## 1 Introduction  
In recent years, technological developments, including digitalization and intelligence in many sectors, have led to the fourth manufacturing revolution, and a new type of industry--Industry 4.0--has emerged. Industry 4.0 aims to digitalize, optimize, and automatize the production process within an energy-efficient communication environment including human-machine and machine-to-machine (M2M) interactions. In this way, it adds significant value to the product life cycle and positively affects the business economy [1]. Figure 1 shows different areas where the implementation of Industry 4.0 can lead to great improvements in terms of smartness, performance, and quality [2]. Nevertheless, its success depends on complex technologies, such as the Internet of Things (IoT), Industrial IoT (IIoT), cloud computing, and software-defined networks (SDNs) [3]. In this scenario, the efficient implementation of these critical enablers has become a crucial subject of research. The IoT concept entails massively connecting smart devices that sense, transmit, process, and feedback data through the Internet. The IIoT, which is the subset of the IoT technology focused on manufacturing applications, is one of the key pillars of Industry 4.0 [4, 5]. Generally speaking, the IIoT is devoted to the industrial value chain where an enormous number of sensors, robots, and devices are connected to the Internet for smart manufacturing purposes. In particular, it is based on M2M interfaces, which enable sensing, monitoring, collecting, transmitting, exchanging, and analyzing data without human intervention [6]. Thus, communication technologies are the foundation of IIoT. Within its framework, three communication layers--sensing, network, and application--can be distinguished. The network layer constitutes a communication neck. This layer includes technologies based on wired sensors, such as Ethernet, and wireless sensors, such as Zigbee, WIFI, Bluetooth, Internet Protocol Version 6 (IPv6), Long Range Wide Area (LoRa), and IPv6 over Low Power Wireless Personal Area Networks (6 LoWPAN) [7; 8]. In general, the need for mobility and wireless connections in the industrial environment is supported by wireless sensor networks (WSNs) and wireless actuator networks (WANs). Wireless sensor networks and WANs are a group of interconnected sensors and actuators that enable data sensing, information gathering, and intelligent processing with less interaction with human actions and decisions. Their capacity to address different challenges in the production line, such as integration, flexibility, and scalability, which help to reduce optimizing costs, makes them essential in the intelligent industry [9]. Wireless sensor networks have been successfully implemented in several industrial scenarios, from environment monitoring to cyber-physical cloud systems [10]. Nevertheless, the increasing demand for WSN-based solutions within the context of the IIoT technology to support Industry 4.0 requires further improvements in their quality of service (QoS). Nowadays, the data generated from factories' stations are massive according to the number of sensors and data required to sense; thus, the energy consumption becomes extraordinary. In this scenario, the need for an efficient implementation of WSNs in terms of energy and end-to-end delay increases are accruing with improvement in the Industry 4.0 field. According to the authors of [11], it is possible to improve power consumption with minor delays, thus solving both issues by efficiently controlling the energy consumption. In this line, different strategies--power control schemes, data aggregation schemes, and energy-efficient routing protocols--have been proposed in the literature to deal with industrial WSNs' energy constraints imposed by their small batteries and processor memories [12; 13].