Real Time Systems
What is a Real-Time System?
Real-time systems have been defined as:
"those systems in which the correctness of the system depends not only on
the logical result of the computation, but also on the time at which the
results are produced".
Examples : ABS, aircraft control,
ticket reservation system at airport, over-temperature monitor in nuclear power
station, mobile phone, oven temperature controller, Doppler blood-flow monitor,
ECG/arrhythmia monitor.
Failure of Real-Time System
Failure is inability to perform according to specification. In the
case of real-time systems the ‘failed’ specification may be lack of correctness
or the failure to produce the response by the required time. A real-time system
is one whose correctness is based on both the correctness of
the outputs and their timeliness. The ‘novelty’ here is that the
system is time critical.
Real-Time Characteristics
•
Real-time
systems often are comprised of a controlling system, controlled
system and environment.
1.
Controlling system: acquires information about
environment using sensors and controls the environment with actuators.
•
Timing constraints derived from physical
impact of controlling systems activities. Hard and soft constraints.
1.
Periodic
Tasks: Time-driven recuring at regular intervals.
2.
Aperiodic:
event-driven.
Typical Real-Time System
Digital
control systems - periodically performs the following job:
- senses
the system status and
- actuates
the system according to its current status
Deadlines: Hard vs. Soft
Hard deadline
1.
Disastrous
or very serious consequences may occur if the deadline is missed
2.
Validation
is essential: can all the deadlines be met, even under worst-case scenario?
3.
Deterministic
guarantees
Soft deadline
1.
Ideally,
the deadline should be met for maximum performance. The performance degrades in
case of deadline misses.
2.
Best
effort approaches / statistical guarantees
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