Hard and Soft Real time systems

As in general, Real time systems have to finish a particular job within the specified time. In other words, Real time systems have time restrictions and they have to pursue the deadline. According to the following factors, they can be classified as Hard and Soft real time systems:

• the permissible(tolerance) level of not meeting the deadline
• the usefulness of the result of the job obtained after the deadline is expired
• the severeness of the penalty paid when the deadline is not met

 In Hard real time systems, the permissible level of not meeting the deadline is almost zero. In other words, the deadline must be met. The result obtained after the deadline is useless. The penalty paid when the deadline is not met is destruction or failure of the system.

In soft real time systems, the permissible level of not meeting the deadline is not zero. The usefulness of the result obtained after the deadline is not zero. It depreciates gradually over period of time. Even when the deadline is not met, the effect is not so destructive physically.

So, Hard real time systems have almost zero flexibility and they have to meet the deadline at any cost. Meet the deadline or failure of the system. And the failure of the system causes extremely high penalty, even loss of human life. The result obtained after the deadline is almost useless. For example, a car engine control system is a hard real-time system because a delayed signal may cause engine failure or damage. Other examples of hard real-time embedded systems include medical systems such as heart pacemakers and industrial process controllers.

Though soft real time systems also have to meet the deadline, they have flexibility. They can change the flexibility level or set an average value. Though there is no damage when the deadline is not met, depend on the application it will have its own cost proportional to the delay. Live audio-video systems are also usually soft real-time; violation of constraints results in degraded quality, but the system can continue to operate.