## 3 Related Work  
Recently, mobility in wireless sensor networks has attracted significant interest, especially with the evolution of mobile robotics and smart mobile robots [10]. In addition, mobility is considered one of the essential solutions for energy consumption and improvement of the data collection process in this kind of network [1; 15; 18]. In the literature, there are three mobility types for nodes in the networks, which are the mobility of sensor nodes, mobility of collectors or sinks, and the mobility of sensor nodes and collectors together. As we mentioned before, we are interested in the mobility of the collector node or the sink, which is named mobile sink (MS) in this paper. For large-scale networks, clustering is proposed to organize the wireless sensor network [12; 13; 14]. In the cluster-based wireless sensor network, CHs receive data sensed by sensor nodes in a single hop then the MS collects these data directly from CHs. To do so, the MS needs a trajectory plan to pass by all CHs in the network and gather data. Few works dealt with the problem of finding the best path to collect data in wireless sensor networks [15; 19; 20; 21]. The moving plan of MS through the network field is considered as a problem in [15; 19; 21; 22], where they refer this problem to the well-known problem traveling salesman problem (TSP) which is considered an NP-hard class problem. These research works studied the problem of finding an optimal path for the MS using optimization methods such as metaheuristics algorithms for the sake of improving the data collection process and extend the network lifetime. In this paper, we use a mobility model based on metaheuristic algorithms used in a previous publication [15], in which MS collects data from each sensor nodes in the network. We adapt this mobility model to cluster-based wireless sensor networks in order to make MS collects data directly from CHs in order to reduce latency. This mobility model gave significant results regarding data collection and energy saving. While the MS is moving through the network field, interruption of communication may occur between the MS and CHs. To handle this problem, the handover mechanism is proposed. Many works in the literature such in [4; 23; 24; 25; 26], study the handover mechanism in the classical wireless sensor networks and a few of them address the handover problem in the hierarchical wireless sensor networks such as cluster-based wireless sensor networks. Most of these research works deal with the handover  
Figure 1: Proposed Handover Mechanism CLR-MSPH in Cluster-Based Wireless Sensor Network. at the MAC layer where they propose seamless handover mechanism based on some existed MAC protocols such as BMAC [24], XMAC [23], MA-MAC [24], SMAC [27] and RI-MAC [26]. In [23], authors proposed a seamless handover based on XMAC protocol at the MAC layer for a classical wireless sensor network with MS, named in this paper HXMAC. The source node starts sending the data stored in the buffer to the receiver (MS). The MS replies by an acknowledgment (ACK) to the source node. Meanwhile, the mobility estimation scheme at the background evaluates the RSSI values of the received ACK packets. The initiation of the handover in the proposed HXMAC depends on the value of the RSSI, the speed of the MS, and the minimum distance between the source node and the receiver [26]. The handover mechanism used in HXMAC starts by broadcasting a handover request and waits for a handover response from other sensor nodes in the network. The source node selects the closest node which replied with a handover response in order to perform the handover and route data packets in two hop routing. The mobility model used in this paper is the random waypoint model with varying the speed of the MS from 1 m/s to 5 m/s.