# A New Association Scheme for Handling Node Mobility in Cluster-Tree Wireless Sensor Networks
## 5 Conclusions  
This paper proposes a new association and re-association scheme to handle mobile nodes in IEEE 802.15.4 cluster-tree networks. We first presented the state-of-the-art on handover mechanisms for WSNs through a systematic literature review. Next, a proposal for the improvement of the association mechanisms was systematized using a set of strategies. We also assessed the protocol constraints for network formation, such as BI and beacon scheduling and superframe duration allocation. The communication infrastructure was validated via extensive simulation tests, exploring the proposed improvements. It was possible to prove the efficiency of the proposed anticipation method, in relation to packet delivery rates as maintaining the connectivity of the mobile nodes with the network. Simulation results show that it is possible to improve the IEEE 802.15.4 standard association mechanisms, to support wide-scale WSN applications with mobile nodes. The following key conclusions can be summarized:  
* Current WSN standards do not efficiently support node mobility, especially if they are being used to support critical applications with real-time and reliability requirements;
* Currently, available Wireless Body Area Network (WBAN) / WSN technologies and standards do not support high packet delivery rates when multiple hop topologies are used;
* Patient mobility in health monitoring applications such as physiotherapy requires high packet delivery rates combined with small and predictable end-to-end communication delays. Therefore, it is essential to ensure efficient handover mechanisms so that patients do not lose connection from the network while moving around the coverage area;
* This paper has shown that by using a combination of strategies, it is possible to enhance the IEEE 802.15.4 standard association mechanisms to support node mobility and, therefore, to meet the health monitoring applications requirements.  
In summary, results reported in this paper represent a step forward when compared to the IEEE 802.15.4 standard association mechanisms, guaranteeing that mobile nodes remain connected during longer periods of time with coordinator nodes. As future work, we intend to integrate an optimized active period allocation mechanism for cluster-tree networks and to evaluate the proposed mechanism by experimental analysis, comparing the power consumption and other metrics.  
[PERSON] and [PERSON] conceived of and designed the proposal, experiments and simulation scenarios. [PERSON] and [PERSON] performed the simulation assessments and analyzed the resulting data. [PERSON] wrote the paper. [PERSON], [PERSON], and [PERSON] provided guidance for writing and revised the paper. All authors have read and agreed to the published version of the manuscript.  
Figure 12: Average end-to-end delay at 5 m/s.  
Funding:This work has been partially supported by CAPES, The Brazilian Agency for Higher Education, under the project PrInt CAPES-UFSC \"Automation 4.0\". It was also supported by FCT/Portugal, under the Project UIDB/50022/2020, and the CNPq/Brazil, under the Universal Project 443711/2018-6. Conflicts of Interest:The authors declare no conflict of interest.