## 2 Background and Related Work
### Related Work
SEP ensure the uniform energy consumption of all the nodes in the network to prevent the early depletion of nodes. In [24], an energy efficient multi level and distance aware clustering (EEMDC) was proposed. EEMDC splits the entire network into three levels based the number of hops from the base station. These levels comprised of first level clusters which have hop counts of one to two, the second level clusters having hop counts of three to five and third level clusters which have hop counts of six or more. The different levels determine the distance from base station (e.g., the nodes in level 1 are closer to BS than that of nodes in level 3). In EEMDC, CH declaration is based on residual energy and hop-count. EEMDC ensures the smallest route towards the BS in order to optimize the energy consumption and enhance the overall network lifetime. An energy-aware routing algorithm (ERA) has been proposed in [25]. In ERA, the CH declaration mechanism is based on residual energy of nodes. Meaning that, the nodes which have high residual energy are elected as CH. ERA generates directed virtual backbone (DVB) of CHs to relay the data from CHs towards the sink. Sink initiate process of DVB generation by broadcasting a route request message towards the CHs. On recieving route request message, the CH(s) increment its level by one, higher than that of sink (e.g., consider the level (L) of the sink node is zero e.g., L(sink) = 0, then L(CH) = L(sink) + 1) and then re-broadcast the message to other CH nodes in their vicinity and creates a DVB. A CH selects the relay nodes based on the ratios of the average residual energy of the CHs in different levels and distributes all the aggregated data packets towards the sink node sequentially. Energy and coverage aware distributed clustering (ECDC) has been proposed in [26]. In ECDC, the node declare itself as a CH based on its residual energy and coverage. Each node share its residual energy and coverage information with its neighboring nodes. The node which has high residual energy and coverage is selected as a CH. ECDC achieves a lower energy consumption of nodes and better in-coverage performance compared to other protocols. A decentralized energy-efficient hierarchical cluster-based routing algorithm (DHCRA) was proposed in [27]. In DHCRA, CHs are selected at tree edges based on residual energy and distance from BS. DHCRA minimizes control overhead and optimizes the energy consumption of nodes. In [28], an adaptive clustering algorithm (TCAC) has been proposed to enhance the lifetime of the network. TCAC enables the CHs to adjust their power level to achieve the desired network connectivity. TCAC generate the balanced clusters across the whole network and improves the network lifetime. However, in TCAC, the periodic transmission of range updates and competition-based CH selection increases the network complexity. Link-aware clustering mechanism (LCM) [29] for WSNs has been proposed. In LCM, the CHs are elected based on the condition of the links and status of sensor nodes. In LCM, predicted transmission count (PTX) has been introduced to evaluate the condition of the node(s). PTX is calculated based on the residual energy, transmission power, and the link quality with a specific neighbor. The node which has a large PTX value has significant chance to become a CH. Cluster chain weighted metrics (CCWM) has been proposed in [30] to optimize the energy consumption and enhance the performance of WSNs based on weighted metrics. In CCWM, the CHs are elected based on weighted metrics. In this scheme, the child nodes forward the sensed data directly towards the CH whereas the CH transfer the aggregated data towards the neighbor CHuntil the data reach the BS. CCWM improves the network lifetime however, the direct intra-cluster communication may leads to unfair energy distribution. A non-probabilistic multi-criteria based CH selection mechanism was proposed in [31]. In this scheme, analytical network process decision tool is used for CH is selection.