# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### Network Coverage
The Energy Zone LEACH (EZ-LEACH) protocol [99] is an enhancement of the LEACH protocol. This protocol differs from previous ones in which the location of nodes is utilized to select a CH after dividing the sensing area into several zones. The advantages of dividing a network into zones are to ensure the equivalence of distances from a CH to other nodes that belong to each zone and to select a fixed number of CHs according to the number of zones in each round. To select a CH, the distance from each node to the center of the zone is the main used parameter. The BS selects nodes that have a minimum distance to the center in each zone. Then, all other nodes join with a suitable CH. Each node sends the information of its distance from the zone center with the energy level, and a new CH is selected according to Equation (17):  
\[Z=\{z\mid\forall\ sensors\in cluster,\ E_{sensor}\geq Thre\ Value\} \tag{17}\]  
\[\forall\ z\in Z:if\ d_{z}<d_{z+1} \tag{18}\]  
\[\ Index\ \gets z \tag{19}\]  
\[\ New\ Cluster-\ Head\ \leftarrow\ Sensor\ Index \tag{20}\]  
where _sensor_ is any sensor in a cluster and \(E_{sensor}\) is the remaining energy in it. \(z,z+1\), and _index_ are indices. The EZ-LEACH protocol prolongs networks lifetime and network coverage due to the efficient way of forming clusters, that depends on the number of zones. Summary of these protocols based on network coverage in cluster formation is presented in Table 8. The strengths and weaknesses of each protocol are also provided. Figure 6: Cells and clusters. The process of sending the data from a source to a destination using an appropriate path is called routing. In WSN, many protocols have been proposed to select an efficient routing path in order to increase the network lifetime. The main issue in routing protocols is the selection of the efficient routing path. Sending data from a CH to the BS using an efficient way is important in prolonging the network lifetime and reducing the power consumption during transmission. The following paragraphs discuss in detail the cluster formation-based routing protocol belonging to this subcategory. A protocol called Cross-Layer LEACH (CL-LEACH) [100] has been proposed by utilizing the cross-layer technique to maximize the lifetime of the WSN. This protocol comprises four stages. In the first stage, which called cluster formation stage, a CH is selected depending on the distance to the BS and the energy level. In the second stage, a route will be created by using the routing mechanism. To find the route, distance is computed using the following equation:  
\[D=\sqrt{\left(x_{2}-x_{1}\right)^{2}+\left(y_{2}-y_{1}\right)^{2}} \tag{21}\]  
where \(D\) is the distance, \((x_{1},y_{1})\) are the coordinates of the source node, and \((x_{2},y_{2})\) are the coordinates of the destination node. During the third stage, the residual energy of each node is compared with a threshold; if it is greater, then the node is considered to be a relay node in the path. If a route between the source and destination is damaged, then the route maintenance technique finds the broken node in a route and substitutes it with another one. This protocol is better than LEACH based on energy consumption and live nodes, but the overhead and complexity are its main drawbacks. Distance is important when forming clusters in WSNs to maximize the network efficiency. When the distance from the CH in each cluster to the BS is short, then energy consumption decreases. Familiar protocols that depend on the distance factor when forming clusters are discussed in detail in the next paragraphs. In this subcategory, the first protocol is the Dual-hop Layered LEACH (DL-LEACH) [101] protocol that uses the multi-hop routing technique in two layers according to the distance to the BS. Lower layer contains nodes that are near to the BS. When these nodes wish to send data, they compare the distance to the CH and to the BS.