Optimal query assignment for wireless sensor networks

Increasing computing capabilities of modern sensors have enabled current wireless sensor networks to process queries within the network. This complements the traditional features of the sensor networks such as sensing the environment and communicating the data. Query processing, however, poses Quality of Service challenges such as query waiting time and validity (age) of the data. We focus on the processing cost of queries as a trade-off between the time queries wait to be processed and the age of the data provided to the queries. To model this trade-off, we propose a Continuous Time Markov Decision Process which assigns queries either to the sensor network, where queries wait to be processed, or to a central database, which provides stored and possibly outdated data. To compute an optimal query assignment policy, a Discrete Time Markov Decision Process, shown to be stochastically equivalent to the initial continuous time process, is formulated. A comparative numerical analysis of the performance of the optimal assignment policy and of several heuristics, derived from practice, is performed. This provides a theoretical support for the design and implementation of WSN applications, while ensuring a close-to-optimum performance of the system.