Modem systems are characterized by a multifunctional capability. They are designed to accomplish not one but a series of missions, each one requiring the performance of certain functions. Each of those functions requires the support of some system elements. The fact that an element is not available at certain point in time by no means implies that the entire system is "down" at that moment as the traditional availability definition and formulation requires. Depending on what the mission-function and function-element support requirements are, the "down" condition of a certain element may prevent the accomplishment of some system missions, but some others will still be available. Moreover, the traditional approach assumes that the time to failure and the time to repair associated to each element follow both a negative exponential distribution. Therefore, a more comprehensive treatment of the concept of system availability is required.

All the necessary assumptions to enable the definition and quantification of availability figures of merit are listed. Then, definitions are established for availability and degraded availability at different levels in the system structure, from element to system. In addition, some related concepts such as mission reliability and dependability are defined. The developed model enables the prediction of the defined availability figures of merit. The foundation of the model is the renewal process associated with each system element and the links that specify the mission-function and function-element support requirements. The formulation for some related concepts is also presented.

Some well-known pairs of distributions are considered and the general expressions are particularized for them. Finally, an example is conducted in order to show the applicability of the derived expressions and to compare the obtained results with those obtained using the traditional approach.