# A New Association Scheme for Handling Node Mobility in Cluster-Tree Wireless Sensor Networks
## 2 Related Works
### Improving Connectivity between MN and AP  
[PERSON] and [PERSON] [8] investigated the question of coordinator discovery and association for mobile end nodes, which need to perform these operations due to these frequent cluster changes. The authors propose a scheme named Coordinator-Assisted Passive Discovery and Association for mobile end devices, which is intended to provide rapid discovery of coordinators and to reduce the association time during a cluster change. In this scheme, a parent endpoint coordinator uses LQI to check if any mobile node is moving away from its cluster. If so, the coordinator asks the neighboring coordinators to assign a temporary network address to the mobile device. This action reduces the association time by eliminating timeout responses when exchanging messages in the association procedure.  
In another work, the same authors [27] propose a modification of the IEEE 802.15.4 re-association procedure that can decrease the frequency of re-association and latency for a beacon-enabled WSN with cluster-tree topology and mobile nodes. The scheme named Preemptive Re-association was designed to preventively detect loss of connectivity. In addition, the method performs a fast re-association procedure and selects coordinators that can provide increased connectivity during re-association. In the case of networks with a high beacon order parameter, the method also attempts to decrease the beacon interval of coordinators that are close to the mobile devices. In short, a make-before-break approach is first used to reduce the delay in connectivity loss detection after cluster switching. The orphan scan is not used, since in this approach the MN is connected to the AP when scanning, thus avoiding becoming an orphaned node. Passive scanning is performed only on the current channel to decrease the scan time. Finally, the Beacon Order (BO) parameter of neighboring coordinators is decreased when the parent coordinator perceives weak connectivity with any of its child nodes. This helps a child device with low connectivity to quickly select a new coordinator. To reduce the handover frequency, a new criterion for coordinator selection is proposed that takes into account the LQI between the mobile node and the coordinator. As a result, connectivity can be improved by selecting the coordinator with the best signal quality.  
In [PERSON] et al. [28], both a WSN architecture and a scheme for tracking mobile nodes in outdoor scenarios are proposed. The architecture assumes a ZigBee network, where there is always an intersection in the coverage areas of neighboring AP nodes. According to the authors, there is no need for a handover process because all the AP nodes are configured with a unique PAN identifier, and the AP with higher RSSI value will receive the messages from mobile nodes and send them towards the coordinator. The architecture was assessed in a real urban environment where AP nodes were installed at semaphores, and communicate with MNs installed in vehicles. The scheme showed promising results in most of the cases. However, it would be interesting to have further results for a larger number of nodes to assess scalability problems.