Active and passive replication models constitute an effective way to achieve the availability objectives of distributed real-time (DRE) systems. These two models have different impacts on the application performance. Although these models have been commonly used in practical systems, a systematic quantitative evaluation of their influence on the application performance has not been conducted. In this paper we describe a methodology to analyze the application performance in the presence of active and passive replication models. For each one of these models, we obtain an analytical expression for the application response time in terms of the model parameters. Based on these analytical expressions, we derive the conditions under which one replication model has better performance over the other. Our results indicate that the superiority of one replication model over the other is governed not only by the model parameters but also by the application characteristics. We illustrate the value of the analytical expressions to assess the influence of the parameters of each model on the application response time and for a comparative analysis of the two models.

This paper examines timing constraints as features of realtime systems. It investigates the various constructs required in requirements languages to express timing constraints and considers how automatic test systems can validate systems that include timing constraints. Specifically, features needed in test languages to validate timing constraints are discussed. One of the distinguishing aspects of three tools developed at GTE Laboratories for real-time systems specification and testing is in their extensive ability to handle timing constraints. Thus, the paper highlights the timing constraint features of these tools.

Abstract This paper describes a formal model for expressing the functional requirements of the man‐machine interfaces of interactive systems. It also shows how this model can facilitate the automation of other useful activities such as checking for inconsistency, redundancy, and incompleteness in the specification, and validating the implementation of the interface against its original requirements. Finally, the paper comments on the authors' experience in developing an interactive system using this formal model.

The paper introduces a new transformation on ordered trees which is used to compute a few average properties of ordered trees. The properties that are studied are: (a) the average path length of an ordered tree with n nodes, (b) the average number of terminal nodes of an ordered tree with n nodes and (c) its average height. An algorithm is also presented to effect this transformation on any ordered tree T. The algorithm is linear in the number of nodes in T.