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A Unified Performability Evaluation Framework for Computer and Communication Systems

A Unified Performability Evaluation Framework for Computer and Communication Systems Aad P. A. van Moorsel Boudewijn R. Haverkort
Tele-Informatics and Open Systems University of Twente P.O. Box 217, 7500 AE, Enschede, The Netherlands moorsel@cs.utwente.nl

Abstract
In this paper we discuss a unified approach towards model-based quantitative evaluation of both computer systems and communication systems. In the area of fault-tolerant computer systems, dependability evaluation has been recognized as being a topic of importance, both to judge a system on its merits and to provide trust in the actual dependability of the system. In communication systems, the need for identifying and evaluating quality of service parameters is becoming more and more apparent because of increasing demands on for instance speed and availability. In this paper we construct a framework, the so-called performability evaluation framework, within which the quantitative evaluation of both types of systems can be discussed. The framework closely resembles the performability framework introduced by Meyer [18]. We present a general view, a system view and a modelling view on performability evaluation, leading to a framework which naturally fits known measure definitions, modelling methods and solution techniques. We will especially discuss the importance of evaluating useful performability measures. In this respect we will distinguish between system measures and task measures. Task measures directly relate to evaluation of the service, and we will argue that in performability evaluation task measures should be evaluated. We relate the performability evaluation framework with known concepts in both the areas of computer and communication systems.

1. Introduction
In this paper we develop the so-called performability evaluation framework (PEF). The goal of the PEF is twofold. Within the PEF we motivate our approach to model-based quantitative evaluation of computer and communication systems and it will form a

A Unified Performability Evaluation Framework for Computer and Communication Systems

framework within which modelling methods, definitions of measures and solution techniques can be discussed. We construct the PEF in three phases. First, we identify that our first aim should be to do meaningful quantitative evaluation. This gives us the basic definition and motivation of performability evaluation. Secondly, we create abstractions from the application area of computer and communication systems by defining concepts such as system, service and user. In terms of these concepts we will redefine performability. The third step consists of a model definition, in which we distinguish between system and service (or task) issues. Again, we will make the definition of performability more specific by reformulating it in terms of the modelling concepts. Relevant measures and modelling and solution techniques can be discussed in the resulting framework. One will recognize in this paper several ideas similar to earlier suggested related concepts and frameworks. In the area of fault-tolerant computer systems, the concern of evaluating the so-called dependability of the system has lead to a conceptual framework of Laprie [17], and to the performability framework of Meyer [18] [20]. Especially, the concept of performability evaluation as introduced by Meyer is closely related to the presented PEF. This explains why we have adopted the term performability for our framework. Our starting point is somewhat different, however. In [18] the need for a more expressive measure than pure reliability motivated the definition of the combined performance/reliability measure “performability” to evaluate the quality of service of degradable systems. In this paper we take a more global view and try not to identify what measure is actually of interest, but what can be of interest in general. However,...
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