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 and 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 emergencies (e.g. calling 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 efficiency. Reliability theory is the set of mathematical techniques to apply in order to perform this evaluation. This chapter presents these techniques, giving numerous 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. Conversely, the mission fails once the system breaks down. We will calculate 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 equipment that is generally repairable, another important feature is the long-term proportion of time the service is available; we then refer 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 the preceding states...
