Link Between Real-Time Scheduling and Time-Triggered Networks

We demonstrate that, for periodic systems with offsets (tasks or flows of messages within a time-triggered network), the simulation cycle can be confined to the range of [0, hyperperiod) only under the condition that an idle point exists at the hyper-period. Furthermore, we establish that ensuring both (1) contention-freedom and (2) that no offset exceeds the value of the period minus duration, is a sufficient condition to guarantee the presence of an idle point at the hyper-period. Most contemporary methods aiming to eliminate latency are implicitly based on these properties and fail to propose a schedule if the input system does not allow a contention-free solution. Consequently, we propose a heuristic approach to scheduling periodic flows of frames within time-triggered networks. Our method focuses on minimizing latency, without necessitating a solution where every frame at every output port is contention-free, and it effectively manages the cyclicity problem.

I. Introduction

This paper is a product of extensive deliberations between researchers with expertise in real-time scheduling and those in real-time networks. Since the advent of time-triggered networks, an increasing number of papers address what real-time schedulers call an offset-free problem (where the designer chooses the offset). We have periodic flows of frames to be sent over a network, and the question to be addressed is how to assign an offset to the flows in order to be able to schedule them with low or no latency over the output ports of the encountered switches. While offset-free systems in real-time scheduling have been mainly studied from the late 1990s to the early 2000s, in works such as [1], offset-free studies in networks are more recent [2], but are more focused on recent standards, especially regarding Scheduled Traffic in Time-Sensitive Networking (TSN).