Sensors on speaking terms: Schedule-based Medium Access protocols for wireless sesor networks

Wireless sensor networks make the previously unobservable, observable. The basic idea behind these networks is straightforward: all wires are cut in traditional sensing systems and the sensors are equipped with batteries and radio’s to virtually restore the cut wires. The resulting sensors can be placed closely to the phenomenon that needs monitoring, without imposing high wiring costs and careful engineering of the position of the devices. Sensors can even be attached to mobile objects. As a result, monitoring can be done more efficiently and at higher sensing resolution compared to traditional sensor systems. Yet, these are not the only advantages of wireless sensor networks. We —as user of the wireless sensor network– are not interested in constant streams of sensor readings, but we are more interested in the interpretation of the sensor readings. This is exactly the big potential of wireless sensor networks. Due to intelligence, added locally to the wireless sensors, and collaboration between the individual sensors, the network by itself can carry out complex tasks related to observing. To achieve these goals, wireless sensors must be ”on speaking terms” i.e. the nodes must be able to exchange information. The aim of this thesis is to provide a set of communication rules —commonly known as medium access control (MAC) protocol– that organizes efficient communication through a shared wireless medium and is well suited for the inherent characteristics of wireless sensor networks. The MAC protocol determines when a wireless sensor transmits its sensor information and is thereby in control of one of the most energy consuming components in the wireless sensor hardware architecture. The lifetime of the battery operated wireless sensors is thus heavily dependant on the efficiency of the MAC protocol. This thesis provides a self-organizing, schedule-based medium access approach. In general, this class of medium access is recognized for its energy-efficiency and robustness against high peak loads. These aspects are required by wireless sensor networks. Its drawbacks are message delay, over-provisioning and required time synchronization between wireless sensors. In our approach, wireless sensors analyse local medium usage and autonomously choose when to access the medium i.e. transmit. In the analysis, medium usage of second order neighbours is also taken into account. Therefore, our approach does not suffer from the well-known hidden terminal problem, and additionally, the wireless medium is spatially reused without energy-wasting conflicts. The medium access schedule of wireless sensors is adjusted when —e.g. due to mobility of nodes– conflicting schedules exist. The correctness of the general medium access control protocol is