# An Energy-Efficient Mobile Sink-Based Unequal Clustering Mechanism for WSNs
## 2 Related Works
in [32] introduced an approach to alleviate the energy hole problem in a sink-centric traffic pattern network. In their model, each layer of the network is divided into equal clusters so that CHs are responsible for forming the clusters in cluster rings. In the uniform node distribution, the innermost CHs can preserve some energy for the inter-cluster traffic, if clusters belonging to the innermost layer have a smaller size than outer layers. However, this principle is not fulfilled in the method proposed in [32]. In addition, one goal of applying clustering is to decrease the energy consumption of nodes. However, this aim will not be realized if the cluster size value is not properly chosen. In fact, the clustering technique will only be effective if the consumed energy ratio by nodes is decreased after applying clustering. Therefore, the energy consumption of nodes before clustering should be considered to determine the cluster size. However, this rule was not considered in previous works. Furthermore, there are several strategies proposed in order to mitigate the energy hole problem. The authors of [12] proposed the Energy Balancing Cluster Head (EBCH) method which is based on adjusting the data transmission range of nodes. In their model, CHs forward the aggregated data packets after bisecting them, so that data packets will be sent to the base station by two different data transmission manners; single- and multi-hop. In [33] the Archimedes spiral node deployment strategy is proposed to optimize the network lifespan. For this reason, the authors introduced a routing aware clustering strategy to balancing the energy expenditure among CHs. However, non-uniform node deployment strategies are not practical for networks which are inaccessible to humans. Among the proposed methods, sink mobility is an important technique in improving network lifespan and energy efficiency [34]. Using mobile sinks can reduce the burden of energy consumption from the sensors to sinks, which are typically considered to have unlimited energy supply and larger computational power. In addition, this method has been accepted as an effective technique for mitigating the energy hole problem by avoiding extreme transmission overhead at the nodes located around the base station. Generally, using mobile sinks in the network offers many benefits such as mitigating the energy holes, increasing the network lifetime, reducing the drop packet ratio, decreasing the energy consumption, increasing the security and providing connectivity in the network [35]. With regards to the advantages of using mobile sinks, the outcome of the network can depend on type of sinks are used. The basic idea of mobile sinks was proposed by [PERSON] et al. [36] where mobile sinks are called \"data mules\". In their work, the mules perform a random walk in their close vicinity to aggregate the data packets and then they drop off the data at some access points. Since the transmission range of nodes is short, their energy consumption can be greatly reduced. [PERSON] et al. [37] proposed a Mobility-Based Data Collection Algorithm in order to improve the network lifetime. In their strategy, the area of the network has a circular shape with MSs working in a back-and-forth motion in the periphery of the circle. In [38], [PERSON] et al. introduced a mobility-based technique in order to optimize the clustering algorithm and sink node deployment strategy under the smart home network concept. They considered two different conditions with a different number of mobile sinks. First, they used a single sink which has a circular motion in a different radius. Then, they used multi mobile sinks to find the optimal number of mobile sinks in a circular area. They considered that the mobile sinks moved with constant velocity. One important parameter of mobility-based strategies is the velocity of the mobile agent. In [39], the velocity of the mobile sink is reduced to guarantee message delivery.