# Green IoT: A Review and Future Research Directions
## 3 Green Hardware IoT Framework  
This section represents a brief overview of the four (4) green hardware IoT frameworks and includes a summary of proposed energy-saving technology classifications in Figure 3. The following sections cover more detailed information on these topics. * **Machine-to-Machine (M2M):** M2M technology refers to the ability of smart devices to communicate with each other and exchange data without the need for human intervention. It forms the basis of IoT and allows for the creation of smart systems that can operate and make decisions independently [16]. One of the key applications of M2M technology is in the field of smart cities [11], where it can be used to optimize the performance of infrastructure such as traffic systems, public transportation, and utilities. M2M communication can also be utilized in industrial settings to improve efficiency and productivity, as well as in healthcare to enable remote monitoring and treatment of patients. In order to achieve energy efficiency in M2M systems, various strategies can be employed (as shown in Figure 3). These may include the use of low-power communication protocols, the implementation of energy-efficient protocols for data transmission, and energy-harvesting techniques to power M2M devices. Overall, M2M technology plays a crucial role in the advancement of the IoT and has the potential to substantially improve the efficiency and sustainability of a wide range of systems. * **Wireless Sensor Networks (WSNs):** WSNs are networks of small, energy-efficient devices equipped with sensors that are capable of wirelessly communicating with each other and with a central hub. WSNs can be used to monitor and collect data from various environments and are commonly deployed in a broad range of applications, including industrial monitoring, healthcare, and environmental monitoring. In order to make WSNs more energy-efficient, various strategies can be employed. These may include the use of energy-efficient communication protocols, the implementation of energy-harvesting techniques to power the sensors, and the use of energy-efficient storage and data processing methods (as depicted in Figure 3). One promising research context in the field of WSNs is the use of machine learning algorithms to improve energy efficiency. These algorithms can be applied to streamline data transmission and lower the overall network energy usage. Overall, WSNs have the potential to greatly boost efficiency and sustainability and will likely continue to be an important area of research in the field of IoT [17]. * **Radio-Frequency Identification (RFID):** RFID is a wireless technology that identifies and tracks objects using radio waves. It consists of a small chip, known as an RFID tag, which is associated with an object and a reader that is able to detect and communicate with the tag. RFID technology has a wide range of applications, including inventory tracking, supply chain management, and asset management. The technologies for improving energy efficiency may include the use of low-power communication protocols, the implementation of energy-efficient data processing techniques, and the use of energy-harvesting methods to power RFID devices. One promising area of research in the field of RFID is the development of passive RFID tags, which do not require an energy source and can operate indefinitely as long as they are within range of an RFID reader [18]. * **Microcontroller Units (MCUs) and Integrated Circuits (ICs):** MCUs and ICs are key components of many electronic devices, including those used in the IoT. MCUs are small computers that are used to control and monitor the operation of a device, while ICs are electronic circuits that are used to process and transmit information. The strategies for enhancing the energy efficiency of MCUs and ICs may include the use of low-power design techniques, the implementation of energy-efficient data processing algorithms, and the incorporation of energy-harvesting methods to power the devices.