Abstract
Traditionally, availability has been seen as an atomic property asserting the average time a system is “up” or “down”. In order to model and analyse the availability of computerized systems in a world where the dependency on and complexity of such systems are increasing, this notion of availability is no longer sufficient. This paper presents a conceptual model for availability designed to handle these challenges. The core of this model is a characterization of availability by means of accessibility properties and exclusivity properties, which is further specialized into measurable aspects of availability. We outline how this conceptual model may be refined to a framework for specifying and analysing availability requirements.