Communication Support for Location- Centric Collaborative Signal Processing in Sensor Networks Parmesh Ramanathan University of Wisconsin, Madison Acknowledgements:K.-C. Wang, K. K. Saluja, T. Clouqueur

What is a sensor network? A large ad hoc network of low-cost, smart devices Devices communicate over wireless channels Devices can sense only a small area around them Need collaboration among devices to carry out most meaningful tasks

Sensor Network Characteristics Commands/queries are typically issued to a geographic region and not to specific nodes Compute the average temperature in a region Are there any unidentified objects in a given region? Track an object within a region Only devices in the specified geographic region need to participate in executing a command/query

Ad Hoc Wireless LANs Nodes are the addressable entities Commands are issued to specific nodes Typical challenge is to how to maintain ongoing interactions between a given set of nodes even as they move

Open Question What is the best programming abstraction and the underlying communication support suited for sensor networks?

Prior Work Programming abstraction Subscribe-Publish model [USC/ISI/MIT/LL] Communication support Directed Diffusion [USC/ISI]

Subscribe-Publish Model Nodes disseminate the attributes of the information they need (Subscribe) Nodes also disseminate the attributes of the information they can provide (Publish) An interaction between nodes is established when there is match between their respective subscription and publication

Directed Diffusion Nodes diffuse interest messages identifying the attributes of the information they need Nodes with the data respond over one or more routes identified by the interest messages Reinforcement messages are used to converge a good route Intermediate nodes may use filters to aggregate information as it passes through the network

Our Programming Abstraction Location-centric Computing All nodes are aware of their current location Addressable entity is a geographic region Regions play the traditional role of a node A region must be created before a command or query can be issued Each region has a manager region responsible for coordinating intra-region activities Each node maintains the list of regions to which it belongs and participates only in the activities of its regions

Location-centric Model

Location-centric Communication Primitives Data exchange primitives Motivated by the well-known distributed computing library called MPI 1.1 Send, Receive, Reduce, Barrier, Multicast, Broadcast,… Administrative primitives Create region and delete region

Location-centric Communication Primitives Example: SN_Send Sends a message from a node to all nodes in the addressed region Used to send commands and data Example: SN_Reduce Aggregates data within a region at the manager region Aggregation is in the form of min, max, average, sum, …

Location-based Routing Each node maintains a routing table identifying next hop to reach a destination region Routing entry for a region is created on demand using RouteRequest (RREQ) and RouteReply (RREP) RREQ and RREP use an approach similar to Location-aided Routing [Vaidya] to limit the scope of flooding

Location-Based Routing: Inter-Region Send: 1. message sent from a source node to a region.

Location-Based Routing: Inter-Region Send: 1. message sent from a source node to a region. 2. Message flooded to all nodes in the region.

Target Detection and Tracking t=0 t=20 t=60 t=40 81 sensors evenly spaced over 800mx800m square area Target emits power sensible within 100m

Location-centric solution Create region(s) at expected entry area(s) Send detect & track command to the region(s)

Location-centric solution Manager Nodes in the region use Reduce to aggregate sensor readings

Location-centric solution Predict the track and create the next region Initiate detection & tracking in the created region

Subscribe/Publish Solution I Each node subscribes to track info from neighboring nodes

Subscribe/Publish solution Duplicate copies of track info from same node are suppressed.

Subscribe/Publish solution Track info from different nodes are not suppressed.

Regional Subscribe/Publish Solution subscribe Each node subscribes to track info from neighboring regions.

Regional Solution with Subscribe/Publish Model Track info propagates to subscribers using directed diffusion Data suppression more effective.

Evaluation We implemented the three approaches in ns-2. Counted the total number of messages exchanged for the tracking scenario Routing messages Application payload

Initial Results

Summary Collaboration in sensor networks are quite different from that in conventional wireless ad hoc networks Compared performance of two different approaches for collaboration in sensor networks Initial results show that a location-centric based approach is better in terms of number of messages for target tracking application

Ongoing Work Physical and link layer aware communication schemes to improve energy and bandwidth usage Incorporate fault-tolerance in sensor fusion algorithms Develop collaborative signal processing algorithms for typical sensor network applications