Sensors on speaking terms: Schedule-based medium access control protocols for wireless sensor 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 \emph{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 verified with a formal analysis tool, the model checker Uppaal. Based upon the general schedule-based medium access approach, EMACs and LMAC are designed. Both protocols have been implemented on prototype wireless sensors designed in this thesis. The prototype hardware platforms are used to demonstrate that time synchronization is sufficiently attainable between wireless sensors. The EMACs protocol includes clustering techniques to determine the role of wireless sensors concerning wireless communication. To maintain a connected communication structure, potentially not all sensors are required to actively participate in multi-hop communication. Therefore, two roles are introduced. Active sensors create a connected backbone, which is used by passive sensors. Passive sensors can conserve energy and the lifetime of the wireless sensor network is extended. With the introduction of these roles, we actively target one of the drawbacks of schedule-based access: over-provisioning. The LMAC protocol is a simplified version of the EMACs protocol. In wireless sensor networks, sensor readings are most often forwarded to so-called gateway nodes. These nodes present the findings of the network to an exterior user. The LMAC protocol includes the necessary functionality to transport messages to gateways without requiring additional routing protocols. Using the LMAC protocol, we demonstrate that message delays can be significantly reduced. Our methods are applicable on our general schedule-based medium access approach, thereby addressing one of the drawbacks of schedule-based medium access: latency. The LMAC protocol can be easily compromised when attackers have full knowledge of the protocol. Encrypting messages, applying source authentication and authenticating message content, makes the task of undermining a wireless sensor network more difficult, yet security keys are easily compromised. Wireless sensor networks are even more easily attacked with jamming. We present such (energy-efficient) jamming attacks for our schedule-based medium access approach and in particular LMAC. For this class of attacks little knowledge is required about the MAC protocol, which makes it a realistic threat. We show its effectiveness on LMAC and present countermeasures. Additionally, consequences for other schedule-based MAC protocols are discussed.