MDP for query-based wireless sensor networks

Increased sensors availability and growing interest in sensor monitoring has lead to an significant increase in the number of sensor networks deployed in the last decade. Simultaneously, the amount of sensed data and the number of queries calling this data significantly increased. The challenge is to respond to the queries in a timely manner and with relevant data, without having to resort to hardware updates or duplication. In this chapter we focus on the trade-off between the response time of queries and the freshness of the data provided. Query response time is a significant Quality of Service for sensor networks, especially in the case of real-time applications. Data freshness ensures that queries are answered with relevant data, that closely characterizes the monitored area. To model the trade-off between the two metrics, we propose a continuous-time Markov decision process with a drift, which assigns queries for processing either to a sensor network, where queries wait to be processed, or to a central database, which provides stored and possibly outdated data.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 discrete-state Markov decision process, shown to be stochastically equivalent to the initial continuous-time process, is formulated. This approach provides a theoretical support for the design and implementation of WSN applications, while ensuring a close-to-optimum performance of the system.