The so-called Industrial Internet of Things (IIoT) is expected to transform our world, and in depth modernize very different domains such as manufacturing, energy, agriculture, construction industry, and other industrial sectors. The need for low power radio networks first led to low duty cycle approaches where nodes turn off their radio chipset most of the time to save energy. The medium access control (MAC) has thus been largely investigated over the last fifteen years. Unfortunately, classical contention access methods use a random access and are unable to provide guarantees. In the meantime, some dedicated standards have emerged (e.g. IEEE 802.15.4-2006, IEEE 802.15.4-2015), combining Time Division Multiple Access (TDMA) with slow channel hopping in order to enable reliability and energy efficiency. Slow channel hopping allows each node to use different channels for a frame and its possible retransmissions with a low-cost hardware. To provide high-reliability, these protocols rely on a common schedule in order to prevent simultaneously interfering transmissions. In this context, we clearly observe a strong growth of the number of proposals in the last years, denoting a strong interest of the research community for deterministic slow channel hopping scheduling for the IIoT. We categorize here the numerous existing solutions according to their objectives (e.g. high-reliability, mobility support) and approaches. We also identify some open challenges, expected to attract much attention over the next few years.

Abs tract. Semantic clustering is a recent technique for saving energy in wireless sensor networks. Its mechanism of action consists in dividing the network into groups (clusters) formed by semantically related nodes and at least one semantic collector, which acts as a bridge between its internal nodes and the sink node. Since semantic collector nodes need to perform more tasks than normal nodes, they deplete their energy budget faster, so it is necessary to use efficient mechanisms for electing semantic collectors to prolong the network lifetime. Our hypothesis is that an effective choice of semantic collectors allows a longer network lifetime. To test it, we start from a previous work of the authors of this article and we propose an algorithm for electing semantic collectors in a distributed way based on a fuzzy inference engine. The inputs of the inference engine are the residual energy of nodes and their received signal strength indicator (RSSI). Simulation results confirm our hypothesis, since the algorithm provides (i) an improvement of 17.4% in relation to another proposal of the related literature, and (ii) a gain of 68.8% over the time life of the network's original work.

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Industrial networks differ from others kinds of networks because they require real-time performance in order to meet strict requirements. With the rise of low-power wireless standards, the industrial applications have started to use wireless communications in order to reduce deployment and management costs. IEEE802.15.4-TSCH represents currently a promising standard relying on a strict schedule of the transmissions to provide strong guarantees. However, the radio environment still exhibits time-variable characteristics. Thus, the network has to provision sufficient resource (bandwidth) to cope with the worst case while still achieving high energy efficiency. The 6TiSCH IETF working group defines a stack to tune dynamically the TSCH schedule. In this paper, we analyze in depth the stability and the convergence of a 6TiSCH network in an indoor testbed. We identify the main causes of instabilities, and we propose solutions to address each of them. We show that our solutions improve significantly the stability.