TABLE OF CONTENTS This chapter presents the major elements of reliability theory, and more generally what is called dependability.
Dependability encompasses all these aspects of reliability, availability, maintainability, which play a major role in communication systems and networks. Indeed ensuring the continuity of service is one of their first missions, especially in case of emergency (e.g. calling the fire brigade), and in any disaster situation. To reach such a goal, operators must deploy secured architectures. As this increases the cost of the system, it is worth evaluating their efficiency. Reliability theory is the set of techniques to apply in order to perform this evaluation. This chapter presents these techniques, giving numerous illustrating examples.
According to the standardization and particularly to the IEC (International Electrotechnical Commission), reliability is "the probability that a system will perform its intended function for a specified time interval under stated conditions". This will correspond to the "success" of the mission. On the opposite, the mission fails once the system breaks down. We will compute the probability of success at time t, or probability of working for a given time interval t.
In the telecommunication domain, and particularly for terrestrial equipments that are generally repairable, another important feature is the long-term proportion of time the service is available; one then refers to this concept as availability. The availability function is defined as the probability that the system is working (into service) at any instant of time t, whatever could be...
