Synchronous Protocols for Low-Power Wireless: Towards Reliable and Low-Latency Autonomous Networking for the Internet of Things and Wireless Sensor Networks

Licentiatavhandling, 2017

With the emergence of the Internet of Things, autonomous vehicles and the Industry 4.0, the need for reliable yet dynamic connectivity solutions is arising. Many of these applications build their operation on distributed consensus. For example, networked cooperative robots and UAVs agree on manoeuvres to execute, and industrial control systems agree on set-points for actuators. Many applications are mission- and safety-critical, too. Failures could cost lives or incur economic losses. Any wireless network connecting safety-critical devices must be reliable, and often energy-efficient, as many devices are battery powered and we expect them to last for years. It shall be self-forming and self-fixing as well, to allow for reliable autonomous operation; as many applications cannot afford to stop and wait for external configuration. In this context, synchronised communication has emerged as a prime option for low-power critical applications. Solutions such as Chaos or Time Slotted Channel Hopping (TSCH) have demonstrated end-to-end reliability upwards of 99.99 percent. In this thesis, we design and implement protocols to support highly reliable and low latency communication in low-power wireless settings. First, we present a standard-based solution that integrates with the 6TiSCH stack (IPv6 over TSCH) without the need of static scheduling or schedule negotiation. Second, we identify key challenges when it comes to implementing the 6TiSCH stack, and demonstrate how these challenges can be addressed. Then, we take a step beyond the standards and focus on synchronous network flooding such as Glossy and Chaos. We show how to enhance them by adding time-slotting and frequency diversity to achieve high reliability and low latency under interference. Finally, we design and realise a network stack that combines and extends ideas from TSCH and synchronous transmissions to achieve highly reliable data delivery with a loss rate lower than x and achieve network-wide consensus with a radio duty cycle of 0.5 percent. On top of this robust kernel, we enable two- and three-phase commit protocols to provide network-wide consensus. We implement our protocols, evaluate them on public testbeds of sensor nodes equipped with IEEE 802.15.4 compatible radios and compare to state-of-the-art protocols. We contribute the source code of our main protocols to the community as a step towards enabling ubiquitous connectivity in the context of the Internet of Things.