Independent WCRT analysis for individual priority classes in Ethernet AVB

Abstract

In the high-tech and automotive industry, bandwidth considerations and widely accepted standardization are two important reasons why Ethernet is currently being considered as an alternative solution for real-time communication (compared to traditional fieldbusses). Although Ethernet was originally not intended for this purpose, the development of the Ethernet AVB standard enables its use for transporting high-volume data (e.g. from cameras and entertainment applications) with low-latency guarantees. In complex industrial systems, the network is shared by many applications, developed by different parties. To face this complexity, the development of these applications must be kept as independent as possible. In particular, from a network point of view, progress of all communication streams must be guaranteed, and the performance for individual streams should be predictable using only information regarding the stream under study and the general parameters of the communication standard used by the network. Initial methods to guarantee latency for Ethernet AVB networks rely on the traditional busy-period analysis. Typically, these methods are based on knowledge of the inter-arrival patterns of both the stream under study and the interfering streams that also traverse the network. The desired independence is therefore not achieved. In this paper, we present an independent real-time analysis based on so-called eligible intervals, which does not rely on any assumptions on interfering priority classes other than those enforced in the Ethernet AVB standard. We prove this analysis is tight in case there is only a single higher-priority stream, and no additional information on interference is known. In case there are multiple higher-priority streams, we give conditions under which the analysis is still tight. Furthermore, we compare the results of our approach to the two most recent busy-period analyses, point out sources of pessimism in these earlier works, and argue that assuming more information on the sources of interference (e.g. a minimal inter-arrival time between interfering frames) has only limited advantages.