# A New Association Scheme for Handling Node Mobility in Cluster-Tree Wireless Sensor Networks
## 2 Related Works
### Selecting Appropriate Coordinator  
The IEEE 802.15.4 standard does not define criteria for selecting the coordinator, since these criteria vary according to the application needs. With the aim to save power and improve latency, some papers have proposed criteria for the selection of the most suitable coordinator. Some criteria for optimizing the topology formation step of IEEE 802.15.4/ ZigBee WSNs were proposed by [PERSON] and [PERSON] [18]. These criteria are of two types: the first focused on single-hop topologies and the other on multi-hop topologies. Both types of criteria aim to balance the nodes' energy consumption and message delays. A re-association method was also proposed, which assumes that the WSN topology can be dynamically changed, taking into account the nodes' energy consumption over time. [PERSON] and [PERSON] [19] proposed a handoff scheme named the Burst Loss Algorithm, which considers the Received Signal Strength Indication (RSSI) and link loss when triggering the handoff procedure. This scheme initially uses an RSSI value threshold; whenever the RSSI is below this predefined threshold, the MN will start searching for a new AP. A hysteresis bound is then used for link loss: whenever the average link loss value is above a predefined value, the MN starts searching for another AP. In addition to this RSSI-based solution, the authors also propose a handover decision where mobile nodes must be associated with two links, the current and the new candidate. The quality of these links are constantly monitored, through the sending of burst probes, in order to use this information for the handover process. [PERSON] et al. [20] propose a handover process based on a probabilistic model, where the RSSI, node speed, energy level, traffic load, number of hops to the AP and link quality metrics are evaluated. They demonstrate the impact of these parameters to decide when the handover should be triggered. Results show a significant improvement in the mobility support of low-power networks. [PERSON] et al. [21] assessed the performance of the IEEE 802.15.4, considering a scenario composed of static APs and MNs operating at different speeds. Their preliminary results showed a need to improve the handover procedure. Then, three different decision schemes to start a handover process were proposed: blind, predictive, and threshold-enabled predictive (TEP) schemes. The blind scheme allows nodes to select the AP based only on the RSSI values. In the Predictive and TEP schemes, the MN estimates its distance from the APs and the movement direction through the RSSI values, selecting the appropriate AP to associate. [PERSON] and [PERSON] [22] have proposed a new re-association approach for WSN to guarantee the connectivity of MNs through a reliable handover mechanism. The main idea of this work is to choose an adequate coordinator for each one of mobile nodes in order to obtain appropriate child-parent relations and optimized network topologies. The handover process can be triggered by both the coordinator node (based on its remaining residual energy) and the MN (based on location change). Simulation results showed that the proposed scheme saves energy consumption and improves the packet delivery ratio and network throughput. In [23], the same authors propose a procedure, where the MN anticipates the handover whenever it detects an LQI degradation. It then performs a fast re-association based on a'smart' criterion. This criterion is given by the sum of the inverse of the remaining energy of the coordinators in the path to the sink node and allows the selection of the shortest path with the maximum remaining energy power, thus optimizing power consumption and decreasing latency. [PERSON] et al. [24] classify the handover mechanisms according to how the process has been initiated, that is, by the coordinator or MN. When the process starts in the MN, whenever it moves away from the coordinator's coverage area, it starts a search for another candidate coordinator.