| Abstract: |
Safety-critical sensor networks are pervasively embedded in our surroundings. Such networks impose strong requirements in terms of reliability and latency of sensor readings and do for
instance allow to monitor buildings for detecting fires and intrusions. The network requires a costly and cumbersome installation of wires for connecting the distributed sensors, which is
sometimes not even possible. This suggests adopting the emerging technology of wireless sensor networks (WSN) to be used in a safety-critical context. With this technology, the wires connecting
the sensors can be replaced by a radio and a battery pack. A WSN is a collection of embedded sensor nodes with wireless networking capabilities. Collectively the sensor nodes establish a wireless
ad-hoc network for transferring, processing and monitoring the sensed data. In order to ensure a small form factor, the sensor nodes are highly integrated and provide minor processing
capabilities and limited memory. More stringent, the battery-powered nodes have to carefully orchestrate the power-hungry radio device if a yearlong independent operation is targeted. To make
matters even worse, wireless communication is inherently unreliable and limited in range. Altogether this makes it a very demanding task to ensure a reliable, timely and energy efficient
transport of the sensed data over possibly multiple hops. Reliability is of utmost importance in a safety-critical environment. Additionally, there are often regulations imposing strong demands
in terms of message latency and the availability of the sensor nodes. In particular, this thesis refers to the exemplary case of a wireless fire-alarm application, in which an alarm must be
reported to a control station within 10 seconds, and a failed node has to be detected by the network within 5 minutes. These requirements are exacerbated by the fact that the nodes have to power
off the radio more than 99% of the time, in order to enable an independent operation for several years with a small battery. This thesis contributes towards adopting WSN technology for
safety-critical applications. It focuses on communication aspects, and makes the following major contributions: - The novel communication strategy, Dwarf, ensures a robust and timely forwarding
of alarm messages, despite having the sensor nodes powered off most of the time. The maintenance protocol DiMo allows the monitoring of the nodes and the network topology with minimal
communication overhead. In conjunction, Dwarf and DiMo enable safety-critical networking. - This thesis contributes an analytical framework for analyzing and comparing WSN MAC protocols. The
framework provides deep insight into the behavior of WSN MAC protocols and provides the first available solution for benchmarking. This provides the means for selecting the most suitable MAC
protocol for an application at hand. - This thesis contributes the NoSE protocol enhancement. NoSE allows for considerable energy savings while maintaining the sensor network and allows for a
swift and dependable initialization. NoSE is beneficial for specialized applications and protocols like Dwarf and DiMo that are optimized for yearlong operation, but exhibit a reduced energy
efficiency and responsiveness during maintenance and initialization. |
