I. Introduction
The increased use of information systems also brings about increased demands on them [1–3]. One such requirement, particularly for the operation of information systems in autonomous mode, is functional reliability [4, 5]. In other words, an information system that is put into operation must perform the assigned tasks [6] for a specified period, despite its own degradation and various negative influences. Therefore, there arises a need for a formal description of the functional reliability of systems.