## 1 Introduction
### Clustering Based Approaches  
Ordinarily, clustering is defined as grouping of sensor nodes and it is referred as cluster. It is a two-layered strategy which separates the network into two levels. For the process of effective communication initially nodes are divided into clusters, and the best node is chosen as a cluster head (CH). This process becomes very essential because a wide diversity of information is furnished in WSN, here clustering is used to connect and aggregate that information which leads to reduce the energy consumption and so to improve the efficiency of the network. A typical application of a clustered WSN is presented in Figure 4.  
Figure 3: Various issues in a wireless sensor network (WSN)—Broad Classification.  
As Figure 4 shows, layer 1 nodes will do field sensing and data generation then it is transmitted to the CH in the next layer. Here the process of data aggregation is performed finally the information is sent to the BS in a multi hop manner. The major specifications of CH are monitoring the cluster count, cluster size and the cluster communication model such as inter and intra cluster communication. Many earlier research concentrated the CH selection section in order to improve the performance of the network. The key parameters for CH selection are node movement, localization, roles, process of clusters, cluster head selection, algorithm intricacy, and the dynamic nature of the cluster [10,11].  
Characteristics of CH:  
Existence: The major type of existence of WSN clustering protocols are CH based and non-CH based. Difference of Capabilities: Founded of energy, the homogeneous and heterogeneous networks are created. In the homogeneous network the energy is equally distributed among the nodes and in the heterogeneous network it is randomly distributed [12].  
Role: The primary roles are cluster heads are traffic management and data aggregation.  
Collision Avoidance: In order to reduce the collision in the network the multi-hop clustering model is introduced. It consists of both the intra-cluster and inter- cluster communication [13].  
The Figure 5 demonstrates the classification of earlier clustering algorithms in WSNs. It can clearly be seen that the clustering algorithms is divided into four main approaches which are: Heuristic approaches, Weighted approaches, Hierarchical approaches, and Grid approaches. In the Heuristic approaches, we can cite a few examples of clustering algorithms, such as LCA, LCA2, Max-Min, and D-cluster. On the other hand, the Weighted approaches, here we can talk about, \"A Weighted Clustering Algorithm for Mobile Ad Hoc Networks (WCA). While other protocols like PEGASIS and GROUP belong to Grid approaches. On the other hand, in the Hierarchical approaches, this will be the focus of our attention, as it is considered an effective method to reduce energy consumption in the cluster and successfully solve the battery problem. Here, we can introduce different hierarchical routing protocols, such as Low Energy Adaptive Clustering Hierarchy (LEACH), Two-Level Hierarchy for Low Energy Adaptive Clustering Hierarchy (TL-LEACH), Threshold service Energy Efficient sensor Network (TEEN), A Hybrid Protocol for Efficient Routing and Comprehensive Information Retrieval in Wireless Sensor Networks (APTEEN) and Hybrid Energy Efficient Distributed Clustering (HEED).  
Figure 4: A typical application of a clustered WSN.  
Furthermore, other protocols include \"A Regional Centralized Clustering Routing Algorithm for Wireless Sensor Networks\", \"Multiple Sink Network design problem in large scale wireless sensor networks (MSNDP)\", \"Energy efficient Dynamic Clustering (EEDC)\", Energy Efficient Clustering using Interconnection (EECI), and Cluster Based Data Aggregation Scheme [14; 15; 16].