Schedulability analysis of real-time traffic in WorldFIP networks: an integrated approach

Abstract

The WorldFIP protocol is one of the profiles that constitute the European fieldbus standard EN-50170. It is particularly well suited to be used in distributed computer-controlled systems where a set of process variables must be shared among network devices. To cope with the real-time requirements of such systems, the protocol provides communication services based on the exchange of periodic and aperiodic identified variables. The periodic exchanges have the highest priority and are executed at run time according to a cyclic schedule. Therefore, the respective schedulability can be determined at pre-run-time when building the schedule table. Concerning the aperiodic exchanges, the situation is different since their priority is lower and they are bandied according to a first-come-first-served policy. In this paper, a response-time-based schedulability analysis for the real-time traffic is presented. Such analysis considers both types of traffic in an integrated way, according to their priorities. Furthermore, a fixed-priorities-based policy is also used to schedule the periodic traffic. The proposed analysis represents an improvement relative to previous work and it can be evaluated online as part of a traffic online admission control. This feature is of particular importance when a planning scheduler is used, instead of the typical offline static scheduler, to allow online changes to the set of periodic process variables.