A Systematic Review on the Energy Efficiency of Dynamic Clustering in a Heterogeneous Environment of Wireless Sensor Networks (WSNs)  
[PERSON]  
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2 Department of Computing, Institute of Informatics and Computing in Energy,  
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to SNs takes a long time and consumes lots of energy. In multi-hop connectivity, nodes far from the base station (BS) do not have to send information directly over long distances by creating multi-level clusters. When many SNs attempts to connect a specific BS, each method will overcrowd the BS. Because of its high energy consumption, the single-level method is rarely used. Clustering is a more secure method of saving energy in a single SN. A network's lifetime can be extended by arranging nodes in such a way that they form a cluster model while successfully minimizing energy consumption [4]. Each sensor node in the cluster is wirelessly connected to the Cluster Head (CH), which is in turn wirelessly connected to the Base Station (BS) through wireless inter-cluster communication and the CH as well as between the BS and the CH [5]. Each cluster contains CHs that collect data from other nodes in the cluster and transmit it to the base station directly or indirectly via other nodes [6]. Clustering is generally defined as dividing a specified territory into small sectors and assigning a single node as the CH. CH is a critical component in developing an energy-efficient WSN model. In practice, the CH may change during certain iterations to improve network performance [7]. Furthermore, with the assistance of clusters, specific specialist nodes such as CHs make all major decisions on behalf of SNs. Clustering can be classified as multi-hop or single-hop. The disadvantages and advantages of single-to-multi hop communications are numerous; for example, the energy losses of single-hop communication due to increased range [8; 9]. Meanwhile, the multi-hop approach can improve energy consumption to handle the scalability issue in the wireless sensor network, hence enhancing both energy consumption and scalability. The energy-efficient protocols for WSN can be classified into four classes: reliable routing, topology-based, network structure, and communication model as shown in Figure 1[10; 11]. These four techniques are used to route messages while considering energy consumption. Furthermore, how they minimize the energy consumption and the lifetime of the networks are also taken into consideration. The routing protocols for the network structure are categorized as flat or hierarchical. The communication model can be either Query-based, Coherent and non-coherent based, or Negotiation-based. Location-based/Mobile Agent-based are considered in the category of topology-based protocols [12]. Lastly, QoS-based or Multipath-based belongs to the reliable routing protocols. Classification of routing algorithms is broken down into additional sub-categories as depicted in Figure 1.  
The identification of CH selection parameters is the main objective of clustering. Low-Energy Adaptive Cluster Hierarchy (LEACH) is a probabilistic and hierarchical technique that has shown great performance in clustering [13]. LEACH has been proposed as a standard clustering technique for topology control. The CH was chosen at random in a  
Figure 1: Classification of routing protocols in WSN (The scope of this paper covers grey colored boxes only). cyclic method so that the energy is distributed evenly across the nodes. The purposes are to increase the network lifespan and at the same time decrease energy consumption. The cluster head is chosen at random to maximize energy efficiency. Numerous research in the literature has been conducted to modify LEACH, which results in a variety of different configurations to increase network performance. However, the node degree and energy metrics were not taken into account during multi-hop data transmission across the CHs [14; 15]. Among the approach that has been proposed to solve the issue are classical methods [16] and numerous meta-heuristic algorithms [17; 18; 19; 20].