Open Access

Dario Fanucchi

• With current low-power wireless standards, the mistrust about wireless technology for industrial applications is unjustifiable. For instance, a Medium Access Control (MAC) technique, called Time Slotted Channel Hopping (TSCH), has made wire-like end-to-end reliability, certified security, and over a decade of battery lifetime a reality in Industrial Wireless Sensor Networks (IWSNs). TSCH is integrated into the IEEE~802.15.4 standard, and it is a cornerstone of the open standardised protocol suite for the Industrial Internet of Thing (IIoT) proposed by IETF. Specifically, the IETF 6TiSCH stack combines the industrial performance of TSCH with a set of higher layers protocols providing IPv6-connectivity to constrained devices. Thus, it promises, above all, interoperability between vendors and seamlessly integration of IWSNs into the Internet. Despite these high potentials and the high reputation of 6TiSCH in industry and academia, challenges remain, and some of its limitations needWith current low-power wireless standards, the mistrust about wireless technology for industrial applications is unjustifiable. For instance, a Medium Access Control (MAC) technique, called Time Slotted Channel Hopping (TSCH), has made wire-like end-to-end reliability, certified security, and over a decade of battery lifetime a reality in Industrial Wireless Sensor Networks (IWSNs). TSCH is integrated into the IEEE~802.15.4 standard, and it is a cornerstone of the open standardised protocol suite for the Industrial Internet of Thing (IIoT) proposed by IETF. Specifically, the IETF 6TiSCH stack combines the industrial performance of TSCH with a set of higher layers protocols providing IPv6-connectivity to constrained devices. Thus, it promises, above all, interoperability between vendors and seamlessly integration of IWSNs into the Internet. Despite these high potentials and the high reputation of 6TiSCH in industry and academia, challenges remain, and some of its limitations need to be understood. In this thesis, we focus on the issues related to the harmonisation of the asynchronous IPv6-based upper layers with the synchronous TSCH technique,which rely on control plane primitives such as the network bootstrap procedure, the management of communication resources and the collection of network statistics. We identify in which circumstances the 6TiSCH standardised control primitives, which should lay the foundations for a reliable operational 6TiSCH network, exhibit limitations. After explaining the shortcomings and their causes, we design refinements and validate them simulatively. The focus is not on data transmissions but on mechanisms for a dependable exchanging of control messages. Nevertheless, these have been designed without significantly reducing the available bandwidth for data applications and the lifetime of power-constrained nodes. Accordingly, we provide the following main contributions. First, we analyse the interplay between the scheduling of control messages and the multi-hop route computation during the network bootstrap phase, pointing out the limits of the current guidelines that may preclude or penalise the 6TiSCH network's operational state. Indeed, an improper choice of the protocol parameters may lead to a very long and energy-consuming network formation and stabilisation phase (e.g. more than 30 minutes even in small 5x5 grid networks). Second, we examine different resource allocation strategies for bootstrapping a 6TiSCH network. Here, we design, implement and evaluate a scheduling mechanism for coordinating the transmission of control messages among neighbouring nodes in a dynamic and distributed way. This mechanism has exhibited a significantly faster network formation than the default configuration, even in challenging chain network topologies, where it consumes at most 0.4% of the battery's charge of a sensor node. Finally, we investigate how to obtain an accurate Link Quality Estimation (LQE) in 6TiSCH. We demonstrate that state-of-the-art strategies, which are not designed having TSCH in mind, are too inaccurate for guaranteeing a reliable and stable 6TiSCH network setup. Indeed, internal interference hampers their link measurements. To overcome this issue, we propose a LQE strategy that allows a collision-free transmission of broadcast probe messages even during the network setup. This proposal improves the estimation accuracy dramatically, exhibiting a quite perfect estimation of at least 90% of the links in different network topologies and in a short time (i.e. in order of minutes) We are firmly convinced that 6TiSCH is an IIoT key enabler. Despite that, we forewarn the risk of its blind adoption as one-size-fits-all solutions in this work. Addressing some limitations in its control primitives and providing essential enhancements, we believe this thesis supports the future wide adoption of 6TiSCH in the industry.…