During the last decade, cybersecurity assessment has become a key issue among all security problems encountered in designing and managing complex systems. Today, for dealing with cybersecurity, many sophisticated models are available. Still, the classic CIA triad of confidentiality, integrity, and availability remains actual and useful. In this article, following the CIA model, we investigate cybersecurity of control systems. After some research, we make it clear that availability is the most critical property of cybersecurity in the operational technology context, and so can be used as an indicator of cybersecurity. Conducting a review of availability definitions, we elect a proper interpretation of availability with a delay as a measure for its evaluation. For calculations, we use a theory of deterministic queueing systems Network calculus, but in contrast to the common practice of delay calculation through the bounds of the arrival flow and service, we investigate another critical case where a delay can be estimated from the knowledge of the backlog bound and the framework of service curves. A problem for systems with a linear relationship between the minimum and maximum service curves and a special case of rate-latency service curves are examined.