STEP 3: Allocate actuators Routes with longer expected lengths should be allocated with more actuators Adaptive Delay-Minimized Route Design (PROUD)

16 STEP 1: Forming R-clusters Sensors construct MSTs locally by communicating with neighbors Weight of each edge e in MST is smaller than communication range R s We refer such an MST as an R-Cluster, RC(V,E), which includes all the sensors that are within R s to some sensors in RC(V,E) The cost of the R-cluster is denoted by Cost(RC), which is the sum of w(e) in RC(V,E) Distributed Implementation

17 STEP 2: Connecting R-clusters We divide the network into M subareas, each of which is explored by one actuator Each actuator looks for the R-clusters in its area and connects them if they are within a certain distance, say |RC1,RC2| <= ξ The new cluster is formed with cost Cost(RC1) + Cost(RC2) + |RC1,RC2| Similarly, the actuators also connect their R-clusters/clusters with those in their neighboring areas: Distributed Implementation

18 STEP 3: Allocating Actuators Actuators are then allocated to the clusters, such that each cluster is assigned to at least one actuator and no clusters are unassigned Distributed Implementation

19 Enhancements Actuators with Variables Speeds By adjusting the speeds of the actuators, we can ensure sensors with the same visiting probability can achieve similar inter-arrival times Assume that node i on Rj has a probability pi of being visited by actuator j every cycle. Its average actuator inter-arrival time Ai can be calculated as Enhancements

20 Load Balancing Multi-Route Improvement Algorithm Actuators on routes with similar expected length have more balanced energy consumption Consider two routes R 1 and R 2 involved in multi-route improvement Their new expected route lengths become ideal if E[R’ 1 ] = E[R’ 2 ] = (E[R 1 ] + E[R 2 ])/2 We provide an approximation method to transfer a proportion of sensor locations ζ from MST 1 to MST 2 Enhancements

21 Load Balancing Task Exchange Algorithm In certain scenarios, it is more energy efficient for one actuator to take up more load than another in the overall energy consumption point of view It may happen in a network that involves clusters with different sizes or weights Load balancing among the actuators can be achieved by exchanging their routes Enhancements

22 Simulations Simulation parameters Algorithms: Partitioning based scheduling algorithm (PBS) [Secon’05] Bounded event loss probability (BELP-2D) algorithm in 2D case [MobiCom’06] PROUD (Our proposed algorithm)

23 Actuator Inter-arrival Time Uniform random Cluster-based uniform N = 100, M =5 Simulation Results

24 Actuator Inter-arrival Time Cluster-based non-uniform Eye Topology Simulation Results

25 Minimum Actuator Speed Uniform random Cluster-based uniform N = 100, M =5 Simulation Results Cluster-based non-uniform

26 Load Balancing Multi-route improvement Task exchange Simulation Results

27 Conclusion We focused on WSN with multiple actuators and their route design We studied the route design problem and proposed delay- minimized and cooperative solution to coordinate actuators and collect data efficiently We proposed an adaptive route design (PROUD) algorithm It differentiates the visiting frequency to sensor locations with different weights and balances the workload of actuators The proposed algorithm adapts well to the dynamic change of the network and balances the energy consumption of actuators effectively Simulation results suggested that the algorithm remarkably reduces the average inter-arrival time Conclusion

28 Q & A Thank You!