1. COMPUTING AGGREGATES FOR MONITORING WIRELESS SENSOR NETWORKS Jerry Zhao, Ramesh Govindan, Deborah Estrin Presented by Hiren Shah

2. Outline Motivation and GoalsMotivation and Goals ContributionsContributions –Architecture –Computing Network Digests – Impact of Packet Loss ExperimentsExperiments ConclusionConclusion

3. Motivation and Goals Every Network requires monitoring.Every Network requires monitoring. Sensor Network is no exception !!!Sensor Network is no exception !!! Monitoring in traditional networks – SNMP.Monitoring in traditional networks – SNMP. Need for a new architecture.Need for a new architecture.

4. Architecture

5. Architecture (contd.) Three software components of the architecture.Three software components of the architecture. – Dumps Detailed node states. E.g. logs with raw data reading, packet level details, energy details for a node.Detailed node states. E.g. logs with raw data reading, packet level details, energy details for a node. Dumps are costly to collect.Dumps are costly to collect. – Scans Abstract views of resource consumption in the network without referring to individual node.Abstract views of resource consumption in the network without referring to individual node. Derived using in-network aggregation.Derived using in-network aggregation.

6. Architecture (contd.) Network DigestsNetwork Digests –Energy cost of computing scans over entire network is very high. –Scans should be invoked only when necessary. –A digest is an aggregate of some network property. E.g. size of the network, average energy left at a node etc.E.g. size of the network, average energy left at a node etc.

7. Architecture (contd.) Digests indicate WHEN should scans be invoked.Digests indicate WHEN should scans be invoked. Scans indicate WHERE should dumps be collected from.Scans indicate WHERE should dumps be collected from.

8. Computing Digests Naive centralized approach does not scale well and has a single point of failure.Naive centralized approach does not scale well and has a single point of failure. Some Requirements of Digest ComputationSome Requirements of Digest Computation –Should be energy efficient. – Digests should be available all the time and everywhere in the network. Typical Digest functions are Node with max energy, count of no. of nodes, Sum of node energies, Avg. residual energy. Typical Digest functions are Node with max energy, count of no. of nodes, Sum of node energies, Avg. residual energy.

9. Computing Digests (contd.) These functions are decomposable.These functions are decomposable. Hence partial result can be added to get overall result.Hence partial result can be added to get overall result. Hence one can use in-network aggregation.Hence one can use in-network aggregation.

10. Computing Digests (contd.) One can use hierarchical approach for computing the digests. E.g. clusters like LEACH.One can use hierarchical approach for computing the digests. E.g. clusters like LEACH. However this involves overhead in computing leaders and maintaining the hierarchy.However this involves overhead in computing leaders and maintaining the hierarchy. Authors propose Digest Diffusion.Authors propose Digest Diffusion.

11. Digest Diffusion Nodes periodically send tuple (Mi, Si, Hi).Nodes periodically send tuple (Mi, Si, Hi). Initially node sets Mi to its perceived maximum value e.g. its own residual energy.Initially node sets Mi to its perceived maximum value e.g. its own residual energy. Si is source of maximum. Initialized to i.Si is source of maximum. Initialized to i. Hi is hop distance of the source of maximum.Hi is hop distance of the source of maximum.

12. Digest Diffusion Upon receiving a tuple from neighbor a node performs following processing.Upon receiving a tuple from neighbor a node performs following processing. – If Mj >Mi then set Mi=Mj ; Si=Sj ; Hi=Hj+1; – Also set parent Pi=j; Algorithm converges in steps proportional to network diameter.Algorithm converges in steps proportional to network diameter. Approach is scalable. Periodic messages can be PIGGYBACKED.Approach is scalable. Periodic messages can be PIGGYBACKED.

13. Computing other digests

14. Hence a tree is needed without any overlaps.Hence a tree is needed without any overlaps. Digest diffusion establishes a tree rooted at the node with the maximum value.Digest diffusion establishes a tree rooted at the node with the maximum value. Every node i aggregates values from its children and passes the partial result to its parent along the tree.Every node i aggregates values from its children and passes the partial result to its parent along the tree. The root will have the final aggregate value.The root will have the final aggregate value.

15. Computing other digests Metric for establishing the tree should be carefully chosen such that the tree is relatively stable. E.g. link degree of a node is a bad choice since it fluctuates a lot with node failures.Metric for establishing the tree should be carefully chosen such that the tree is relatively stable. E.g. link degree of a node is a bad choice since it fluctuates a lot with node failures.

16. Digest Tree Maintenance With node failures the tree should be suitably changed.With node failures the tree should be suitably changed. Packet loss needs to be taken into account since absence of heartbeat does not mean the neighbor has failed.Packet loss needs to be taken into account since absence of heartbeat does not mean the neighbor has failed. Let T0 be the interval between broadcasts.Let T0 be the interval between broadcasts. If heartbeat is not received from a parent or child in Tp= 4To seconds change the tree.If heartbeat is not received from a parent or child in Tp= 4To seconds change the tree.

17. Impact of Packet Loss Packet losses are frequent in wireless environments.Packet losses are frequent in wireless environments. Packet losses make aggregation tree unstable and hence affect the quality of aggregate digest.Packet losses make aggregation tree unstable and hence affect the quality of aggregate digest.

18. Impact of packet loss

21. Solution Link Quality Profiling and RejectionLink Quality Profiling and Rejection –“blacklist” links with poor quality and asymmetry. –A node chooses as parent a node with which it has good and symmetric communication. Use packet sequence number to estimate how many of the packets sent by the neighbor are getting lost.Use packet sequence number to estimate how many of the packets sent by the neighbor are getting lost. Exchange “ I CAN HEAR YOU “ lists to identify asymmetric links.Exchange “ I CAN HEAR YOU “ lists to identify asymmetric links.

22. Experiments Used REAL Berkley Motes !!!Used REAL Berkley Motes !!! Studied performance of various schemes against packet loss.Studied performance of various schemes against packet loss. Used root mean square error to quantify performance.Used root mean square error to quantify performance.

23. Experiments

24. Conclusion Proposed an architecture for monitoring sensor network.Proposed an architecture for monitoring sensor network. Suggested design for computing network digests.Suggested design for computing network digests. Carefully handling lossy and asymmetric links reduces error in digest computation.Carefully handling lossy and asymmetric links reduces error in digest computation. Future work includes experiments on larger testbed and designing a full-fledged monitoring suite.Future work includes experiments on larger testbed and designing a full-fledged monitoring suite.

25. Questions ?