## 1 Introduction  
The so called _Internet_ of Things (IoT) is widely considered as one of the most disruptive paradigms accelerating the transition to a novel technological and societal era [1]. An IoT domain can connect a large number of resource-constrained devices (namely the _things_)with heterogeneous characteristics, in terms of category, functionality, manufacturer up to the adopted communications protocols, then requiring novel and flexible network architectures to meet a variety of new applications and users's requirements in real-time [2]. In particular, IoT ecosystem could be considered an extremely relevant and promising component of the future industrial processes, that are alternatively indicated as the Industrial Internet or the Industry 4.0. Industry 4.0 is converting traditional industrial automation and control systems into cyber-physical manufacturing systems by integrating Information Technology (IT) and Operational Technology (OT). In this vision, industrial wireless sensors and actuators networks (IWSANs), may contribute to overcame the drawbacks and constraints of the wired communications technologies usually implemented in industrial scenarios. IWSANs decrease the installations and maintenance costs since they do not requires to deploy or protect cables even in hostile industrial areas. Moreover, they allow easy reconfiguration and, possibly, the support for devices mobility, and, consequently, the control of robot. However, IWSANs and in general the Industrial IoT (IIoT) systems are becoming more complex with growing scales, which leads to a number of significant challenges that need to be addressed, such as increasing energy consumption [3, 4]. In order to provide a general purpose IIoT platform to easily set up a distributed application, it has been investigated the adoption of the so called Web of Things (WoT)approach [5]. The information exchange occurring as an WoT domain interacts with the physical world is characterised by specific features, mainly dictated by the _constrained_ devices which suffer of limited processing power, network bandwidth, and intermittent connectivity. In addition, communications are sporadic and often event-triggered updates, with continuous multiple data flows [6]. To address these limitations, the Constrained Application Protocol (CoAP) has been firstly proposed to adapt HTTP for constrained devices and to enable more advanced interaction patterns w.r.t. HTTP, e.g., the support for multicast [7]. Another promising candidate is represented by the Message Queue Telemetry Transport (MQTT) protocol designed for monitoring applications. It is based on the publish-and-subscribe paradigm, by which Publishers (e.g., sensors) transmit data messages to a Broker, which in turn delivers such messages to interested entities, called Subscribers [8]. This flexible approach places complexity in the Broker. Furthermore, MQTT defines a lightweight header format and requires a small code footprint. Recently, the industrial Open Platform Communications Unified Architecture (OPCUA) adopted the publish/subscriber model in its release 14, and the MQTT messages are used for the broker-based case. Moreover, a version of this protocol, called MQTT for Sensor Networks (MQTT-SN), has been specifically designed to face the constraints of a typical IoT domain with suitable architecture, interfaces and components [9]. In a smart manufacturing environment endowed with IWSANs, actuators and sensors are usually battery powered. This requires a specific maintenance cycle which can became a daunting task with hundreds of devices installed in harsh and hardly accessible locations. As a consequence, an energy-saving approach for IWSANs should be considered to improve the networks lifetime. In principle, two possible methodologies for supplying energy to devices can be considered: Energy Harvesting (EH) and Wireless Power Transfer (WPT) [10]. Both the approaches represent promising methodology but differ conceptually.