# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### Residual Energy
Given that WSNs suffer from dynamic and reselecting CHs in each round, distance is important in selecting a CH due to the main function of a CH, which is sending aggregated data to the BS. Thus, the distance will affect the process of data transmission. The following paragraphs discuss CH selection-based protocols that focus on the distance factor. Improved-LEACH is another enhancement of LEACH, and it is one of the protocols based on distance when selecting a CH [90]. Two factors are considered in this protocol when selecting a CH: the level of energy in nodes and the distance from each node to the BS. Accordingly, Equations (14) and (15) are used to find a lasting energy for each node in a cluster, and the distance to the BS, respectively:  
\[SCRN1=\ \frac{E_{current}}{E_{max}} \tag{14}\]  
where \(E_{current}\) denotes the energy level in a node and \(E_{max}\) represents the maximum energy, and:  
\[SCRN2=\ \frac{d_{bs}}{d_{far}} \tag{15}\]  
where \(d_{bs}\) denotes the distance from a node to the BS and \(d_{far}\) denotes the distance from the farthest node in a cluster to the BS. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Protocol** & **Strengths** & **Weaknesses** \\ \hline EEM–LEACH [86] & Energy consumption and packet delivery & Packet delay increases because of computing the residual energy for relay nodes \\ \hline EMOD–LEACH [87] & Efficient CH replacement scheme enhances network lifetime & Unsuitable for periodically sensing data \\ \hline EHA–LEACH [88] & Using energy harvesting nodes increases network lifetime & High complexity \\ \hline LEACH–MAC [89] & Energy efficiency due to consider the number of CH advertisements. & Complexity and extra overhead. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Summary of energy efficiency-based CH selection protocols. for the cluster. Afterward, the neighbor nodes will be sorted according to their residual energy by executing a sorting algorithm. This protocol has several advantages. For example, it selects nodes with the highest remaining energy to send data to the BS, thus increasing the network lifetime. Moreover, this protocol provides a good ratio of packet delivery. However, complexity increases due to the use of several techniques. Energy Efficiency  
This factor is important because of its influence on the design of routing protocols. The main goals of protocols belonging to this subcategory are to prolong network lifetime and minimize energy consumption by using factors that help in achieving these goals. The following paragraphs discuss these protocols in detail. The intra balanced LEACH protocol (IB-LEACH) [93] was presented to minimize the energy consumed between sensor nodes and their CH inside a specific cluster and to reduce the load on a CH by distributing its tasks among all nodes in the cluster. IB-LEACH has several rounds as mentioned before in the LEACH protocol, but in IB-LEACH, each round comprises three stages. Set-up and steady-state stages are similar to the stages in LEACH. In the new stage, which is called the pre-steady, nodes in each cluster are tagged as CH, sensor nodes, and aggregator. Sensor nodes sense events and then transmit data to the aggregator. The main jobs of the aggregator node are the aggregation of data and the transmission of them. The task of a CH is to manage the activities, such as creating the TDMA schedule and distributing it among nodes. In this manner, the tasks are distributed among all nodes in the cluster, which results in minimizing the load on the CH. The main advantages of the IB-LEACH protocol are as follows: (1) network lifetime significantly improves (2) and energy consumption declines because of the identical distribution of energy in a cluster. However, the drawback of this protocol is the extra overhead in messages that are circulated to select CHs and aggregators. To equally split the energy among nodes in wide area networks, and to increase the performance of data transmission, Partition-LEACH was presented in [94].