# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### Residual Energy
A CH selection-based routing protocol that concentrates on residual energy is not energy-efficient because it does not consider other factors, like the communication cost, energy consumption, and energy harvesting. Moreover, many existing routing protocols select the CH depending on residual energy. Specifically, CogLEACH-C, V-LEACH, I-LEACH, and LEACH-1R use only a residual energy parameter. EEM-LEACH and EMOD-LEACH use a residual energy parameter with a transmission cost metric, EHA-LEACH uses a residual energy factor with energy harvesting capacity, and LEACH-MAC uses a residual energy criterion with the number of CH advertisements. The following paragraphs present routing protocols that emphasize the energy efficiency factor for selecting CHs. To improve CH selection in LEACH and enhance the transmission of data, a new protocol called energy efficient multi-hop (EEM-LEACH) was proposed [86]. To select a CH, a threshold is calculated by specifying the residual energy. The new equation of the threshold value is:  
\[T(n)=\left\{ \begin{array}{cl}\frac{P}{1-P\times\left(r\;mod\;\frac{1}{P} \right)}\times P(RE)&:&\text{if }n\;\in\;G\\ 0&:&\text{otherwise}\end{array} \right\} \tag{5}\]  
where \(P(RE)\) is the residual energy of the node and is calculated as follows:  
\[P(RE)=\left\{ \begin{array}{cl}\frac{E_{res}-E_{avg}}{E_{res}}:&\text{if }E_{res}>E_{avg}\\ 1-E_{avg}&:&\text{otherwise}\end{array} \right\} \tag{6}\]  
In this protocol, the communication cost is computed by each CH when a CH advertisement is received from the BS. The communication cost of node \(u\) can be calculated as follows:  
\[c=\left\{ \begin{array}{cl}C(n)+d_{nm}^{4}&:&d_{nm}\geq\;d_{0}\\ C(n)+d_{nm}^{2}&:&d_{nm}<d_{0}\end{array} \right\} \tag{7}\]  
where \(d_{nm}\) denotes the distance from node \(n\) to node \(m\) and \(C(n)\) helps in constructing a path from a CH to the BS. This enhancement protocol provides an efficient technique for reducing energy consumption and increasing packet delivery. However, packet delay increases significantly because of the calculations in computing the communication cost for each node to select a CH and in computing the nodes remaining energy for specifying the nodes in the path. Moreover, the distance from the BS to a CH is not considered. Enhanced modified LEACH (EMOD-LEACH), a new method that implements the idea of effective CH replacement and double transmitting power with an intracluster transmission manner, was proposed in [87]. To replace a CH, the algorithm checks the energy level in the current CH. If the \begin{table}
\begin{tabular}{c c c} \hline
**Protocol** & **Strengths** & **Weaknesses** \\ \hline CogLEACH–C [79] & Balances energy load in a network via the use of idle channels with residual energy. & Inappropriate for wide networks. \\ \hline V-LEACH [80] & Ensures data delivery because it uses two CHs. & Uses an extra CH which leads to more overhead, single hop communication. \\ \hline I-LEACH [84] & Minimizes average energy consumption for each sensor node. & Computation overhead. \\ \hline LEACH–1R [85] & Network lifetime and energy consumption. & Energy hole problem because of changing a CH after it runs out of energy. \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of residual-based CH selection protocols. level is greater than the specific value, then the current CH will still the head in the following round. Otherwise, the set-up phase will be run again, and the current CH will be replaced to minimize power consumption. The proponents of this protocol proposed two specified values (hard and soft) to minimize data consumption in redundant value sensing in addition to the residual energy. Data transmission process in this method depends on the value of the sensed event if this value is greater than the predefined hard value, and the predefined soft value is less than the difference between the current sensed and the previously sensed values; then the sensor node transmits the sensed data.