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Definition and configuration of reliable event detection for application in wireless sensor networks
(2010)
Ubiquitous systems based on wireless sensor networks will amazingly increase our quality of life. These systems are to be deployed in large areas with high density where hundreds or thousands of nodes are used. Certainly that demands to use low cost devices with limited resources, which in turn are prone to faulty behaviour. This work introduces a novel concept for wireless sensor network configuration considering fault tolerance, energy efficiency and convenience as primary goals while being tailored to user needs. It allows to ignore low-level details like node resources, network structures, node availability etc. and enables the programmer to work on a high abstraction level, namely the event itself including event related constraints. The definition of events characterising real world phenomena is of prominent use in sensor networks. The presented concept autonomously configures and monitors events, even if it requires to organise collaboration between nodes to deliver the results. The contribution of this work is threefold. An intuitive XML-based ESL simplifies event configuration to a level that is even suitable for non-professionals. It features hardware independent description elements to define complex phenomena and enhances these by tailor-made voting schemes and application constraints. Based on that, a novel, fully decentralised mechanism to autonomously set up distributed event detection called EDT and a cost efficient means to maintain such EDT, are presented. EDTs can be efficiently constructed on every device by using a tiny generating finite state machine requiring eight states only. It enables every node to self-divide event queries according to its own resources and self-adapt to the tasks assigned. Simultaneously, the EDT provides the interface for efficient collaboration using a lease-based publish/subscribe approach. The simulations clearly show that this concept works well and the applied collaboration scheme outperforms even idealised acknowledgement-based approaches. On top of the EDT, a means is developed that enhances the reliability of detection beyond the scope of Boolean event decision. It examines behavioural trends in sensor readings to indicate the significance of actual measurements in relation to the configured event. Measured data is investigated in detail to finally attach a significance indicator "is" to each event. This automatically generated indicator shall support users or overlaying systems in decision-making. In the example scenario based on data of real test cases, the "is" indicates a flaming fire 88 seconds and a smouldering fire 48 seconds before the threshold-based method triggers the alarm.