# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### Residual Energy  
The CH selection process in the clustering protocols can be conducted depending on the lasting energy of each node in a cluster. Residual energy exerts a major impact on the performance of a network and the networks lifetime. Specifically, if the node selected to be a CH has the highest level of energy among all nodes in the cluster, this will maximize the network lifetime, and the probability of a CH dying early will decrease. The subsequent paragraphs discuss the protocols that belong to this group in detail. The centralized cognitive LEACH (CogLEACH-C) protocol overcomes a major problem in WSNs, namely, the residual energy in a CH [79]. A centralized spectrum-aware algorithm for the cognitive radio sensor network (CogLEACH-C) is used to address the level of energy to select a CH, in addition to the number of idle sensed channels. This protocol is centralized. Thus, the BS is responsible for determining if a node will become a CH or not. First, each node transmits a message to the BS with its ID, its position in the network, and its residual energy. Second, the BS computes the total remaining energy and how many idle channels in a network. Third, the decision for each node to be a CH or not is determined by the BS. The major advantage of the CogLEACH-C protocol is that network lifetime is significantly increased because of the mechanism used to select a CH. Thus, the node with a high level of energy has a high probability to become a CH, thereby prolonging network lifetime. However, CogLEACH-C suffers from disadvantages. It uses location information metrics in messages sent to the BS to select the CH node; this process is costly because of the usage of GPS. Moreover, the nodes' residual energy is the key metric in CogLEACH-C. Therefore, CH selection is directly affected by the total residual energy in the network. CogLEACH-C also relies on the BS in calculating the probability of nodes to become CHs. Therefore, message overhead in the network increases because of the information sent from each node to the BS. Another protocol in this subcategory is vice CH (V-LEACH) [80]. Different from LEACH, this protocol uses a vice CH node via the residual energy of nodes, thereby increasing the efficiency of selecting the CH. Figure 5 shows the architecture of the V-LEACH protocol. Figure 5: V–LEACH architecture [80]. In V-LEACH, a new mechanism is proposed to avoid the early death of CHs due to consuming more energy than member nodes do. Thus, nodes are reorganized and CHs are reselected based on the vice CH (VCH) concept [81; 82; 83]. When the CH has the lowest level of energy, it selects a VCH depending on the residual energy and then sends the VCHs ID to all other nodes in a cluster. Thus, normal nodes will begin to transmit data to the VCH instead of the previous CH, which will become a standard node. In V-LEACH, network lifetime is significantly increased due to selecting a vice CH instead of a re-clustering operation, thereby saving energy in the entire network. Moreover, the efficiency of sending data to the BS is increased because the data are sent to the BS even the death of a CH by using the vice CH. Thus, sending packets to the BS is guaranteed in this protocol. However, the transmission cost is not considered in this protocol. The improved LEACH (I-LEACH) protocol concentrates on the lasting energy of sensor nodes in a cluster [84]. In the LEACH protocol, a CH is selected depending on a predefined threshold without considering the remaining energy of a node. This approach has a seriously negative influence on the network lifetime when a little energy node becomes a CH. The threshold formula in LEACH is updated by introducing the remaining energy factor, as presented in Equations (2) and (3) [84]:  
\[T(i)=\left\{\begin{array}{c}\frac{  
Energy Efficiency  
One of the basic issues in routing protocols is selecting a CH with the highest energy efficiency. This selection exerts a significant influence on the overall network lifetime.