Timing Constraints in Real-time System

Timing constraints is a vital attribute in real-time systems. Timing constraints decides the total correctness of the result in real-time systems. The correctness of results in real-time system does not depends only on logical correctness but also the result should be obtained within the time constraint. There might be several events happening in real time system and these events are scheduled by schedulers using timing constraints. Classification of Timing Constraints : Timing constraints associated with the real-time system is classified to identify the different types of timing constraints in a real-time system. Timing constraints are broadly classified into two categories: 1. Performance Constraints : The constraints enforced on the response of the system is known as Performance Constraints. This basically describes the overall performance of the system. This shows how quickly and accurately the system is responding. It ensures that the real-time system performs satisfactorily. 2. Behavioral Constraint : The constraints enforced on the stimuli generated by the environment is known as Behavioral Constraints. This basically describes the behavior of the environment. It ensures that the environment of a system is well behaved. Further, the both performance and behavioral constraints are classified into three categories: Delay Constraint, Deadline Constraint, and Duration Constraint. These are explained as following below.

1. Delay Constraint - A delay constraint describes the minimum time interval between occurrence of two consecutive events in the real-time system. If an event occurs before the delay constraint, then it is called a delay violation. The time interval between occurrence of two events should be greater than or equal to delay constraint. If D is the actual time interval between occurrence of two events and d is the delay constraint, then

   D >= d

   
2. Deadline Constraint - A deadline constraint describes the maximum time interval between occurrence of two consecutive events in the real-time system. If an event occurs after the deadline constraint, then the result of event is considered incorrect. The time interval between occurrence of two events should be less than or equal to deadline constraint. If D is the actual time interval between occurrence of two events and d is the deadline constraint, then

   D <= d

   
3. Duration Constraint - Duration constraint describes the duration of an event in real-time system. It describes the minimum and maximum time period of an event. On this basis it is further classified into two types:
   • Minimum Duration Constraint: It describes that after the initiation of an event, it can not stop before a certain minimum duration.
   • Maximum Duration Constraint: It describes that after the starting of an event, it must end before a certain maximum duration elapses.