Nowadays, industrial control systems are becoming more digital, more complex, and more interconnected causing growing anxiety about their safety, security, and especially cybersecurity. For dealing with all security problems including cybersecurity assessment, security programs are utilized where the properties of confidentiality and integrity are characterized in detail. But the availability attribute often suffers due to a lack of attention, which makes the assessment of availability grow into one of the thorniest issues. The article investigates cybersecurity in the industrial control systems context, clarifies the great value of availability, and explains a reasonable shift between cybersecurity and availability assessment problems. A delay of the signal transmission is discovered to be a suitable measure of the quantitative availability assessment, and a theory of deterministic queuing systems Network calculus is advocated to be a relevant tool for the delay estimation and availability modelling. A reference model for the availability assessment and also an appropriate metric based on delay and system dependency are proposed. The results of the verification of the applicability of Network calculus to solving the delay estimation and cybersecurity assessment problems are presented.