A GENERAL VIEW - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 9

In critical applications, a result produced after the deadline is not only late but wrong!
Depending on the consequences that may occur because of a missed deadline, a real-
time task can be distinguished in three categories:
Hard : A real-time task is said to be hard if producing the results after its deadline
may cause catastrophic consequences on the system under control.
Firm : A real-time task is said to be firm if producing the results after its deadline
is useless for the system, but does not cause any damage.
Soft : A real-time task is said to be soft if producing the results after its deadline
has still some utility for the system, although causing a performance degradation.
A real-time operating system that is able to handle hard real-time tasks is called a
hard real-time system . Typically, real-world applications include hard, firm, and soft
activities; therefore a hard real-time system should be designed to handle all such
task categories using different strategies. In general, when an application consists
of a hybrid task set, all hard tasks should be guaranteed off line, firm tasks should be
guaranteed on line, aborting them if their deadline cannot be met, and soft tasks should
be handled to minimize their average response time.
Examples of hard tasks can be found in safety-critical systems, and are typically re-
lated to sensing, actuation, and control activities, such as the following:
Sensory data acquisition;
data filtering and prediction;
detection of critical conditions;
data fusion and image processing;
actuator servoing;
low-level control of critical system components; and
action planning for systems that tightly interact with the environment.
Examples of firm activities can be found in networked applications and multimedia
systems, where skipping a packet or a video frame is less critical than processing it
with a long delay. Thus, they include the following: