Conventional and early token release scheduling models for the IEEE 802.5 token ring

Abstract

We develop analyticalscheduling models for both the original IEEE 802.5 token ring protocol and a recent extension to the original protocol that allows early token release (ETR). A scheduling model is an abstraction that supports reasoning about the timing correctness of a given set of real-time messages scheduled on the network. Scheduling analysis of the original IEEE 802.5 token ring protocol has previously been discussed in Strosnider and Marchok (1989) and Pleinevaux (1992) in the context of improving responsiveness of soft deadline aperiodic messages. In contrast, this paper develops schedulability conditions for arbitrary periodic message sets. The main contributions of this work are: Scheduling models for both the original protocol and ETR protocol; comparison for maximum achievable utilizations for the two protocols; comparison between the original protocol and ETR from a schedulability viewpoint. We also demonstrate the utility of our scheduling models to select network operating parameters such as maximum packet size, and to quantify effects of parameters such as the number of stations, and network size on schedulability.