# Energy Efficiency for Green Internet of Things (IoT) Networks: A Survey +
## 3 Literature Review
### WSN Communication Architecture  
A WSN is similar to a wireless ad hoc network, since both are self-organized and multihop networks [16, 56]. WSN is used to monitor and record specific phenomena and cooperatively pass data wirelessly through a gateway (base station/sink) to a central location as shown in Figure 6. The more modern WSNs are bi-directional (two-way communication), thus enabling control of the activity of the sensors [57].  
In large-scale networks, the sensory network is often partitioned into subgroups, with each group having both sensor nodes and a single cluster-head (CH) node. This clustering approach has several advantages for sensor networks in terms of energy consumption, delay, network communication, etc. [58].  
Sensor nodes establish connection with each other wirelessly and individually collect data from the surrounding environment, perform simple computation processes and then forward the information to its associated CH node via a single hop or intermediate nodes [59]. The intermediate node serves as a data sender and path for other sensor nodes towards the CH node. These nodes make forwarding decisions (i.e., routing) based on their knowledge of the network [60]. The CH nodes can be elected randomly or based on one or more criteria such as number of neighboring nodes, transmission distance to the final destination, residual energy, where the BS is the master node which gathers the data  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Type of Application** & **Military** & **Habitat** & **Business** & **Public/Industrial** & **Health** & **Environment** \\ \hline \multirow{3}{*}{Tracking} & Enemy & Animal & Human & Traffic & Patient & Tormado \\  & Tracking & Tracking & Tracking & Tracking & Tracking & Tracking \\ \hline \multirow{3}{*}{Monitoring} & Security & Animal & Inventory & Machine & Patient & Weather \\  & Detection & Monitoring & Monitoring & Monitoring & Monitoring & Monitoring \\ \hline \hline \end{tabular}
\end{table}
Table 1: Examples of WSNs Applications.  
Figure 6: WSN communication architecture.  
from all sensor nodes and processes it, and then disseminates these data to the intended destination [61]. The task of the CH node is to gather the information from its member nodes, compress it and then disseminate it to the base station. Most of the literature on WSNs is related to a search for proper clustering, optimal path, and aggregation methods that can significantly minimize the energy depletion and lengthen the network lifetime. Figure 7 reveals a WSN communication architecture which splits the sensing field into two groups and each group has several nodes that are linked with each other to main CH node and these CH nodes are connected to the BS.  
Load balancing technique is an important aspect to maximize the network lifetime of the sensors by minimizing energy consumption. According to an investigation by [62], authors use a new cluster formation scheme in the sensing field to manage the load balancing issue in the deployed area. The CH nodes in the proposed scheme are selected dynamically for a fixed interval. Although the static route is created in the designated clusters by applying the AODV routing algorithm. The scheme balanced the energy consumption of the nodes and thus extended the network lifetime. In another study [63], authors introduce a novel routing protocol called content-based adaptive and dynamic scheduling (CADS). The protocol is used two ways communication model for the WSNs. The CADS is avoided redundant data and thus reduced the forwarding of unnecessary data packets, which as a result, prolongs the network lifetime.