Impact of time-triggered transmission window placement on rate-constrained traffic in TTEthernet networks

Abstract

Safety-critical Ethernet-based networks are receiving significant attention in avionics, automotive and industrial domains. Time-Triggered Ethernet (TTEthernet AS6802) provides safety-critical transmission guarantees via a high priority, time-triggered (TT) traffic class and a lower priority, rate-constrained (RC) traffic class. TT traffic is transmitted between synchronized nodes of a TTEthernet network in offline scheduled TT transmission windows. In this work, we analyze the impact of different placement strategies for these TT transmission windows on end-to-end delay and jitter of RC messages on the same path segment. We show that, depending on the placement of TT transmission windows in a schedule, the end-to-end delay and jitter of RC messages can vary significantly. We further introduce link-based offsets, a new placement strategy for TT transmission windows which allows to reduce the impact of TT transmission windows on RC traffic. In this strategy offsets are applied to all TT transmission windows in a physical link schedule to reduce the amount of time that an RC message on the same physical link is delayed by TT traffic. The link-based offsets strategy can be implemented in the TTEthernet scheduler and does not require hardware modifications. We show that the link-based offset strategy can reduce the end-to-end delay and jitter of RC traffic, and evaluate our claims using an OMNET++ simulation.