A "soft" real-time system (video streaming) can tolerate rare missed frames. A "hard" real-time system (airbag deployment) cannot. Liu provides the formulas to calculate worst-case response time (RTA), accounting for release jitter and interrupt latency.
Not all real-time systems are event-driven. Liu dedicates significant space to Time-Division Multiple Access (TDMA) and static cyclic scheduling, which is still used in avionics (ARINC 653) and automotive (FlexRay). Real-time Systems By Jane W. S. Liu Pdf
The text is built upon several foundational pillars: A "soft" real-time system (video streaming) can tolerate