Performability analysis of a tramway system with virtual tags and local positioning

Current solutions for tramway Interlocking Systems are based on physical sensors (balizes) distributed along the infrastructure which detect passing of the trams and trigger different actions. This approach is not easily scalable and maintainable, and it is costly. The Regional Project SISTER aims at designing new architectural solutions for addressing the previous problems based on the virtualization of the sensors and on the local positioning of each tram. The idea is to trigger actions when the computed local position corresponds to a virtual tag. However, the computed position can be affected by errors, compared to the real one. Therefore, it is important to understand the impact of these new solutions on the performability of the system. This paper focuses on the analysis of the performability of a tramway system, based on the SISTER architectural solutions. We build a model using Stochastic Activity Networks and run sensitivity analyses on i) the accuracy of the positioning, ii) the different SISTER parameters (e.g., those triggering the activation of manual procedures). This analysis allows us to properly set and fine-tune the key architectural parameters, to understand the impact of the accuracy on the positioning, and to understand the impact of failures.