## 3 Advantages and Objectives of Clustering
On the other hand, in the multi-hop clustering model, a WSN is divided into clusters and data communications between sensor nodes comprise two modes, _i.e._, intra-cluster and inter-cluster, respectively for data collection and for data transmissions. Accordingly, resources can be allocated orthogonally to each cluster to reduce collisions between clusters and be reused cluster by cluster [48]. As a result, the multi-hop clustering model is appropriate for large-scale WSNs. _Latency Reduction_: When a WSN is divided into clusters, only CHs perform the task of data transmissions out of the cluster. The mode of data transmissions only out of the cluster helps avoiding collisions between the nodes. Accordingly latency is reduced. Furthermore, data transmission is performed hop by hop usually using the form of flooding in flat routing scheme, but only CHs perform the task of data transmission in clustering routing scheme, which can decrease hops from data source to the BS, accordingly decrease latency. _Load Balancing_: Load balancing is an essential consideration aiming at prolonging the network lifetime in WSNs. Even distribution of sensor nodes among the clusters is usually considered for cluster construction where CHs perform the task of data processing and intra-cluster management. In general, constructing equal-sized clusters is adopted for prolonging the network lifetime since it prevents the premature energy exhaustion of CHs. Besides, multi-path routing is a method to achieve load balancing. _Fault-Tolerance_: Due to the applicability of WSNs in a good many dynamic scenarios, sensor nodes may suffer from energy depletion, transmission errors, hardware malfunction, malicious attacks and so on. With applications such as hurricane modeling and tracking envisioned to utilize a large number of small sensor nodes, the cost of each sensor node is constrained. Owing to significant constraints on the cost, and therefore on the quality of sensor motes, and the often hostile environments in which they are deployed, sensor networks are prone to failure. Thus, fault-tolerance is an important challenge in WSNs [49]. In order to avoid the loss of significant data from key sensor nodes, fault-tolerance of CHs is usually required in this kind of applications, thus effective fault-tolerant approaches must be designed in WSNs. Re-clustering is the most intuitive method to recover from a cluster failure, though it usually disarranges the on-going operation. Assignment of CH backup is a viable scheme for recovery from a CH failure. _Guarantee of Connectivity_: Sensor nodes usually transmit data to one or more BSs via a single-hop or multi-hop routing in WSNs, thus whether or not the data is successfully delivered to the BS is mainly determined by the connectivity of each node to its next hop node along the path. Furthermore, sensor nodes that cannot communicate with any other sensor node will get isolated and their data can never be transmitted to the BS. Therefore, guarantee of connectivity is an essential goal of clustering routing protocols in WSNs [3,4]. An important example is when some information concerning all the sensor nodes needs to be collected by a designated fusion node in clustering routing protocols [50]. _Energy Hole Avoidance_: Generally, multi-hop routing is used to deliver the collected data to a sink or a BS. In those networks, the traffic transmitted by each node includes both self-generated and relayed traffic. Regardless of MAC protocols, the sensor nodes closer to the BS have to transmit more packets than those far away from the BS [51]. As a result, the nodes closer to the BS to deplete their energy first, leaving a hole near the BS, partitioning the whole network, and preventing the outside nodes from sending information to the BS, while many remaining nodes still have a plenty of energy. This phenomenon is called energy hole [52].