# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### Network Coverage  
Network coverage is important when designing routing protocols. It is necessary for each routing protocol to cover the entire network and efficiently distribute the nodes in the target space to achieve this type of coverage. The protocols that depend on network coverage are discussed in detail in the following paragraphs. In [96], LEACH and PEGASIS protocols were combined to develop a new reliable protocol for routing data in WSNs called P-LEACH. The idea of data transmission in clusters was utilized along with chain formations that are used in LEACH and PEGASIS to increase the network coverage. In P-LEACH [96], the node with the maximum level of energy is selected by other nodes to be a CH. After determining the CHs in each cluster, a chain is created with the CHs. The nearest CH to the BS becomes the leader of the chain. When a CH collects data that are sent by other nodes in a cluster, it transmits these data to the next CH in the chain until it reaches the BS. In P-LEACH, if a CH has minimum energy and cannot send data, then the node with the second highest value of energy will be a new CH to ensure efficient operation in the network and keep the coverage of the network stable. The integration of the best features in LEACH and PEGASIS into the P-LEACH protocol leads to good performance and improved network coverage in WSNs. Another protocol in this subcategory is the Cell LEACH protocol (C-LEACH) [97], which was proposed to increase network coverage in WSNs. Different from P-LEACH, C-LEACH divides the entire network into sets of cells with many sensor nodes in each cell. Every adjacent seven cells are grouped together and form one cluster. In each cell, there is a private node named the cell head. The function of this node is to allocate a time slot using TDMA to cell members, aggregate data from other nodes, and send these data to the respective CH. The same thing is performed to transmit data to the BS using the optimal route. This protocol has many advantages. It has good network coverage due to the technique of dividing networks into cells. Moreover, it increases the energy efficiency. The disadvantages of this protocol are the increased of control packet overhead and the complexity due to the different head nodes in this protocol, which are cell heads and cluster heads. Another protocol that provides high network coverage is Orphan LEACH (O-LEACH) [98]. Figure 6 shows the technique used in this protocol to partition the network into cells and to form a cluster with seven nearby cells. \begin{table}
\begin{tabular}{c c c} \hline
**Protocol** & **Strengths** & **Weaknesses** \\ \hline IB-LEACH [93] & Uniform energy distribution in each cluster. & \begin{tabular}{c} Control message overhead for \\ selecting CHs and aggregators. \\ \end{tabular} \\ \hline Partition-LEACH [94] & \begin{tabular}{c} Balanced distribution of energy among nodes \\ and increase data transmission efficiency. \\ \end{tabular} & \
\begin{tabular}{c} Extra overhead because of the \\ mobility of the sink. \\ \end{tabular} \\ \hline FT-LEACH [95] & Fault tolerance &
\begin{tabular}{c} Re-clustering and duplicate data \\ management are not discussed. \\ \end{tabular} \\ \hline \end{tabular}
\end{table} 
Table 7: Summary of energy efficiency-based cluster formation protocols. Sensor nodes are connected to CHs in each cluster, but several nodes are still not connected. These nodes are considered orphan nodes. These nodes transmit data to gateway nodes. Afterward, the gateway nodes play the role of a CH by gathering and transmitting data to the BS. Another way that has been proposed in this protocol is to form a special cluster of orphan nodes and to select a private CH, which belongs to that special cluster. The task of this CH is the same task as the traditional CH but in this case, the communication is done between the special CH and the gateway. This protocol provides great network connectivity and good coverage, but the problem is the delay in data delivery.