# Variants of the Low-Energy Adaptive Clustering Hierarchy Protocol: Survey, Issues and Challenges
## 2 Background and Overview of WSNs
### WSNs  
WSNs are sets of sensors deployed for sensing and monitoring an environment to collect data on natural attributes, such as temperature and pressure [34; 35]. The data collected by each sensor are transmitted to a BS, in which analysis of the collected data is conducted [36]. WSNs are useful to monitor and collect data in areas where humans cannot do so.  
These areas include healthcare (monitoring patients and related medical management), providing emergency services in cases of disasters (e.g., earthquakes), and traffic control [37; 38]. WSNs differ from traditional networks because the former consists of many small nodes called sensors, but these sensors have many limitations [39]. Thus, WSNs have distinctive characteristics, which are listed in Table 1.  
Challenges are also present in the WSN field [47; 48], and researchers continue to develop new methods and strategies to address these challenges. Several of these challenges are summarized in Table 2.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Characteristic** & **Description** \\ \hline Computing capabilities & Sensor nodes in WSNs are small in size and memory and have a limited battery power [40]. \\ \hline Battery energy & Sensor nodes consume energy to operate functions, such as sensing, gathering and transmitting data. Power consumption is one of the challenges in WSNs. Given that the sensors are usually deployed randomly in a large area, if one of these nodes depletes its energy, replacing the power supply is difficult, especially in hostile and inaccessible environments [41]. \\ \hline Communication capabilities & Sensor nodes are affected by their surroundings, including rain, storms, mountains, and buildings. The communication distance is short, and the communication bandwidth is narrow [42]. \\ \hline Dynamic & In WSNs, several sensor nodes may be added or removed from the network according to task requirements. A sensor node may exit from the network when its energy is exhausted or because of other failures, thereby changing the topology of the network [43]. \\ \hline No center, self-organization & In WSNs, installing network infrastructure is unnecessary because a sensor node can adapt itself, and an independent network can automatically be formed after nodes switch on [44]. \\ \hline Multi–hop communications & In WSNs, a sensor node directly communicates with its neighbors, but if a node needs to communicate with other nodes, this communication must be via a multi–hop route through intermediate nodes. In contrast to traditional wired networks that need gateways and routers for multi–hop communication, nodes in WSNs act as routers in addition to their main functions as data collectors and senders [45]. \\ \hline Application relevance & WSNs are different from traditional networks in application relevance. WSNs are highly dependent on applications; thus, the topology, routing protocols, and physical signals in one network cannot be applied to others [46]. \\ \hline \hline \end{tabular}
\end{table}
Table 1: WSN’s characteristics.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Challenge** & **Description** \\ \hline Limited battery power & One of the most significant obstacles in WSNs is power limitation because each sensor node has a limited power supply. Developing methods to maximize network lifetime by saving energy and balancing energy consumption remains a challenge [7; 49]. \\ \hline Limited computation capability & Owing to the small processors, memory modules, and computational capabilities in sensor nodes, designing efficient algorithms that minimize complexities is a challenge [6]. \\ \hline Limited communication capability & The communication range in WSNs is roughly several hundred meters. Thus, developing algorithms to complete the transmission, analysis, and mining of sensed data with minimal communication complexity is important in WSNs [10]. \\ \hline \hline \end{tabular}
\end{table}
Table 2: WSN’s challenges.