A hierarchical approach to systems with heterogeneous real-time requirements

Abstract

This paper presents a family of real-time executives, designed by Telettra, for telecommunication, telecontrol, process control and supervisory applications.

These applications are subjects to two different types of real-time requirements: deadlines and throughput. Rather than designing a single executive capable to face both requirements, a two level, hierarchical approach has been taken. Two executives coexist on a single processor: a low level, periodic executive for tasks with strict deadline constraints, and a higher level, multitask executive for tasks with throughput constraints, or tasks with weaker deadline constraints that can be specified and dealt with at application level with little support from the executive.

Thanks to the hierarchical approach, simple mechanisms are sufficient to support communications between the two levels. Facilities are also provided to support load control in the deadline oriented environment, according to policies that are defined by the multitask level application.

The presence on all computational nodes of a multitask environment is a key characteristic, since it allows a highly modular style of programming, and facilitates the construction of distributed systems.

The paper shows how these ideas are applied in the design of the peripheral processor of a telephone switching system.