Analysis of Timeliness Requirements in Safety-Critical Systems Rogdrio de Lemos, Amer Saeed and Tom Anderson Computing Laboratory University of Newcastle upon Tyne, NE1 7RU, UK Abstract Requirements analysis plays a vital role in the development of safety-critical systems since any faults in the requirements specification will corrupt the subsequent stages of system development. Experience in safety-critical systems has shown that faults in the requirements can and do cause accidents. This paper presents a general framework for the analysis of timeliness requirements in safety-critical systems. The analysis is performed in two distinct phases; for each phase we propose different formalisms and time structures. The specification of the timing constraints is based on an event~action model To illustrate the proposed approach an example based on a train set crossing is presented. Keywords: safety-critical systems, requirements analysis, timeliness requirements, formal models, time modelling. 1. Introduction A safety-critical system is a system for which there exists at least one failure that can be adjudged to cause a catastrophe (e.g. loss of life). A major motivation for the work presented in this paper is to extend the framework for the requirements analysis of safety-critical systems, previously introduced in/Saeed 91/, to allow analysis of timeliness requirements. The aim of the framework is to locate and remove faults related to timing issues introduced during the requirements analysis in the development of software for safety-critical systems. (Although "safety" is an attribute of the system rather than just software, in this paper attention is restricted to problems related to "software safety".) The approach to be followed for the analysis of the requirements is based on a clear separation of the mission and the safety requirements. The mission requirements focus on what the system is supposed to achieve in terms of function, timeliness and some dependability requirements - namely the attributes of reliability, availability and security. On the other hand, the safety requirements focus on the elimination and control of hazards, and the limitation of damage in the case of an accident; thus they are related to the safety attribute of dependability/Laprie 90/. In the proposed framework we are concerned with the timeliness requirements that are related to the safety attribute of dependability.