An Adaptive Zone-based Storage Architecture for Wireless Sensor Networks Thang Nam Le, Dong Xuan and *Wei Yu Department of Computer Science and Engineering, The Ohio State University, Columbus *Computer Science Department, Texas A&M University, College Station IEEE Global Telecommunications Conference, GLOBECOM 2005 Reporter: Shin-Wei Ho

Outline Introduction System Model Zone Division Algorithm Query Processing Performance Evaluation Conclusion

Introduction A sensor Limited amount of energy Limited amount of storage space Over a period of time, a wireless sensor network can generate a large a mount of data.

Introduction Sensor network applications must devise an efficient scheme Storing the collected data Retrieving the collected data

Introduction Desired properties Minimizing overall energy consumption Maximizing network lifetime Load-balancing Resilience to node or network failures

Introduction Some Application examples A monitoring sensor network powered by solar energy. In the summer, the primary concern is storage space. In the winter, the priority will be given to energy consumption or load-balancing.

Introduction The desired performance of the network Customizable to adapt to the changes in the network or the external environment.

Network Model The nodes in the network are laid out uniformly. All sensors have equal transmission range. Nodes are unaware of their physical location. Query Type Node-specific What is the current noise level of node 10. Network-wide Report temperature 30 minutes ago for all nodes. The sensor network is designed to keep data for a fixed period of time. The applications in the domain have high query rates.

Overall approach of solution Dividing a large network into zones Nodes store their data into a specific storage node selected by the zone leader The zone leaders form an overlay When the query source point is fixed, it changes the mesh topology of the overlay to a tree structure

Zone Division Algorithm Let G = {V, E} denote the wireless sensor network For a sensor V i, we define its contribution potential P(V i ) P(V i ) = f (IP(V i ), EP(V i )) IP(V i ) = internal contribution potential A function on a set of attributes of node Vi, energy level, storage space, connection degree, number of serving slots as zone leaders, others.. EP(V i ) = external contribution potential of V i A function of internal contribution potential of all neighbors of V i.

Zone Division Algorithm Step 1 Each node V i in the network will exchange its contribution potential IP(V i ) and then P(V i ) with its neighbors. A node V i will select a neighbor V j as an uplink node if P(V j ) = max { P(V i ) ∪ P(V x ) : V x ∈ set of neighbors of V i }. It informs the uplink node about itself.

Zone Division Algorithm Step 2 Nodes without an uplink node will become zone leaders. The zone leaders broadcast a zone formation message containing the zone ID (its ID) with TTL(as the maximum zone radius) to its downlink nodes..

Zone Division Algorithm Step 3 Nodes without zone will reinitiate the zone finding process until all nodes belong to a certain zone.

Zone Division Algorithm The selection of potential function (f) will decide how the zone leaders are selected. Node degree The overall data transmission energy is minimized for intra- zone data storage. Node energy/Storage space The lifetime of the network is maximized. Serving slot The nodes with lowest serving time (as zone leader) are selected. This ensures that nodes in the network serve as zone leaders for a similar number of timeslots. The solution aims towards providing load balancing.

Zone Division Algorithm It can preset the potential function to Have storage space as the dominating term during the summer Give more weight to the number of serving-slots during the winter The authors do not give a specific formula for the potential function.

Query Processing Intra-zone Data Storage Information Retrieval Query Source Adaptation Fault Tolerance and Load Balancing

Query Processing -- Intra-zone Data Storage Periodically, the zone division process is performed. A zone leader selects from its direct children the node with highest contribution potential as a storage node for its zone.

Query Processing -- Intra-zone Data Storage The selected storage node announces its presence to the zone. A broadcast message with TTL as R+1. This broadcast message also serves as a route initialization message. Nodes periodically send their data to the storage node using the established path.

Query Processing -- Intra-zone Data Storage Nodes can avoid expensive cross- network data transmission External storage Hash-based approaches

Query Processing -- Intra-zone Data Storage the zone leader also serves as the resource directory for the network. keeps track of all storage nodes. if a former storage node discovers that it belongs to a different zone, the node registers its previously stored data with the new zone leader. Zone(A) Leader Storage node(A) Zone(B) Leader Storage node(B) register

Query Processing -- Intra-zone Data Storage The zone leader can avoid expensive broadcast to find the resource as queries arrive. By maintaining a list of local resources within the zone.

Query Processing -- Intra-zone Data Storage Intra-zone routing from storage nodes (both previous and current) to the zone leader by Multicast manner based on the temporal property of the query.

Query Processing -- Information Retrieval The set of zone leaders forms an overlay over the physical sensor network.

Query Processing -- Information Retrieval To establish path between overlay nodes, border nodes overhear broadcast to decide if they have any neighbors in different zones.

Query Processing -- Query Source Adaptation The authors improve the query routing efficiency. Converting the overlay topology from Mesh-based to Tree-based.

Query Processing -- Fault Tolerance and Load Balancing Fault Tolerance Zone structure When a node detects that it is no longer connected to its parent, it informs its children and reinitiates the zone formation process. This process can be done locally.

Query Processing -- Fault Tolerance and Load Balancing Load Balancing There is no centralized storage for the whole network. Select one or more storage node.

Performance Evaluation A network with a diameter of 500x500m. The number of nodes varies from 200 to 600. Maximum zone radius is set at 4 hops. There is a single query source at (0,0).

Performance Evaluation -- Zone formation overhead

Performance Evaluation -- Total system overhead for query processing

Performance Evaluation -- Energy distribution after 100 time slots

Performance Evaluation -- Number of timeslots until node failure occurs

Conclusion An adaptive zone-based storage architecture for wireless sensor networks. Node contribution potential A distributed clustering algorithm The system has low overhead and is efficient in terms of query processing.

Thank you! Question?

Performance Evaluation -- Zone characteristics (transmission range: 50m)