Citation: Leonardi, L.; Lo Bello, L.; Patti, G. RESEMBLE: A Real-Time Stack for Synchronized Mesh Mobile Bluetooth Low Energy Networks. Appl. Syst. Innov. 2023, 6, 19. https:// doi.org/10.3390/asi6010019 Academic Editor: Christos Douligeris Received: 6 January 2023 Revised: 21 January 2023 Accepted: 24 January 2023 Published: 26 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article RESEMBLE: A Real-Time Stack for Synchronized Mesh Mobile Bluetooth Low Energy Networks Luca Leonardi , Lucia Lo Bello * and Gaetano Patti Department of Electrical, Electronic and Computer Engineering, University of Catania, Viale A. Doria 6, 95125 Catania, Italy * Correspondence: lucia.lobello@unict.it Abstract: Bluetooth Low Energy (BLE) is a wireless technology for low-power, low-cost and low- complexity short-range communications. On top of the BLE stack, the Bluetooth Mesh profile can be adopted to handle large networks with mesh topologies. BLE is a promising candidate for the implemention of Industrial Wireless Sensor Networks (IWSNs), thanks to its wide diffusion (e.g., on smartphones and tablets) and the lower cost of the devices compared to other wireless industrial communication technologies. However, neither the BLE nor the Bluetooth Mesh specifications can provide real-time messages with bounded delays. To overcome this limitation, this work proposes RESEMBLE, a real-time stack developed on top of BLE that is able to realize low-cost IWSNs over mesh topologies. RESEMBLE offers support to both real-time and non-real-time communications on the same network. Moreover, RESEMBLE provides clock synchronization, thus allowing for Time Division Multiple Access (TDMA) transmissions. The clock synchronization provided by RESEMBLE can be also exploited by the upper layers’ industrial applications to implement time- coordinated actions. Keywords: Bluetooth Low Energy; mesh networks; real-time networks 1. Introduction and Motivation Thanks to Industry 4.0, factories have significantly evolved in recent years. New devices and tools, such as smart sensors, tablets, advanced displays and monitors with effective user interfaces and virtual assistants, are gradually replacing traditional devices and conventional interfaces [1–4]. Moreover, novel functions, such as the monitoring of production levels, control and predictive maintenance of the plant basic facilities and digitalized management of the supply chain, are gaining ground. In this context, low-cost communication systems are one of the enabling technologies for modern industrial applica- tions, as they make it possible to monitor all phases of the production chain [5]. However, industrial communications pose several challenging requirements, such as predictability (i.e., the ability to provide guaranteed upper bounds on the delivery times of time-critical messages), mobility support (i.e., connectivity for mobile nodes) and high reliability (i.e., low error probability and low packet loss). To meet these requirements, industrial commu- nication systems adopt specifically devised protocols that are typically quite expensive and not used beyond the scope of industrial scenarios. This consideration is the motivation behind this work, which presents a real-time stack developed on top of Bluetooth Low Energy (BLE). The proposed approach, called RESEMBLE, enables the use of BLE, i.e., a low-cost, widespread consumer communication technology, in the industrial field. What is new in RESEMBLE is the ability to support real-time communications and node mobility in BLE networks with mesh topologies. The current Bluetooth Mesh specifications do not include support for real-time traffic; therefore, Bluetooth Mesh is not suitable for Industrial Wireless Sensor Networks (IWSNs). To solve Appl. Syst. Innov. 2023, 6, 19. https://doi.org/10.3390/asi6010019 https://www.mdpi.com/journal/asi