Real-time scheduling - Distributed scheduling

The complexity of the processes to be controlled or supervised, the large number of data and events to be processed, the geographical distribution of processes, and the arrival on the market of industrial local area networks over the last few years have all led to a rethinking of centralized real-time applications. Today, the notion of distributed real-time architectures is widely accepted in industrial circles. By way of illustration, the application domains that commonly make use of real-time and distributed systems are :

• telecommunications (switching systems, etc.) ;

• the medical field (assistance and monitoring of patients, etc.);

• inspection of vehicle equipment (engine, brakes, suspension, etc.);

• urban traffic control and regulation ;

• industries (process control) ;

• military applications (missile trajectory tracking, etc.);

• aerospace (satellite tracking, automatic piloting, etc.);

• multimedia (teleconferencing, teleshopping, etc.) ;

• home automation (home security, etc.).

A computer system designed to control or supervise operations is usually made up of several processing units (computers or programmable logic controllers), sensors, actuators, peripherals for visualization and dialogue with operators. All these elements are interconnected by a network, or a whole host of interconnected networks (industrial networks, office networks, field buses, etc.), as shown in figure 1 . This type of system is known as a distributed real-time system.

In this type of system, task and message scheduling play a fundamental role. Centralized task scheduling is covered in the article "Real-time scheduling. Centralized scheduling" of this treatise. Message scheduling is the subject of this article.