## 1 Introduction  
Industries now represent a rising 22% of the total applications of the Internet of things (IoT) [1], and the industrial Internet of things (IIoT) has become an extremely active research area. The IIoT has application opportunities in a wide variety of fields, from process and manufacturing to logistics [2]. One of the main needs to be addressed is reducing the energy consumption of the IIoT [3]. The information and communications technology (ICT) sector consumes approximately 3.6% of the worldwide energy and produces 1.4% of the CO\({}_{2}\) emissions [4], in which the IoT constitutes an increasingly substantial portion. Projections indicate the proliferation of tens of billions of devices in the coming decade [5]. To overcome these challenges, a new paradigm called the Green IoT has emerged, which focuses on ensuring that the growth of the IoT is sustainable [6], mainly by reducing its energy consumption. The Green IoT relies on three principles: energy provision (the sources of the energy used), energy transfer (the transfer of green energy to the device), and energy efficiency (the minimum energy usage). These three principles in relation to the notion of sustainability are shown in Figure 1.  
The Green IoT involves the application of several techniques, such as energy harvesting, more efficient communication hardware and protocols, or the better usage of the computing capabilities of edge and far-edge devices. The technique on which we focus in this review is wake-up radio (WuR), which is a technique used to explore and enable energy-efficient and eco-sustainable wireless sensor networks in the Green IoT [1]. WuR technology enables a device to wait for a wireless signal prior to activation, independent of an eventual duty-cycle schedule. The duty cycle indicates the fixed fraction or percentage of times a resource is busy, though it is a harsh constraint on device communication capability and reactivity. The transceivers consume most of the energy in IoT devices [7]. In more common duty-cycle systems, a constant trade-off exists: a low duty cycle increases latency, whereas a high duty cycle increases energy consumption. By allowing a device to keep a part of its radio system on while consuming little energy, WuR technology provides a solution to this trade-off issue and allows low latency and low energy consumption [8]. This combination considerably enhances reliability and battery longevity, thereby ensuring low energy consumption and enabling use in batteryless applications [9]. In addition, WuR facilitates innovative and efficient techniques for gathering data on-demand [10]; it is viable as a substitute for conventional communication systems, particularly for time-sensitive applications in Industry 4.0 [8]. In 2021, the IEEE 802.11 standard was amended [11], adding the possibility of WuR operation in standard wireless local area networks (WLANs). The next release of 3 GPP, Release 18, is also expected to include WuR specifications. By using WuR to wake up devices on-demand, synergies could be created with the on-demand IoT (OD-IoT) or service-oriented IoT [12], conceptualized recently as the \"fog of things\" [13], which reuse existing IoT resources to provide infrastructure support to new applications. OD-IoT is a concept that should be extended further and integrated from the outset in devices, as we show that it can improve energy savings, data collection, and process flexibility. We begin by presenting our proposal for the classification of communication-initiation mechanisms used in the IoT and how they enable the analysis of the current IIoT landscape. This is followed by a discussion of the state of the art of WuR technology, including its operating principles, current implementations, and dedicated protocols. We also discuss the connection between WuR and energy harvesting, as well as its implementation in novel data collection methods that enable the OD-IoT. Additionally, we provide a concise overview of unconventional WuR methods employed in specific domains.