Real-time fault tolerant operation of the 802.5 token ring

Abstract

Scheduling a hard real time application on a token ring requires that the delays to capture the token be known. Previous studies considered only a small subset of the token ring mechanisms in schedulability analyses. In this paper, we study the influence of three types of faults on the time to capture the token. These are token losses, failures of the active monitor and cable faults, We also show the influence of the stack mechanism on the delay to capture the token. The paper ends with an evaluation of these delays in various ring configurations in which the transmission speed, token hold time and ring latency are variables. We conclude that the token ring may be very difficult to schedule for hard real time applications because we do not know if other mechanisms or combinations of phenomena can introduce additional delays that make the operation of the ring unpredictable in the present state of knowledge.