In this work we provide a transport layer solution to entity maintenance and connectivity in sensor networks. Our proposed middleware service provides a novel way for programmers to relate to events in the environment without concern for topological and communication layer details irrelevant to their application. We establish entities in a one-to-one relationship with events to ensure correct and well defined behavior. Entities form and register with interested parties to allow unique identification and communication without regard for an event's location.
The analysis of our architecture demonstrates the effect of both uncontrollable (environment specific) and controllable (architecture specific) parameters on entity formation and maintenance. In accordance with an ideal sensor network, we require that at any time at least one node is capable of sensing the event being tracked. Under this assumption we demonstrate our architecture's capabilities and limitations in tracking events of varied speeds as a function of the pre-specified heartbeat and awareness horizon parameters. For a large field of relatively dense nodes, we show in simulation that our architecture is capable of tracking events that travel over half of the communication radius of a node per second (see Figure 7). Under optimal conditions in our simulation study, our architecture was able to track objects moving at about 88% of the communication radius. We additionally discuss the required settings for tracking events of varied speeds and the tradeoff of increasing the trackable speed and thereby increasing the amount of energy consumed.
In addition to simulation analysis, we provide an implementation of our architecture on the MICA test bed in our lab. In the presence of true fading and message loss we demonstrate the feasibility of our work for a simple application. Implementation results show the importance of a reliable MAC layer. Unlike the simulation, which used 802.11 to access the medium, the MAC layer in the implemented prototype used unreliable transmission. Consequently, the maximum tracked speed was significantly lower. The authors are currently implementing a reliable MAC layer for the motes platform. While our architecture provides the required mechanisms to create, maintain, and communicate with abstract entities in an environment, we have only begun to explore the possibilities of such sensor network related services. The applications and opportunities for sensor networks remain vast and mostly unexplored. This paper is a step towards a comprehensive coverage of research issues motivated by tracking problems in sensor networks.