Impact of Packet Loss and Timing Errors on Scheduled Periodic Traffic with Time-Aware Shaping (TAS) in Time-Sensitive Networking (TSN)

Time-Sensitive Networking (TSN) is a collection of mechanisms to enhance the realtime transmission capability of Ethernet networks. TSN combines priority queuing, traffic scheduling, and the Time-Aware Shaper (TAS) to carry periodic traffic with ultra-low latency and jitter. That is, so-called Talkers send periodic traffic with highest priority according to a schedule. The schedule is designed such that the scheduled traffic is forwarded by the TSN bridges with no or only little queuing delay. To protect that traffic against other frames, the TAS is configured on all interfaces such that lower-priority queues can send only when high-priority traffic is not supposed to be forwarded. In the literature on scheduling algorithms for the TAS there is mostly the explicit or implicit assumption that the TAS also limits transmission slots of high-priority traffic. In this paper we show that this assumption can lead to tremendous problems like very long queuing delay or even packet loss in case of faulty frames. A faulty frame arrives too early or too late according to the schedule, it is missing or additional. We construct minimal examples to illustrate basic effects of faulty frames on a single link and demonstrate how this effect can propagate through the networks and cause remote problems. We further show using simulations that a single slightly delayed frame may lead to frame loss on multiple links. We show that these problems can be alleviated or avoided when TAS-based transmission slots for high-priority traffic are configured longer than needed or if they are not limited at all.