A mobile ad hoc network (MANET) is an autonomous infrastructure less network in which collection of mobile nodes (i.e., mobile, sensor, palmtop, laptop) dynamically communicates with each other through wireless medium within their own transmission range (using single hop or multiple hops) via intermediate nodes . The infrastructure less nature of mobile ad hoc network causes frequent change in the topology of network due to dynamic mobility of mobile nodes, communication management, and creation of a stable network are the most challenging tasks in MANET. Clustering is a possible solution to address these existing challenges. With the help of clustering, nodes are organized into different groups that make the network more robust, durable, and scalable .
The mobile ad hoc network (MANET) can be deployed without requirement of any further extra cost and time. In a MANET, every mobile node plays a role of router along with its job as an ordinary host. MANET still has some challenges like limited bandwidth, limited battery power for each mobile node, and frequent topology changes because of node movement [3, 4]. To list such complex and dynamic environment challenges of mobile ad hoc network, various clustering algorithms have been proposed in the literate . Several routing protocols have been proposed in the literature to handle the one hop and multihops, self-organizing network based on proactive, reactive, and hybrid protocols [1–9]. Routing protocols are categories in three categories which are Proactive, Reactive Active and Hybrid Protocol. Proactive protocol remains activate all the time in network even there is no data to transmit and keeps route information available all the time from source to destination.
Whereas, in reactive routing protocols, route information is available on request. This reduced power consumption in reactive protocols. Hybrid protocol works on the principal of proactive and reactive protocol.
For the large networks, flat routing structure requires excessive information. In order to overcome such problem, hierarchical structure (clustering) plays an important role in MANET reducing network overhead, increasing reusability of bandwidth, providing stability to the cluster structure, reducing battery power consumption of mobile node, and reducing clustering as well as in intracluster communication .
Clustering is about segregating a collection of mobile nodes into different virtual logical groups in a MANET. Each cluster is capable to connect with other cluster using cluster gateway to provide connectivity for a network. Each cluster consists of various mobile nodes such as cluster head, cluster member, and cluster gateway, and they perform different roles in cluster at the time of data communication in mobile ad hoc network.
Each cluster head is a special mobile node in a cluster which acts as local coordinator within the cluster. In a cluster, only one mobile node acts as cluster head at a time. Main responsibilities of cluster head include data forwarding from source node to destination mobile node, intracluster transmission, and managing all member nodes of a cluster. Cluster members in a cluster are treated as normal mobile nodes which can communicate with each other through cluster head. A cluster gateway is cluster member which is used to connect two or more clusters so that each cluster can access its neighbor cluster to send or receive data from other neighboring cluster.
It is evident that MANET is dynamic in nature; due to such nature, performance of network decreases as the size of network increases. Such problem can be reduced by introducing clustering, for such networks. Clustering increases scalability of wireless network and decreases network overhead . Clustering also provides spatial reuse of resource to increase the network capacity. The same frequency code can be used if two clusters are not neighbors (not within the same radio range).
In , centralized metaheuristic based on “tabu search” and a distributed heuristic based on “ant colony” are applied to reduce computation overhead in wireless sensor network (WSNs). Energy-Efficient, Delay-Aware, and Lifetime-Balancing (EDAL) take care to minimize the system lifetime for individual node. It reduces the amount of traffic generated in the network by compressive sensing. The present taxonomy is of opportunistic routing protocols for disruption tolerant networks (DTNs). In  opportunistic routing solutions, number of features which are used to classified DTNs according to mobility, capability, and connectivity of nodes. It also describes opportunistic routing protocols for DTN and basic opportunistic routing building blocks (Fig. 1).
Energy consumption for mobile node is a very important issue in MANET because it reduces network connectivity as soon as mobile node dies due to battery drainage. In  the best next hop, mobile node is selected based on energy efficiency to provide network connectivity. Selecting the best nodes will reduce the path change from source to destination and decreased network overhead. In , optimal clustering and throughput are archived in hierarchical cooperation in mobile ad hoc network. The exact throughput for various numbers of stages can also be achieved . The optimal cluster size is responsible to minimize throughput for all stages. The hierarchical scheme does not handle linear scaling.
In , a weight-based clustering algorithm is described to manage mobile nodes and maintaining the local topology within the network. Each mobile node calculates its weight by using weight function and compares its weight with other mobile nodes which are neighbor nodes within two hops to create the cluster head. The node with the highest weight will be elected as the cluster head and the remaining mobile nodes will be considered as cluster members. In this paper, neighboring mobile node has higher priority to group into the same cluster, which reduces number of cluster, improved cluster stability and reduced clustering overhead. In  various cluster heads, selection schemes have been proposed for WSNs and MANET. The main objectives of such schemes are to elect efficient cluster head for a cluster so that dynamic topology changes may be reduced as cluster head moves from the cluster or dies because of battery drainages. Hence, a solution is required which selects a reliable (stable) cluster head.
In clustering algorithms , routing information is shared with cluster head and cluster gateway. This reduces the total number of transmission for routing information and efficiently manages routing table in a network. A cluster structure increases scalability of network and energy consumption  and decreases network overhead.
In , present a routing algorithm that handles mobility of nodes and clustered wireless network, by a capable gateway selection that is responsible for load balancing capabilities. Virtual hierarchies of clusters are used to explore the contextual proximity of nodes. The protocol also creates application of a kernel-based link quality estimator which agrees to pick the most appropriate gateways with load balancing and disconnection predication capability in each cluster. In , N-layer discrete power control scheme is design to improve larger transmission capacity and spatial reuse factor in a cluster with N-layers. In , problem related to non-uniform load distribution in mobile ad hoc network is studded and proposed light-weighted dynamic channel allocation and cooperative load balancing algorithm based on cluster. Load balancing and dynamic channel allocation method reduced clustering overhead and message passing. In , particle swarm optimization (PSO) is used to reduce network overhead and message drop while maintaining message delivery ratio.
Advantages of clustering: In comparison to the traditional network, clustering has many advantages. Some of them are as follows:
Clustering allows better performance of the protocol for the Medium Access Control (MAC) layer by improving the spatial reuse, throughput, scalability, and power consumption.
Efficient handling of mobility management improve routing at network layer and decrease transmission overhead. Each node stores less information related to network topology and saves energy of mobile node and bandwidth for multipath routing algorithm in mobile ad hoc network .
Disadvantages of clustering: In [9, 23], a large and flat network is managed in MANET using clustering topology. Clustering required construction and maintenance cost in comparison to other topology.
Some side effects of clustering are as follows:
In MANET, mobile nodes moves frequently; this leads to change network topology very quickly dynamically. Because of dynamic movement of mobile node and death of mobile node, reorganization of cluster structure is required. This needs exchange messages inside a cluster that consumes bandwidth and energy of mobile node. Cluster head and gateway node forwards (manage) the message for cluster member, so power consumption of such nodes is higher compared to cluster member nodes. This causes reelection of cluster head and cluster gateway.