## 3 Advantages and Objectives of Clustering  
Compared with flat routing protocols in WSNs, clustering routing protocols have a variety of advantages, such as more scalability, less load, less energy consumption and more robustness. In this section, we summarize these advantages as well as the objectives of WSN clustering as follows:  
_More Scalability_: In s clustering routing scheme, sensor nodes are divided into a variety of clusters with different assignment levels. The CHs are responsible for data aggregation, information dissemination and network management, and the MNs for events sensing and information collecting in their surroundings. Clustering topology can localize the route set up within the cluster and thus reduce the size of the routing table stored at the individual sensor nodes [32, 44]. Compared with a flat topology, this kind of network topology is easier to manage, and more scalable to respond to events in the environment [3]. \begin{table}
\begin{tabular}{|c|c|p{142.3 pt}|p{142.3 pt}|} \hline
**Year** & **Authors** & **Literature** & **Main contributions** \\ \hline
2010 & [PERSON] _et al._ & Cluster-based routing protocols for energy-efficiency in wireless sensor networks [37] & \text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@notemark{\text@@notemark{ \text@@@notemark{\text@@@@notemark{\text@@@@@notemark{\text@@@@@@@@@@@@@@@@@@@@@@@@@  
_Data Aggregation/Fusion_: Data aggregation/fusion, which is the process of aggregating the data from multiple nodes to eliminate redundant transmission and provide fused data to the BS, is an effectual technique for WSNs to save energy [45]. The most popular data aggregation/fusion method is clustering data aggregation, in which each CH aggregates the collected data and transmits the fused data to the BS [46]. Usually CHs are formed a tree structure to transmit aggregated data by multi-hopping through other CHs which results in significant energy savings [47]. _Less Load_: Since sensors might generate significant redundant data, data aggregation or fusion has emerged as an important tenet and objective in WSNs. The main idea of data aggregation or fusion is to combine data from different sources to eliminate redundant data transmissions, and provide a rich and multi-dimensional view of the targets being monitored [3,4]. Many clustering routing schemes with data aggregation capabilities require careful selection for clustering approach. For clustering topology, all cluster members only send data to CHs, and data aggregation is performed at the CHs, which help to dramatically reduce transmission data and save energy. In addition, the routes are set up within the clusters which thus reduce the size of the routing table stored at the individual sensor nodes [32,44]. _Less Energy: Consumption_: In clustering routing scheme, data aggregation helps to dramatically reduce transmission data and save energy. Moreover, clustering with intra-cluster and inter-cluster communications can reduce the number of sensor nodes performing the task of long distance communications, thus allowing less energy consumption for the entire network. In addition, only CHs perform the task of data transmission in clustering routing scheme, which can save a great deal of energy consumption. _More Robustness_: Clustering routing scheme makes it more convenient for network topology control and responding to network changes comprising node increasing, node mobility and unpredicted failures, _etc_. A clustering routing scheme only needs to cope with these changes within individual clusters, thus the entire network is more robust and more convenient for management. In order to share the CH responsibility, CHs are generally rotated among all the sensor nodes to avoid the single point of failure in clustering routing algorithms. _Collision Avoidance_: In the multi-hop flat model, the wireless medium is shared and managed by individual nodes, thus this model can result in low efficiency in the resource usage.