## 5 Conclusions and Outlook  
This article presents a 5G and TSN integrated prototype based on the IEEE 802.1 CB Frame Replication and Elimination for Reliability (FRER) scheme. The prototype has been empirically evaluated for a Smart Sensors for Milling Processes use case. The use case demands highly reliable wireless communication for online monitoring and cloud-based decision-making, which warrants the use of a 5G communication system. While wired communication is not an option in this use case, there is also a need for continuously monitoring the condition of the tool in real-time and reacting accordingly to the condition of the tool, for instance, by adapting machining parameters such as the spindle speed or even exchanging the tool before it is worn out. In industrial environments with challenging radio propagation conditions, 5G communication may undergo packet delay variations, which become undesirable for use cases with reliable low latency communication demands. In this work, we have evaluated the use of redundant paths via FRER implemented on commercial off-the-shelf TSN switches. We have empirically studied the use of redundant 5G paths on the same as well as different 5G systems operating in both 3.7-3.8 GHz and 26 GHz frequency ranges. Our experimental results indicate that while redundant paths require additional hardware and spectral resources, the likelihood of redundant paths concurrently undergoing communication anomalies due to radio propagation and other factors is very rare. The results obtained for the different setups indicate that FRER is very useful in meeting the low latency and high reliability target of the investigated tool condition monitoring use-case with the requirement of lower latency with a 99.99 percentile. Especially with introduced disturbances, FRER greatly improves the reliability in communication in harsh environments such as industrial production. The use of redundant transmission enhances the robustness of communication in the event of a failure and reduces latency and communication behavior. Overall, it can be seen that the usage of FRER enables reliable communication within the requirements set by the industrial use case, which is not the case for most single communication paths. Further, it has been discussed how the requirements described, 10 ms for 99.99% of all packets, are highly relevant for industrial communication. Thus, redundancy enabled by FRER for 5G communication has been shown to meet the demanding communication reliability and latency targets of the Smart Sensors for Milling Processes use-case even for the challenging radio propagation conditions on an industrial shop floor. All the empirical results in this work have been obtained in a real production environment with various 5G systems. We have obtained a large measurement sample size (typically a few million samples) for each experimental test to ensure the high statistical significance of the presented empirical results in this article. Except for the 5G URLLC test system, all the results have been obtained on commercially available equipment, including a 5G network, 5G UEs, and FRER-enabled TSN switches. With these results, future use cases benefiting from reliable wireless communication can be implemented. The integration of 5G-FRER in real scenarios using fieldbus protocols could be one possible future work. Further investigating the impact of FRER on the connectivity of moving devices, for example, on an AGV, would bring further insights. This signifies the applicability of our detailed test results and analysis in today's industrial use cases. We believe that the test results and their analysis in this article provide valuable insights to researchers and engineers aiming to apply deterministic 5G communication in mission-critical use cases. Conceptualization and validation, P.E.K. and J.A. ; methodology, software, formal analysis and research, P.E.K., J.A., M.L., P.M., C.-C.L. and J.-L.Y. ; writing--original draft preparation, P.E.K., J.A., P.M., C.-C.L. and J.-L.Y. ; supervision, R.H.S.