Question

Embedded system.What do you understand by the term "real-time"? How is the concept of real-time different from the traditional notion of time? Explain your answer using a suitable example.

Give an example of soft real-time task and non-real-time task. Explain the key difference between the characteristics of these two types of tasks.

Answer 1

a) When there is a required duration by which a particular system has to finish all the desired tasks, the system is called real-time.

b) In a real‐time application, the notion of time stands for the absolute time which is quantifiable. In contrast to real time, logical time, used in most other applications, deals with a qualitative notion of time and is expressed using event ordering relations. Here, the actual duration of time matters less than the order in which the events occur.

c) For example, the actuator for an automatic door closing mechanism (which is an embedded system) may require that the motor to close the door take no more than 3 seconds in order to prevent unauthorised access. Here, the notion of time is absolute, in that no matter the internal logical ordering of tasks, the door closing mechanism must complete before a certain absolute time deadline.

d) An example of a soft-real-time task is calls on a cellular phone. The voice may be noisy or delayed to a certain extent, but if a call is degraded for a long period of time, it is ulitimately disconnected.

An example of a non-real-time task is a legacy email service, wherein it may have taken hours or even days for an email to deliver to the recipient.

e) Key difference between a soft real-time task and a non-real-time task:

In a soft real-time system, there is a hard deadline (or time bound), but some delays are acceptable if they lie in a pre-defined acceptable range. These tasks are always defined as being a quantitative expression of time. Meanwhile, a non-real time task cannot be expressed as an expression of time, and there are no real detriments caused if the task gets delayed. There are no time bounds for a non-real-time task, though logical event ordering constraints may apply.

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