Mid Sweden University

Nowadays, Wireless Sensor Networks (WSNs) have gained importance as aflexible, easier deployment/maintenance and cost-effective alternative to wired networks,e.g., Fieldbus and Wired-HART, in a wide-range of applications. Initially,WSNs were mostly designed for military and environmental monitoringapplications where energy efficiency is the main design goal. The nodes in the network were expected to have a long lifetime with minimum maintenance while providing best-effort data delivery which is acceptable in such scenarios. With recent advances in the industrial domain, WSNs have been subsequently extended to support industrial automation applications such as process automation and control scenarios. However, these emerging applications are characterized by stringent requirements regarding reliability and real-time communications that impose challenges in the design of Industrial Wireless Sensor Networks (IWSNs) to effectively support time- and mission-critical applications.

Typically, time- and mission-critical applications support different traffic categories ranging from relaxed requirements, such as monitoring traffic to firm requirements, such as critical safety and emergency traffic. The critical traffic is mostly acyclic in nature and occasionally occurs at unpredictable time instants. Once it is generated, it must be delivered within strict deadlines. Exceeding the delay bound could lead to system instability, economic loss, or even endanger human life in the working area. The situation becomes even more challenging when an emergency event triggers multiple sensor nodes to transmit critical traffic to the controller simultaneously. The unpredictability of the arrival of such a type of traffic introduces difficulties with regard to making a suitable scheduling that guarantees data delivery within deadline bounds. Existing industrial standards and related research work have thus far not presented a satisfactory solution to the issue. Therefore, providing deterministic and timely delivery for critical traffic and its prioritization over regular traffic is a vital research topic.

Motivated by the aforementioned challenges, this work aims to enable real-timecommunication for time- and mission-critical applications in IWSNs. In this context, improved Medium Access Control (MAC) protocols are proposed to enablea priority-based channel access that provides a timely delivery for acyclic critical traffic. The proposed framework starts with a stochastic modelling of the network delay performance under a priority-oriented transmission scheme, followed by two MAC approaches. The first approach proposes a random Clear Channel Assessment (CCA) mechanism to improve the transmission efficiency of acyclic control traffic that is generated occasionally as a result of observations of an established tendency, such as closed-loop supervisory traffic. A Discrete-Time Markov Chain (DTMC) model is provided to evaluate the performance of the proposed protocol analytically in terms of the expected delay and throughput. Numerical results show that the proposed random CCA mechanism improves the shared slots approach in WirelessHART in terms of delay and throughput along with better transmission reliability.

The second approach introduces a slot-stealing MAC protocol based on a dynamic deadline-aware scheduling to provide deterministic channel access in emergency and event-based situations, where multiple sensor nodes are triggered simultaneously to transmit time-critical data to the controller. The proposed protocol is evaluated mathematically to provide the worst-case delay bound for the time-critical traffic and the numerical results show that the proposed approach outperforms TDMA-based WSNs in terms of delay and channel utilization.

The Industrial Internet of Things (IIoT) has become a promising technology for the improvement of the productivity, efficiency, and intelligence of the manufacturing process. Industrial Wireless Sensor Networks (IWSNs) represent a main pillar of IIoT to support communications within the field network level. For several IIoT applications, IWSNs are defined by strict communication requirements in terms of latency and reliability to support the proper functioning of the industrial system and avoid production loss. However, there are many challenges in efficiently satisfying these requirements. The key challenges investigated in this thesis are related to the shortcomings of the existing IWSN standards to enable timely delivery of aperiodic critical data, support traffic differentiation, and maintain reliable end-to-end communications. The overall objective of this work is to improve the reliability and real-time communication at the field network level in IIoT applications, particularly in process automation scenarios. Specifically, the proposed solutions represent improvements within the data-link and network layers of the IWSN protocol stack. The work in this thesis introduces the following contributions. The first part of the thesis focuses on improving real-time delivery for critical traffic and enabling traffic differentiation for mixed-criticality systems. The contribution in this part comprises three approaches. The first approach introduces a deterministic priority-based channel access mechanism for emergency data in time- and mission-critical applications. The approach is based on a dynamic deadline-aware schedule to provide a delay-bounded performance for the unpredictable emergency traffic along with efficient channel utilization. In the second approach, a priority-based wireless fieldbus protocol is proposed to enable traffic differentiation in mixed-criticality systems, where each traffic flow is given a transmission priority according to its corresponding criticality level. The third approach presents an optimized retransmission scheme to maximize the probability that an emergency packet is successfully delivered within its deadline bound. The results of the proposed schemes prove their effectiveness in providing real-time delivery for critical traffic and efficient service differentiation for mixed-criticality systems. The second part of the thesis introduces a routing framework to improve the connectivity and the end-to-end communication reliability of 6TiSCH networks. The proposed solutions in this part are mainly designed on the basis of the standard Routing Protocol for Low-Power and Lossy Networks (RPL). The proposed framework comprises the following approaches: 1) a reliable mobility-aware routing scheme to support node connectivity and reliable routing in mobile 6TiSCH networks, 2) a congestion control and detection strategies to enhance packet delivery performance under imbalanced network and heavy load scenarios, 3) a hybrid multi-cast method to maintain downlink connectivity and mitigate routing memory limitations in large-scale 6TiSCH networks. The conducted performance evaluations prove the effectiveness of the proposed approaches to enhance network performance in terms of reliability and delay metrics. The proposed approaches manage to improve routing performance of 6TiSCH networks in terms of connectivity and end-to-end data delivery, which in turn improves the real-time communication in IIoT.