# Energy Saving in Wireless Sensor Networks via LEACH-Based, Energy-Efficient Routing Protocols
## 1 Introduction  
A typical wireless sensor network (WSN) consists of a set of wirelessly interconnected electronic devices, called sensor nodes, along with at least one sink node called base station (BS), which are placed over a given area that is referred as a field of interest (FOI), as illustrated in Figure 1[1, 2, 3]. As depicted in Figure 2, a typical sensor node consists of the following main components:  
* Sensing module: it contains appropriate sensors that are able to monitor ambient conditions. * Processing module: It typically contains a microcontroller or microprocessor and may also comprise additional memory and processing units. The processing unit plays a critical role in handling the data collected by the sensing unit and controlling the data storage and/or transmission. * Communication module: it is responsible for establishing and maintaining communication between the sensor node and other sensor nodes and/or a BS. * Power supply module: This module refers to the energy source that provides the necessary electrical power to the sensor node. Since sensor nodes in WSNs are often  
application-related problems, concerning congestion, connectivity, coverage, security, quality of service, etc., thus posing numerous challenges and optimization needs [19; 20]. However, the particularly constrained energy adequacy of the sensor nodes is by far the most important among all weaknesses that obstruct the operation of WSNs. Actually, energy scarcity is caused by the fact that, in most WSNs, the sensor nodes are powered by batteries that are difficult or even impossible to either recharge or replace. In a WSN, the energy insufficiency of its nodes is a mostly severe concern, because it may result to the rapid cessation of their operation and, thus, the gradual failure of communications within the network until the overall WSN collapses. This is why the enhancement of the energy sustainability of WSNs is necessitated [21]. In order to extend the lifetime of sensor nodes, the elimination of the energy waste that is caused by various phenomena, such as idle listening, overhearing, packet collision, interference, control packet overhead, and over-emitting, is necessitated [22]. Actually, there are numerous hardware-based and algorithm-based methods that pursue energy sustainability of WSNs via not only energy saving but also energy harvesting or even wireless energy transfer [21; 22; 23; 24; 25; 26; 27; 28]. Among them, the use of routing protocols that are energy-efficient has proven to be a widely used energy saving method, because sensor nodes consume most of their residual energy when they transmit and route data. According to their organizational or operational features, the energy-efficient routing protocols are classified into four main categories, namely, the communication model, network structure, topology, and reliable routing, as well as various subcategories [29]. Specifically, protocols belonging to the network structure category are classified as either flat or hierarchical [29; 30]. Flat protocols consider all network nodes as being equal entities that have unique global addresses. These protocols perform well in WSNs that comprise of a few sensor nodes and, thus, have extremely low scalability. On the other hand, in hierarchical protocols, which are further classified as hierarchy-based, zone-based, or fuzzy logic-based, sensor nodes are treated according to the place they hold within the hierarchical architecture of the network. Specifically, all network nodes are distributed in individual groups, called clusters, each of which has one cluster head and a number of cluster nodes. Hierarchical protocols are generally considered to be the most popular among all types of energy-efficient routing protocols, because they enable data aggregation, reduce the communication overhead, and achieve great scalability. For these reasons, hierarchical protocols are ideal for large-scale WSNs that have heavy communication load and cover wide FOIs [31].