# Green IoT: A Review and Future Research Directions
## 5 Energy-Efficient and Eco-Sustainable Wireless Sensor Networks  
Wireless sensors are small, inexpensive devices that run on batteries and are able to detect specific events or send regular updates about their surroundings. They are an important component of smart applications that utilize IoT technology. The deployment of a group of interconnected wireless sensors is referred to as a WSN. A typical WSN consists of multiple source nodes (wireless sensors) connected to a base station (sink node). In WSN topology, one of the sensor nodes acts as the network coordinator and assumes the role of the base station (BS) or head node (HN). The HN performs important network functions such as scheduling, resource allocation, interference management, and routing, and acts as a gateway to the larger network [40]. A summary of low-power wireless communication technologies for sensors can be found in [41]. In addition, Table 2 compares various wireless technologies using key parameters such as standard, energy consumption, frequency band, data rate, transmission range, and cost. Table 3 shows how well these technologies are suited for IoT applications.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameters** & **LoRa** & **Bluetooth** & **LR-WPAN** & **Mobile** & **WiMAX** & **WiFi** \\ \hline \multirow{3}{*}{Standard} & \multirow{3}{*}{LoRaWAN R1.0} & & & 2G-GSM, CDMA & \multirow{3}{*}{IEEE 802.16} & \multirow{3}{*}{IEEE 802.11} \\  & & & IEEE 802.15.4 & & 3G-UMIS, & \\  & & & (ZigBee) & CDMA2000 & & a/c/b/d/g/n \\ \cline{1-1} \multirow{3}{*}{Energy consumption} & \multirow{3}{*}{Very Low} & Bluetooth: & & 4G-LTE-A & \multirow{3}{*}{IEEE 802.11} \\  & & Medium; BLE: & Low & Medium & Medium & High \\ \cline{1-1} \cline{4-5}  & & Very Low & & 868/915 MHz, & 865 MHz– & 2–66 GHz & 5–60 GHz \\ \cline{1-1} \cline{4-5}  & & & 2.4 GHz & 2.6 GHz & 1 Mb/s\(\sim\)1 Gb/s & \\ \cline{1-1} \cline{4-5}  & & & & (Fixed) & \multirow{3}{*}{1 Mb/s\(\sim\)6.75 Gb/s} \\ \cline{1-1} \cline{4-5}  & & & & & (mobile) & \\ \cline{1-1} \cline{4-5}  & & & & & & \multicolumn{1}{c}{} \\ \cline{1-1} \cline{4-5}  & & & & & \multicolumn{1}{c}{} \\
Wireless sensors are used to regularly and autonomously send data over a long period of time, especially in critical applications such as weather forecasting, traffic reporting, water quality monitoring, healthcare, and embedded systems. Wireless sensors are powered by batteries, but the cost of replacing or maintaining exhausted batteries can be prohibitive, especially in remote locations that have difficult terrain, making access to these sites challenging [44]. Due to geographical constraints, battery replacement may prove to be challenging in certain situations. Therefore, it is essential to develop energy-efficient solutions in order to extend battery life and reduce replacement costs for the sustainability of WSN. There have been numerous efforts to propose energy-efficient solutions for WSN. The major existing energy-saving mechanisms for green WSN are summarized in Figure 6. Below, a detailed discussion of the current energy-saving mechanisms will be presented with the goal of creating a more environmentally friendly WSN.