# Green IoT: A Review and Future Research Directions
## 5 Energy-Efficient and Eco-Sustainable Wireless Sensor Networks
### Energy-Efficient WSN Architecture and Routing Protocols  
Cluster architecture is an effective method for increasing the energy efficiency of WSNs and ensuring scalability by maintaining a hierarchical structure. This approach involves grouping sensor nodes into clusters, with each cluster being led by a chosen node known as the cluster head. The cluster head is responsible for managing the activities of the members and communicating with other cluster heads or the base station [74; 75]. This method of organization is illustrated in Figure 9.  
Cluster architecture can improve energy saving in WSN in several ways:  
* It decreases the communication distance within the cluster, which reduces the need for high transmission power. * It reduces the number of transmissions by leveraging data fusion at the cluster head. * It cuts down energy-consuming activities such as coordination and data aggregation by distributing them to the cluster head. * It allows for some nodes to be powered off within the cluster, as the cluster head assumes forwarding responsibilities. * It distributes energy consumption evenly among nodes by rotating the cluster head position. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Technique** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} & **Charging Distance** \\ \hline Magnetic inductive coupling & \(\bigcirc\) & Simple implementation. & \(\bigcirc\) & Short charging distance. & \\  & \(\bigcirc\) & Safe for humans. & \(\bigcirc\) & Needs tight alignment between & \\  & & & & chargers and charging devices. & \\  & & & & Heating effect. & \\ \hline  & \(\bigcirc\) & Loose alignment. & & \\  & \(\bigcirc\) & Charging without the need for a & & \\  & & direct line of sight. & \(\bigcirc\) & Limited charging distance. & \\  & \(\bigcirc\) & Ability to charge multiple devices & \(\bigcirc\) & Complex implementation. & Near-field. \\  & & simultaneously with different & & \\  & & power levels. & & \\  & \(\bigcirc\) & High charging efficiency. & & \\ \hline  & & & & \\  & & & & \\  & & & & \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: A summary of the advantages, disadvantages, and effective charging distance of the energy transfer techniques. Unfortunately, uneven distribution of sensor nodes can result in depletion of energy in certain areas or create energy \"holes\". However, this can be mitigated by properly placing the nodes through optimal distribution or by adding a few relay nodes with increased capabilities. This improves energy balance among the nodes, avoids sensor hotspots, and ensures coverage and connectivity. Many studies have been conducted on finding the minimum number of relay nodes or placing them in the most efficient way to extend the network lifetime [76; 77; 78]. In general, single-path routing protocols are more straightforward compared to multipath routing protocols, However, they can quickly deplete the energy of nodes in the chosen path. Additionally, when a node runs out of power, a new route must be recalculated in single-path routing. On the other hand, multipath routing distributes energy among nodes by alternating forwarding nodes and also improves network reliability by providing alternative routes and allowing the network to recover quickly in case of failure. For more detailed information on multipath routing protocols for WSN, a comprehensive survey can be found in [79]. One energy-efficient multipath routing protocol for WSN is the energy-efficient multipath routing protocol (EEMRP), which is described in [80]. This protocol discovers multiple paths that do not share nodes by using a cost function based on the energy levels and hop distances of the nodes. and then, it allocates traffic to each of the selected paths. Additionally, the energy-efficient and collision aware (EECA) protocol, described in [81], puts forward two routes between a source and a sink that are not only node-disjoint but also free from collisions. [PERSON] et al.