What does Quality of Service (QoS) measure in terms of availability?

Quality of Service (QoS) in the context of availability specifically measures the reliability and uptime of a system. The ratio of time a system is available to total time reflects how often users can utilize the service without encountering downtime. This metric is critical in determining the overall availability and effectiveness of a system, thereby directly contributing to the users' ability to access the necessary resources when needed.

While the other choices touch on relevant aspects of user interaction and satisfaction, they do not directly address the measure of availability. Percentages of user satisfaction and quality of experience over time focus more on user perceptions and experiences rather than factual uptime metrics. Average session length pertains to user engagement duration but does not measure the system's availability directly. Thus, the ratio of system availability provides a clear and quantifiable metric for assessing QoS in terms of availability.