# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### LEACH-Based Routing Protocols in WSNs
#### 2.3.1 Overview of the LEACH Protocol  
The idea of the LEACH protocol [19] is to improve the efficiency of WSNs by reducing power consumption. A rotation technique of CH selection using a random factor has been proposed to achieve this goal.  
The LEACH protocol operates in many rounds [63; 64], and each of them comprises two stages, which are the set-up and the steady-state. The CHs are selected and the clusters (collection of nodes) are formed during the first stage. Afterward, a CH announces itself to the cluster members and assigns a time division multiple access (TDMA) table to each member in a cluster [65]. Figure 3 shows the set-up phase in LEACH.  
To select a CH [66; 67], nodes in each cluster create random numbers between 0 and 1 in each round. A reference number \(T(n)\) is predefined to compare with the created numbers. In which the value of the reference number is greater than the value of the randomly created number, then the node that created the random number becomes the CH. Equation 1 is used to determine the value of the reference number \(T(n)\)[65]:  
\[T(n)=\left\{\begin{array}{cl}\frac{p}{1-p\times\left(r\;mod\frac{1}{p} \right)}&:\text{if }n\in G\\ 0&:\text{otherwise}\end{array}\right\} \tag{1}\]  
Figure 3: Processes of the setup phase.  
Figure 2: Sensor node’s components.  
where \(p\) is the percentage of CHs, \(r\) is the existing round, and \(G\) is a group of nodes that did not become CH in the prior \(1/p\) rounds. In this manner, the energy is distributed among all nodes in the cluster. After CHs are chosen, a decision will be taken by each normal node to decide which CH is suitable to join with to form clusters [68]. CH sends an advertisement message to all other nodes. After that, each node selects the appropriate CH to connect with it depending on the strength of a signal, if the advertising CH is close to a node, then the signal has high strength and vice versa. When a node selects its potential CH, it transmits a message to that CH to be a member of a cluster. Then, the CH sends a timetable to other member nodes in its cluster.  
The table is used to avoid data collision during transmission [65; 66; 67; 68; 69]. During the steady-state stage, each CH collects data, which are sent by sensor nodes in its relative cluster, and transmits them to the BS [70]. The LEACH protocol has many advantages [71]. It balances power consumption among all nodes in each cluster due to the rotation of the CH selection process. This procedure can increase the networks lifetime.  
Furthermore, using the TDMA table allows nodes to be in sleep mode while waiting for their turn to send their data; this condition also increases the network lifetime. By contrast, several disadvantages exist. For example, LEACH does not take into account the residual energy of a node in the selection process of a CH. According to Equation (1), the possibility of a node becoming a CH is not fixed. After several rounds, all nodes have the same probability to become CHs. Thus, nodes with a high amount of energy and nodes with a low amount of energy have an equal chance to become a CH.  
Another problem is the direct transmission from a CH to the BS using single-hop communication. This situation would be a serious issue if the CH and the BS were located in far positions because it uses more energy than a CH that is close to the BS, resulting in energy holes in which isolated nodes are unable to transmit their data. This situation considerably affects the performance of the network [72; 73]. To overcome these disadvantages of LEACH, many improvements have been proposed by modifying LEACH to increase efficiency, as presented in Section 2.3.2.