# An Energy-Efficient Fail Recovery Routing in TDMA MAC Protocol-Based Wireless Sensor Network
## 6 Conclusions  
This paper presented an energy-efficient fail recovery routing algorithm targeted for a tree-topology wireless network whose nodes can fail due to battery depletion. When the recovery process is initiated, MCR algorithm determines a back-up node for each parent from a local subtree. Then, this back-up node employs faulty parent's TDMA slot to forward aggregated data of the subtree to grandparent. In the implementation stage, we observed that some back-up nodes are not able to connect their grandparents due to a distance constraint. We increased the transmission power of back-up nodes and then they were capable of forwarding their data to the grandparents. This small modification of transmission power, however, caused the following problems: (a) The nodes who were out of the interference zone of the back-up node started facing collision if their slots were identical, and (b) the back-up nodes were identified as faulty nodes in further steps since they used higher power to execute each transmission. Thus, we applied our second NE-MCR recovery method to find back-up parent nodes from different branches of the spanning tree. In this phase, we faced other constraints: (c) The back-up parents selected by NE-MCR were only able to accept a limited number of child nodes; (d) slot length was constrained and it was assigned in an earlier scheduling phase. Allocation of longer slots causes additional energy consumption due to the Idle mode of parents who have less child nodes. However, in a denser network scenario, more back-up patents were found, and NE-MCR was able to connect the isolated subtree with the optimal back-up parent. We compared the proposed method with reference algorithms in a wide range of network sizes. In comparison with reference algorithms, NE-MCR provided a substantially higher network connectivity ratio for networks of greater than 400 nodes. When compared with ESCFR and DCFR algorithms, NE-MCR consumed on average 23% less energy, while allowing a 21% longer lifetime for large networks. The proposed algorithm, therefore, is well suited to a fast recovery solution for low power networks. In addition, it offers a non-disruptive recovery solution for TDMA networks since it finds all back up parents without changing the existing scheduling.  
Conceptualization, O.U. and H.W.K.; Methodology, H.W.K.; Software, O.U.; Validation, O.U., H.W.K.; Formal analysis, H.W.K.; Writing--original draft preparation, O.U., H.W.K.; writing--review and editing, H.W.K.; funding acquisition, H.W.K.  
This work was financially supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project, Korea (CISS-2017M3A6A6066117). This work was also supported by IITP grant funded by the Korean government (No. R7117-19-0164, Development of wide area driving environment awareness and cooperative driving technology which are based on V2X wireless communication). It was also supported by SoC platform and SW development Advanced Education for IOT which is funded by the Ministry of Trade, Industry, and Energy (N00011132).  
The authors declare no conflict of interest.