The time-triggered architecture (TTA) provides a computing infrastructure for the design and implementation of dependable distributed embedded systems. A large real-time application is decomposed into nearly autonomous clusters and nodes, and a fault-tolerant global time base of known precision is generated at every node. In the TTA, this global time is used to precisely specify the interfaces among the nodes, to simplify the communication and agreement protocols, to perform prompt error detection, and to guarantee the timeliness of real-time applications. The TTA supports a two-phased design methodology, architecture design, and component design. During the architecture design phase, the interactions among the distributed components and the interfaces of the components are fully specified in the value domain and in the temporal domain. In the succeeding component implementation phase, the components are built, taking these interface specifications as constraints. This two-phased design methodology is a prerequisite for the composability of applications implemented in the TTA and for the reuse of prevalidated components within the TTA. This paper presents the architecture model of the TTA, explains the design rationale, discusses the time-triggered communication protocols TTP/C and TTP/A, and illustrates how transparent fault tolerance can be implemented in the TTA.

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The time-triggered architecture

The Time-Triggered Architecture (TTA) is a computer architecture for distributed real-time systems in safety critical applications, such as computer controlled brakes, or computer assisted steering in an automobile. The TTA is a composable architecture for the design of large real-time systems. Its main characteristics are a common notion of time in all subsystems of the architecture and the provision of fully specified interfaces, called temporal firewalls, between these subsystems. This paper gives an overview of the TTA, discusses the architectural principles, describes the sensor/actuator interfaces in the TTA and informs about the implementation of fault-tolerance in the TTA.

In real-time systems, the timing behavior is an important property of each task. It has to be guaranteed that the execution of a task does not take longer than the specified amount of time. Thus, a knowledge about the maximum execution time of programs is of utmost importance. This paper discusses the problems for the calculation of the maximum execution time (MAXT... MAximum eXecution Time). It shows the preconditions which have to be met before the MAXT of a task can be calculated. Rules for the MAXT calculation are described. Triggered by the observation that in most cases the calculated MAXT far exceeds the actual execution time, new language constructs are introduced. These constructs allow programmers to put into their programs more information about the behavior of the algorithms implemented and help to improve the self checking property of programs. As a consequence, the quality of MAXT calculations is improved significantly. In a realistic example, an improvement fator of 11 has been achieved.

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Calculating the maximum, execution time of real-time programs

This paper summarizes the state of the real-time field in the areas of scheduling and operating system kernels. Given the vast amount of work that has been done by both the operations research and computer science communities in the scheduling area, we discuss four paradigms underlying the scheduling approaches and present several exemplars of each. The four paradigms are: static table-driven scheduling, static priority preemptive scheduling, dynamic planning-based scheduling, and dynamic best effort scheduling. In the operating system context, we argue that most of the proprietary commercial kernels as well as real-time extensions to time-sharing operating system kernels do not fit the needs of predictable realtime systems. We discuss several research kernels that are currently being built to explicitly meet the needs of real-time applications. >

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Scheduling algorithms and operating systems support for real-time systems

Totem: a fault-tolerant multicast group communication system

The authors describe the Maintainable Real-Time System, a fault-tolerant distributed system for process control, developed under the Mars project started in 1980 at the Technische Universitat Berlin. They explore the characteristics of distributed real-time systems and then present the Mars approach to real-time process control, its architectural design and implementation, and one of its applications. The authors focus on the maintainability of the Mars architecture, describe the Mars operating system, and discuss timing analysis. The control of a rolling mill that produces metal plates and bars is examined. >

Distributed fault-tolerant real-time systems: the Mars approach

Verteilte Prozesrechneranwendungen steilen hohe Anforderungen an das Computerkontrollsystem sowohl in bezug auf die Einhaltung von zeitlichen Schranken als auch in bezug auf Zuverlassigkeit und Fehlertoleranz. Das MARS-Entwurfssystem berucksichtigt diese Anforderungen und unterscheidet zwischen dem kreativen Entwerfen (Entwurfsphase) und der Bewertung eines Entwurfes (Bewertungsphase). Beide Phasen werden durch entsprechende Tools mit graphischer Benutzerschnittsteile unterstutzt. Die Entwurfsphase fuhrt durch alternierendes Definieren und Verfeinern von Objekten von der Definition der Anforderungen uber System-, Cluster- und Komponentendesign bis zur Implementierung von Tasks und Nachrichten. Die Bewertungsphase uberpruft, ob der Entwurf eines Systems unter den gegebenen zeitlichen Bedingungen (Antwortzeiten auf externe und interne Stimuli, etc.) realisierbar ist und fuhrt eine Zuverlassigkeitsanalyse (Berechnung von Verfugbarkeit, Zuverlassigkeit, mittlere Zeit bis zum Ausfall, etc.) durch.

C. Senft

Papers

Distributed fault-tolerant real-time systems: the Mars approach

Entwurf und Bewertung fehlertoleranter, verteilter Echtzeitsysteme am Beispiel MARS