# Enhanced Three Layer Hybrid Clustering Mechanism for Energy Efficient Routing in IoT
## 2 Related Work  
Hierarchical transmission in WSN is considered one of the preferred ways for energy efficient routing of data packets toward the destination. In this kind of architecture, sensor nodes are divided into different layers with different tasks. Clustering is a technique where different sensor nodes divided into groups and sub groups, they transmit their sensed data to the CH node and they in turn forward these packets toward the sink in hierarchical fashion. LEACH protocol [3] is considered one of the basic and simple two layer cluster  
Figure 1: Three layer clustering topology. routing technique in WSN, in which single hop communication is used between base station and CH. In LEACH, the CH role is circulated over all nodes one by one randomly; prioritizing those nodes to be selected as CH that had not been selected before. The approach proposed in LEACH lacked efficiency in many aspects, especially in the selection process of CH among deployed homogenous sensor nodes. The authors in [10] take two other factors in consideration before assigning CH role to any sensor node, which are the communication cost and the residual energy of sensor nodes. Enhanced Energy Efficient Adaptive Clustering (EEEAC) protocol [11] is also based on residual energy of sensor nodes and uniformly distributes the CH overload in the network between sensor nodes. While most energy efficient protocols extend network lifetime on the cost of end to end delay or other network parameters, the authors in [12] propose an energy aware protocol that maintains good end to end delay and throughput as well. The authors in [13] prolong the network lifetime by dividing the entire network region intoiers. Sensor nodes of high amount of energy and shortest distance to the sink are selected as CHs in this protocol. In [14], an opportunistic multi path routing mechanism is proposed with aim to minimize the energy consumed due to the selection process of forwarding nodes in order to prolong the network lifetime. A distributed algorithm called Scalable Energy Efficient Clustering Hierarchy (SEECH) is proposed in [15]. SEECH selects low degree nodes as relays while high degree nodes are selected as CHs separately using distance based algorithm. In [16], the size of clusters are chosen to be variable. Thus, clusters with maximum distance to sink have larger size as compared to those that are nearer to sink. A sub tree mechanism is proposed in this protocol so that some parents nodes gather data packets from their neighboring sensor nodes and forward them to CH, which in turn transmits them toward the base station. Cooperative communication improves the capacity of transmission and achieves spectral efficiency by taking advantage of the broadcast nature of communication in WSN [17]. Cooperative communication is basically developed from Multiple Input Multiple Output (MIMO) mechanisms, in which multiple sensor nodes share their resources to cooperatively reach data packets from source to destination without the need to equip each and every sensor node with multiple antennas. Relay selection Based Cooperative Routing (RBCR) is defined in [18], which is a relay selection based cooperative technique. Decode and forward without redundancy check strategy is adopted in RBCR with selection of best paths according to the information of channel state. At the receiving node, all receiving signals are combined again to retrieve the actual sent data packet. Recently, researchers are working on using controllers that are used in the Software Defined Network (SDN) to perform all computational task of WSN. Software Defined Wireless Sensor Network (SDWSN) play important roles in reducing energy consumption and prolonging network lifetime of WSNs. The SDN model also helps in adding many other functionalities and more intelligence to the WSN. A centralized SDWSN model is described in [19]. The main problem in fully centralized WSN or SDWSN is that it is totally dependent over controller, which makes the network less responsive in some conditions.