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.

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We propose several localized sensor area coverage protocols for heterogeneous sensors, each with arbitrary sensing and transmission radii. The approach has a very small communication overhead since prior knowledge about neighbor existence is not required. Each node selects a random time out and listens to messages sent by other nodes before the time out expires. Sensor nodes whose sensing area is not fully covered (or fully covered but with a disconnected set of active sensors) when the deadline expires decide to remain active for the considered round and transmit an activity message announcing it. There are four variants in our approach, depending on whether or not withdrawal and retreat messages are transmitted. Covered nodes decide to sleep, with or without transmitting a withdrawal message to inform neighbors about the status. After hearing from more neighbors, active sensors may observe that they became covered and may decide to alter their original decision and transmit a retreat message. Our simulations show a largely reduced message overhead while preserving coverage quality for the ideal MAC/physical layer. Compared to an existing method (based on hello messages followed by retreat ones and where excessive message loss contributed to excessive coverage holes), our approach has shown robustness in a model with collisions and/or a realistic physical layer.

We propose several localized sensor area coverage protocols, for arbitrary ratio of sensing and transmission radii. Sensors are assumed to be time synchronized, and active sensors are determined at the beginning of each round. The approach has a very small communication overhead since prior knowledge about neighbor existence is not required. Each node selects a random timeout and listens to messages sent by other nodes before the timeout expires. Sensor nodes whose sensing area is not fully covered (or fully covered but with a disconnected set of active sensors) when the deadline expires decide to remain active for the considered round, and transmit a message announcing their activity status. There are four variants in our approach, depending on whether or not negative and retreat messages are transmitted. Experimental results with ideal MAC layer show that, for a similar number of selected active sensors, our methods significantly reduce number of messages to decide activity compared to existing localized protocol, We also consider a MAC layer with collisions, and show that existing compared method, for dense networks, fails to cover the area reasonably. Our methods, however, still remain robust in terms of high area coverage with reasonable amount of active nodes, despite some message collisions.

Envisioned communication densities in Internet of Things (IoT) applications are increasing continuously. Because these wireless devices are often battery powered, we need specific energy efficient (low-power) solutions. Moreover, these smart objects use low-cost hardware with possibly weak links, leading to a lossy network. Once deployed, these Low-power Lossy Networks (LLNs) are intended to collect the expected measurements, handle transient faults and topology changes, etc. Consequently, validation and verification during the protocol development are a matter of prime importance. A large range of theoretical or practical tools are available for performance evaluation. A theoretical analysis may demonstrate that the performance guarantees are respected, while simulations or experiments aim on estimating the behaviour of a set of protocols within real-world scenarios. In this article, we review the various parameters that should be taken into account during such a performance evaluation. Our primary purpose is to provide a tutorial that specifies guidelines for conducting performance evaluation campaigns of network protocols in LLNs. We detail the general approach adopted in order to evaluate the performance of layer 2 and 3 protocols in LLNs. Furthermore, we also specify the methodology that should be adopted during the performance evaluation, while reviewing the numerous models and tools that are available to the research community.

In this paper, we explore the role of simulators and testbeds in the development procedure of protocols or applications for Wireless Sensor Networks (WSNs) and Internet of Things (IoT). We investigate the complementarity between simulation and experimentation studies by evaluating latest features available among open testbeds (e.g., energy monitoring, mobility). We show that monitoring tools and control channels of testbeds allow for identification of crucial issues (e.g., energy consumption, link quality) and we identify some opportunities to leverage those real-life obstacles. In this context, we insist on how simulations and experimentations can be efficiently and successfully coupled with each other in order to obtain reproducible scientific results, rather than sole proofs-of-concept. Indeed, we especially highlight the main characteristics of such evaluation tools that allow to run multiple instances of a same experimental setup over stable and finely controlled components of hardware and real-world environment. For our experiments, we used and evaluated the FIT IoT-LAB facility. Our results show that such open platforms, can guarantee a certain stability of hardware and environment components over time, thus, turning the unexpected failures and changing parameters into core experimental parameters and valuable inputs for enhanced performance evaluation.

Part 3: DTN and Sensor NetworksInternational audienceThis paper presents a description of SensLAB(Very Large Scale Open Wireless Sensor Network Testbed) that has been developed and deployed in order to allow the evaluation through experimentations of scalable wireless sensor network protocols and applications. SensLAB’s main and most important goal is to offer an accurate open access multi-users scientific tool to support the design, the development tuning, and the experimentation of real large-scale sensor network applications. The SensLAB testbed is composed of 1024 nodes over 4 sites. Each site hosts 256 sensor nodes with specific characteristics in order to offer a wide spectrum of possibilities and heterogeneity. Within a given site, each one of the 256 nodes is able both to communicate via its radio interface to its neighbors and to be configured as a sink node to exchange data with any other ”sink node”. The hardware and software architectures that allow to reserve, configure, deploy firmwares and gather experimental data and monitoring information are described. We also present demonstration examples to illustrate the use of the SensLAB testbed and encourage researchers to test and benchmark their applications/protocols on a large scale WSN testbed

Abstract-Sensor networks consist of autonomous nodes with limited battery and of base stations with theoritical infinite energy. Nodes can be sleep to extend the lifespan of the network without compromising neither area coverage nor network connectivity. This paper addresses the area coverage problem with equal sensing and communicating radii. The goal is to minimize the number of active sensors involved in coverage task, while computing a connected set able to report to monitoring stations. Our solution is fully localized, and each sensor is able to make decision on whether to sleep or to be active based on two messages sent by each sensor. The first message is a "hello" message to gather position of all neighboring nodes. Then each node computes its own relay area dominating set, by taking the futhest neighbor as the first node, and then adding neighbors farthest to the isobarycenter of already selected neighbors, until the area covered by neighbors is fully covered. The second message broadcasts this relay set to neighbors. Each node decides to be active if it has highest priority among its neighbors or is a relay node for its neighbor with the highest priority.