# Energy Efficiency for Green Internet of Things (IoT) Networks: A Survey +
## 3 Literature Review
### Taxonomy of Energy Consumption Solutions
#### 3.7.1 Routing Protocols  
One of the most significant current discussions of IoT based on WSNs, is the generation of an unprecedented amount of data [37], and how to select optimal paths for transmitting such vast quantities to the final destination. Energy-efficient routing algorithms are implemented to decrease the energy depletion and lengthen the network lifetime for these sensor networks. Several routing algorithms have been investigated by various authors to conserve energy. Most involve investigating the concept of using clustering the sensing field and CH nodes selection, electing the optimal path from the source node to the required destination, or manipulating the location of the BS [88]. 1. Cluster-Head Node Selection Various strategies are used in the literature for CH nodes election process to optimize energy usage. The most common three are: low energy adaptive clustering hierarchy (LEACH) [84], hybrid, energy-efficient and distributed protocol (HEED) [89] and power-efficient gathering in sensor information systems (PEGASIS) [90]. We present a brief survey of LEACH, HEED and PEGASIS in which nodes are partitioned in many forms for data collection and communication protocols. 2. **Low Energy Adaptive Clustering Hierarchy (LEACH)** LEACH is one of the most interesting strategies, in which the CH node is elected based on a probabilistic approach and the amount of energy remained of the CH and the system is rotated at different time intervals [84]. A sensor node that has already been the CH cannot be elected again for some rounds. The selected CH node broadcasts to the network and creates a schedule for each node in its cluster to send its data. Each node connects to the CH with a single hop and chooses a random number between 0 and 1, then compares the number with a threshold value \(T(n)\). A node becomes a CH in each round if the random number is less than the following threshold: \[T(n)= \begin{cases}\frac{1}{1-P(r\mod 1/p)}&\text{if }n\in G\\ 0&\text{if }n\
ot\in G\end{cases} \] (2) where \(G\) is a group of sensor nodes that have not been picked as CH node in the previous \(1/p\) rounds. \(r\) defines the most recent round, \(P\) is the required percentage of CH. The CH node collects the information from all sensors connected to it, compresses the information and then forwards it to the ultimate receiver. Every node will be in standby mode except when sending to its CH. Figure 13 shows a cluster organization for the LEACH protocol. Figure 13: Example of LEACH protocol architecture. Several studies have been published on modifications of the LEACH protocol, such as LEACH-C and energy-balanced LEACH [91, 92].These studies tried to overcome the problems associated with LEACH (i.e., random process selection of CHs) and further minimize the total energy consumption for WSNs. In the LEACH-C scheme, each node in the sensing field can calculate its energy level and send the information about its location (possibly using GPS) to the intended destination. The intended destination uses a centralized clustering algorithm to select the CH nodes. Once the clusters and related CH nodes are computed, then the BS chooses a node with more energy and broadcasts a packet to all sensor nodes that consist of the ID of each CH node. If the ID matches, then a node is the selected CH node and its intended destination is the BS. Otherwise, a node must gather and forward the information to the CH node. LEACH-C provided better clustering and longer lifetime than the LEACH protocol. However, energy-balanced LEACH (E-LEACH) enhances the CH node election by considering the remaining energy of each node. Initially, each node has the same residual energy, and the CH nodes are elected randomly. From second round, each sensor node with the highest remaining energy will become the CH node of its cluster. The E-LEACH protocol uses master cluster heads (MCH) to relay packets for those CH nodes that are away from the required destination. Similarly, [PERSON], et al.