## 8 Conclusions  
The TSN TG has been working to provide standard Ethernet with real-time guarantees and reliability in layer 2 of the network architecture. These new capabilities are especially relevant for applications such as automotive, industrial automation and aerospace, as these applications are considered to be critical since their failure could have catastrophic consequences in terms of lives and cost. Therefore, these applications must provide a correct service in a continuous manner during their mission time. To that, the systems that support said applications must be reliable in all the levels of the architecture, including the communication subsystem. The TSN TG proposes two fault tolerance mechanisms to increase the reliability of Ethernet networks. On the one hand, the TG has developed a space redundancy mechanism which is standardised in [6; 7] which allows transmitting several copies of each frame in parallel through different physical paths. On the other hand, TSN has standardised an error containment mechanism [8] to reduce the negative impact of incorrect frames in the network. Nonetheless, the TSN TG does not propose any time redundancy mechanisms to tolerate temporary faults in the links. This is especially important as temporary faults in the links are the most common type of fault that can affect the communication network. Even though temporary faults could be tolerated using space redundancy, this is not a cost-effective solution, as space redundancy requires the addition of hardware which increases the cost, size, weight and energy consumption of the system. Instead, temporary faults can be tolerated using time redundancy. In previous works we proposed the PTRF mechanism to provide time redundancy to TSN-based networks. PTRF consists in transmitting several copies of each frame in a preventive manner to increase the probability of at least one copy reaching its destination even in the presence of temporary faults. In this work, we carry out a parametric sensitivity analysis of the reliability achievable by TSN and PTRF, in order to assess the benefits that PTRF can yield to TSN networks. To that end, we model TSN and PTRF using PRISM, a probabilistic model checker tool that allows evaluating the reliability of systems using discrete-time Markov Chains, among other techniques. In this work we have seen that the reliability achievable by any system or subsystem is affected by a myriad of aspects. Therefore, there is not a single solution that can fulfil the reliability needs of any system. Instead, an adequate specific solution must be used for each system. This solution must be configured appropriately in order to provide the required level of reliability, especially in critical systems that operate in harsh environments. The analyses herein presented can help designers in making better decisions when building the communication subsystem of TSN networks. On the one hand, we have corroborated the idea that the BER is determinant in the reliability achievable by TSN, PTRF approach A and PTRF approach B. Furthermore, we have seen that approach A is a more adequate solution than approach B to provide high reliability in networks with low BERs. Not only does approach A provide a higher reliability level, but it is also a cheaper solution as it uses COTS TSN bridges. On the contrary, approach B can reach significantly higher levels of reliability than approach A in networks with high BERs. Thus, in these cases using approach B is more adequate than using approach A, even if the cost of the network is higher. Furthermore, we have seen that the frame size has an important impact on reliability, which increases significantly as the BER increases. Therefore, the size of the frames should be reduced whenever possible, e.g., preprocessing the information in the source. If this is not possible, streams that convey larger frames should have a higher level of redundancy than those that convey smaller frames when high reliability is required. We have also studied how network size impacts reliability.