Section: New Results

Industry 4.0 and Wireless Sensor Networks

By the year 2020, it is expected that the number of connected objects will exceed several billions devices. These objects will be present in everyday life for a smarter home and city as well as in future smart factories that will revolutionize the industry organization. This is actually the expected fourth industrial revolution, more known as Industry 4.0. In which, the Internet of Things (IoT) is considered as a key enabler for this major transformation. IoT will allow more intelligent monitoring and self-organizing capabilities than traditional factories. As a consequence, the production process will be more efficient and flexible with products of higher quality.

Several standards have been designed for industrial wireless sensor (IoT) networks such as WirelessHart and ISA100. Both of them are based on the IEEE 802.15.4 standard for the lower layers. More recently, Time Slotted Channel Hopping (TSCH) which is specified in amendment e of the IEEE 802.15.4 standard, uses a time slotted medium access operating on several channels simultaneously. In addition, radio perturbations are mitigated by frequency hopping. TSCH supports star and mesh topologies, as well as multi-hop communication. It has been designed for process automation, process control, equipment monitoring and more generally the Internet of Things. It is a candidate technology for the Industry 4.0. In fact, Industry 4.0 will use more and more the on-demand manufacturing in a highly fexible and widespread environment. Different supply chains located in various regions need to coordinate their actions in a real-time basis with high fidelity. The IoT communicating in a wireless manner will play a major role to achieve this target. Time Slotted Channel Hopping (TSCH) networks are emerging as a promising technology for the Internet of Things and the Industry 4.0 where ease of deployment, reliability, short latency, flexibility and adaptivity are required. However, the strong requirements in terms of short latency and high reliability of such applications are obstacles to its penetration in the Industry 4.0. That is why in 2017 we made three contributions dealing with:

• how to quickly build a TSCH network;

• how to increase the reliability of end-to-end communications;

• how to efficiently schedule the transmissions made for data gathering.

Building an IEEE 802.15.4e TSCH network

Participants : Ines Khoufi, Pascale Minet.

The IEEE 802.15.4e amendment has been designed to meet the requirements of industrial applications with regard to the wireless sensor networks supporting them. Because of its scheduled medium access and multichannel transmissions, the TSCH mode has received much attention. In this study, we focus on the time needed by a node to detect a beacon sent by a TSCH network, as well as on the time needed to build a TSCH network. These times are important for industrial applications where new nodes are inserted progressively, or when failed nodes are replaced. Both times highly depend on the beacon advertisement policy, policy that is not specified in the standard and is under the responsibility of a layer upper than the MAC one. Since beacons are broadcast, they are lost in case of collisions: the vital information they carry is lost. The main problem is how to avoid collisions between two devices that are not neighbors. That is why we propose DBA, a Deterministic Beacon Advertisement algorithm that ensures a regular transmission of beacons without collisions. The goal of DBA is to ensure that beacons are transmitted on all frequencies used by the TSCH network, regularly and without collision. With DBA, the exact value for the maximum time for a joining node to detect a beacon can be computed easily. We use the NS3 Simulator to evaluate this time as well as the the number of message losses, considering different network topologies (star or multihop). Simulation results show that DBA clearly outperforms existing solutions such as Random Vertical and Random Horizontal, two algorithms existing in the state of the art. In addition, DBA is able to provide the exact value of the maximum joining time.These results have been presented at the EUCASS 2017 conference, see [31].

Increasing the reliability of an IEEE 802.15.4e TSCH network

Participants : Ines Khoufi, Pascale Minet.

Our goal is to improve reliability of data gathering in such wireless sensor networks. We present three redundancy patterns to build a reliable path from a source to a destination. The first one is the well-known two node-Disjoint paths. The second one is based on a Triangular pattern, and the third one on a Braided pattern. The reliability provided by each pattern, the delivery time and the overhead in terms of the number of transmissions generated by each pattern as well as the amount of energy consumed by an end-to-end transmission allows us to conclude that the Braided pattern provides the highest reliability but with an overhead approximately twice the overhead of the Disjoint-path pattern and 4/3 the overhead of the Triangular pattern. These performance results are corroborated by simulations performed with NS3 for various configurations. This result has been presented at the NCA 2017 conference ([21]).

Scheduling transmissions in an IEEE 802.15.4e TSCH network

Participants : Ines Khoufi, Pascale Minet.

TSCH provides a multichannel slotted medium access ruled by a periodic schedule and supports multihop communications. This schedule is repeated every slotframe. A slotframe consists of a set of cells, each cell is identified by a (time slot offset, channel offset) pair. The size of a timeslot (e.g. 10 ms by default) allows the transmission of a point-to-point frame and its immediate acknowledgment. The schedule defines for each cell the nodes allowed to transmit and those that should receive. The channel offset is translated into a physical channel depending on the channel hopping sequence of the TSCH network. Channel hopping allows the TSCH to increase its robustness against external perturbations of the radio signal.

In the paper presented at VTC-Fall 2017 [20], we study how applications with data delivery constraints can be supported by a TSCH network. We first propose a framework based on a multislotframe that allows the coexistence of Data Slotframes and Control Slotframes. We then determine a lower bound on the minimum number of slots required to perform data gathering, taking into account the number of channels, the number of interfaces of the sink, the number of packets generated by each sensor node as well as the number of children of the sink. These feasibility conditions are established for two cases: with spatial reuse and without. We propose a debt-based scheduler that for simple topologies, provides a schedule minimizing the slotframe size. We determine the conditions for which an increase in the number of channels or sink's interfaces leads to a shorter data delivery delay. We compare the number of slots needed by data gathering with and without spatial reuse for small configurations. Finally, we consider a network configuration representative of an industrial application and evaluate the performance of the TSCH network in terms of data delivery delay and queue size for each sensor node, using the NS-3 simulator, where the multislotframe has been integrated. Simulation results Simulation results showed that the maximum theoretical delivery delay is never exceeded and the number of messages in the Transmit queue of each sensor node remains small. In addition, the debt-based scheduler builds a valid schedule with the minimum number of slots for the industrial application considered. we can conclude that TSCH with its time-slotted and multichannel medium access provides an efficient support for data gathering.