## 2 Background and Related Work
### Related Work  
A myriad of energy efficient schemes have been proposed for WSNs. The one with the most hype around is low energy adaptive clustering hierarchy (LEACH) [16]. LEACH is one of the pioneer routing protocols proposed for energy optimization in WSNs. The basic idea of LEACH is to balance the energy consumption by rotating the CH role among the nodes. Each node generates a random number between 0 and 1. If the random number is less than a certain threshold, the node declares itself as a CH. In LEACH, each node in the network has an equal probability to be elected as CH irrespective of its residual energy. In this equal probability CH selection mechanism, there is a possibility of electing CH having lower residual energy compared to the one with a higher energy level. LEACH is then enhanced to R-LEACH in [17], which provides a CH selection mechanism based on the node's initial energy, residual energy and the optimal number of CHs in the network. In [18], low energy adaptive clustering hierarchy centralized (LEACH-C) mechanism was proposed. LEACH-C modifies the existing LEACH protocol. In LEACH-C, each node in the network forwards location and residual energy information to the sink node. Based on the received information, the sink node performs clustering and determines the optimal CH for each cluster. Due to the centralized nature of LEACH-C, communication overhead increases in the re-clustering process. In [19], a sleep-wake energy-efficient distributed clustering algorithm (SEED) for wireless sensor networks was proposed. SEED divides the sensing network into three regions based on energy level (e.g., low energy region, advance energy region, high energy region) to achieve even energy distribution. In SEED, nodes which have high residual energy in each region can only become CHs. However, SEED results in additional control messages overhead that highly effects the lifetime of the network. Moreover, SEED does not take into account the node(s) storage capacity, and computational capability during CH declaration process which may degrade the overall performance of network in high multimedia traffic conditions. In [20], an Energy Centric Cluster-Based Routing Protocol (ECCR) for WSNs was proposed. In ECCR, clusters are predetermined and static, and CHs are selected based on the node's rank where rank comprises of node's residual energy and average distance from member nodes. The nodes with high residual energy and low average distance have a high rank. ECCR also resulted in control messages overhead which increases the power consumption of the nodes. A dual and static CH selection mechanism was proposed in [21] that split the entire network into equal sized static clusters. In this scheme, two CHs are selected in each cluster e.g., one for data aggregation and another for data transmission. The CHs are selected based on residual energy and distance from child nodes and sink node. The CH which has low distance from member nodes is used for data aggregation while the other CH which has low distance from sink node is responsible for data transmission. The selection of two CHs generates additional control overhead in the network. Furthermore, this scheme only considers residual energy and distance for CH selection which may result in inefficient CH selection in a heterogeneous network environment. An energy-driven unequal clustering protocol (EDUC) [22] was proposed to minimize the energy consumption in re-clustering (i.e., CH node rotation). In EDUC, a node can act as a CH only once during the entire network lifetime. Another scheme named as energy and proximity based unequal clustering algorithm (EPUC) [18] was proposed to address the energy imbalance issue of the nodes closer to the base station (BS) due to excessive relaying of data. In this scheme, nodes are selected as CHs based on their residual energy and BS proximity. In [23], an extension of LEACH protocol named as stable election protocol (SEP) was proposed.