## 1 Motivation  
Synergistic mating of wireless communication, sensing and network technology paves the way for the emergence of wireless sensor networks (WSN) [1] that open an enormous range of applications in various domains such as surveillance, tracking, healthcare, and in environmental science [2]. Most of these applications require unattended sensors with non-renewable scarce energy resources to stay operational for a longer period of time. The successful operation of WSN in these applications primarily relies on routing sensed data from sensor nodes to the Base Station (BS). Sensor nodes dissipate most of their energy in routing which limits network lifetime. Therefore, routing protocols must be designed in such a way that minimizes energy consumption and extends network lifetime. Direct communication from sensors to the BS is only feasible for very small WSNs and network size is the function of maximum communication range of nodes. For large scale networks, multi-hop communication provides scalability through the transit nodes' to destine the data to far distantly placed BS. Simulation experiments have shown the effectiveness of multi-hop communication over direct in [3]. For minimum utilization of battery energy, all the steps from node deployment to network architecture (flat/clustered) and from environment sensing to communicating the sensed data to the BS (routing) should be carefully designed. In a clustered network architecture [4], nodes are grouped together in clusters with one node called Cluster Head (CH) designated as their head and other nodes in the clusters designated as Cluster Members (CM). In the clustered network, the process of designating a node as the cluster head, _i.e._, CH election, is usually the initial phase, while establishing the route for communicating the data from the source to the destination is usually its last phase [5], but this sequence is not always the case. Hence, in a clustered network architecture, nodes are designated different roles. Figure 1 shows the possible roles of a node during its lifetime depending upon the underlying clustering algorithm. The node roles that are underlined in the figure are the part of every clustering algorithm. For better understanding of each role, a short description is also given for each one. Energy efficient routing strives to minimize energy consumption by improving various factors of a number of functional aspects such as communications to the sink, cluster design, CH election, inter-cluster and intra-cluster communication style, and CH rotation. Although a clustered network has priority over a flat network for better network performance in many aspects, its cluster design part is still more energy consuming [6]. Recently, a notable effort to design a multilayer cluster (_i.e._, MCDA) was reported in [5], where the authors showed that reducing the broadcasting, decreasing computation and packet overhead all contribute to energy conservation. More-over, direct hop communication from source node to sink node as adapted in [7, 8] is an energy-aware solution for small networks. This limitation is removed by multi-hop communication style, which also has its number of variants. Some algorithms are just two level multi-hop from source to sink as in [9] and some are pure multi-hop without any limit of hop count from source to sink as in [10]. This last fashion of communication is the most preferable choice among the two options, _i.e._, direct hop and two level multi-hop. [PERSON] _et al_. [11] have presented a hybrid solution to further improve the network efficiency with respect to energy consumption in routing processes. The same hybrid technique is analyzed for its dependability and reliability in intra-cluster routing technique in temperature sending and battlefield applications in [3]. A self-optimizing scheme for energy balanced routing in wireless sensor networks using ants to improve the network performance is comprehensively discussed in [12].