SenSys 2003 Differentiated Surveillance for Sensor Networks Ting Yan Tian He John A. Stankovic Department of Computer Science, University of Virginia November 5, 2003

ACM SenSys Differentiated Surveillance Outline Problem Statement Basic Sensing Coverage Protocol Enhanced Protocol with Differentiated Surveillance Evaluation Conclusions

November 5, 2003 ACM SenSys Differentiated Surveillance The Problem Leverage redundancy of deployment to save power and still maintain a specified degree of sensing coverage.

November 5, 2003 ACM SenSys Differentiated Surveillance An Example Even harder to decide schedules when nodes are deployed with a random distribution and distributed decisions.

November 5, 2003 ACM SenSys Differentiated Surveillance Differentiated Surveillance > 100% = 100% < 100% Most Important Important Less Important

November 5, 2003 ACM SenSys Differentiated Surveillance Contributions Developed one of the first protocols to address the differentiated surveillance problem Achieved as much as 50% reduction in energy consumption and as much as 130% increase in the system half-life compared to other state-of-the-art schemes

November 5, 2003 ACM SenSys Differentiated Surveillance Goals Provide an approach for nodes to decide their sleep/work schedules: guarantee different degrees of coverage redundant nodes go to sleep to save energy and extend system lifetime Other features balance energy consumption minimize computation and communication costs

November 5, 2003 ACM SenSys Differentiated Surveillance Assumptions Nodes are not mobile Localization and Synchronization Sensing area: a circle with radius r can be relaxed Communication range > 2r can be relaxed

November 5, 2003 ACM SenSys Differentiated Surveillance Work/Sleep Schedule for a Single Point Global period T and common starting time Point x is covered by at least one node’s sensing area at ANY time A B C Point x Node A Node B Node C Awake time Asleep

November 5, 2003 ACM SenSys Differentiated Surveillance Decide Single Point Schedule Reference randomly selected from [0, T) Each node broadcasts tuple (location, reference) Work Schedule: [n*T + ref - T front, n*T + ref + T end ] Total work time is minimized Full coverage is still guaranteed A B C Point x Schedule of Each Node for Grid Point x refCrefA refBrefC t t T front T end

November 5, 2003 ACM SenSys Differentiated Surveillance Schedules for All Grid Points Similar procedures for other geometric points – cover the target area with a grid and calculate each node’s schedules for the grid points it can cover Grid size selection - neither too large nor too small How to integrate schedules for all grid points on a single node? A B C Grid Point x D Grid Point y Schedules for Grid Point y refA refD t t refA

November 5, 2003 ACM SenSys Differentiated Surveillance Put them all together Choose the UNION as the schedule of the node for ALL the grid points node A is able to cover Self-evident that the full coverage for each grid point is guaranteed Integrated schedule may be longer than needed Node A’s schedules for Grid Point a Grid Point b Grid Point c Grid Point z Node A’s integrated schedule T=100

November 5, 2003 ACM SenSys Differentiated Surveillance Differentiated Surveillance 200% = 100% < 100% Most Important Important Less Important ? ?

November 5, 2003 ACM SenSys Differentiated Surveillance Extension - 200% Coverage A B C Point x Schedules for Grid Point x refCrefA refB refC t t t t 0 We only need to double T front and T end of the integrated schedules for 200% coverage - or shrink them for less than 100% coverage (multiplied by desired degree of coverage alpha ) 120 Node A Node B Node C

November 5, 2003 ACM SenSys Differentiated Surveillance Issue: Energy Balance Energy consumption unbalance among nodes due to random selection of reference numbers Multi-round extension to decrease the variation Each node selects N reference numbers with an iid distribution Get N schedules with the same algorithm Compose these N schedules consecutively

November 5, 2003 ACM SenSys Differentiated Surveillance Issue: Schedule Redundancy An integrated schedule may be longer than needed due to the union operation Second pass optimization to reduce redundancy

November 5, 2003 ACM SenSys Differentiated Surveillance Cost Analysis – Computation and Communication Communication Broadcast only once at the initial phase Local communication Only (location, reference) transmitted Computation - typically 10K~100K inst. For one grid point Calculate distance: * (#neighbor within 2r) Lay out references: c * (#neighbor) Run it for all grid points it can cover: * (#grid) Integrate: c * (#grid)

November 5, 2003 ACM SenSys Differentiated Surveillance Related Work (I) F. Ye et. al., “Energy-Efficient Robust Sensing Coverage in Large Sensor Networks,” UCLA technical report 2002 Each node probes a neighborhood for working nodes each time it wakes up Cons - more communication, holes

November 5, 2003 ACM SenSys Differentiated Surveillance Related Work: Sponsored Coverage D. Tian et. al., “A Node Scheduling Scheme for Energy Conservation in Large WSNs”, Wireless Communications and Mobile Computing Journal, May 2003 underestimated “sponsored sector” per-round communication overhead

November 5, 2003 ACM SenSys Differentiated Surveillance Simulation Configuration Sensing range 10m, Communication range 25m 160X160 Field, nodes deployed with an iid uniform distribution, the inner 100X100 area measured Repeated 100 times with different random references and node deployments, 90% CI < 10% mean Target area Measured area

November 5, 2003 ACM SenSys Differentiated Surveillance Total Energy Consumption The Differentiated Surveillance protocol outperforms the Sponsored Coverage scheme by as much as 50% reduction in total energy consumption.

November 5, 2003 ACM SenSys Differentiated Surveillance Half-Life of the Network The Differentiated Surveillance protocol outperforms the Sponsored Coverage scheme by as much as 130% increase in the half-life of the network.

November 5, 2003 ACM SenSys Differentiated Surveillance Differentiated Surveillance Result Total Energy Consumption – linearly increasing with alpha

November 5, 2003 ACM SenSys Differentiated Surveillance A protocol that achieves both energy conservation and differentiated degree of sensing coverage lower computation and communication overhead longer network lifetime Conclusions

November 5, 2003 ACM SenSys Differentiated Surveillance Questions? Thank you!