Simulation Time and Clock Time

There are different notation of time in simulation study. The most important discussion on time in simulation is about physical time and the simulation time.

Physical time is  a time represented by system such as computer clock. It is also sometime known as clock time.

Where as a simulation time is defined during the experimental design and it may be different then clock time.

Since simulation is a model of real system. During the representation of system in to a simulation model, there may be some gap and all the information may not be included in a model. Only the relevant information are included in a model. Now the time represented by such model may not be equal to the clock time which represent an actual time. The following figure can be used to illustrate this concept.

 
Now the system and its model with time parameters are:
 

Wall clock time
A simulation program can usually obtain the current value of wall clock by reading a hardware clock maintained by some operating system (accurate to some specifiable amount of error).

Simulation time
• It is defined as totally ordered set of values where each value represents an instant of time in the physical system being modeled. Further, for any two values of simulation time T1 representing physical time P1, and T2 representing physical time P2, we have following relations"
              (T2–T1) = K*(P2–P1) for some constant K.
• If (T1<T2), then T1 is said to be occur before T2, and if T1>T2, then T1 is said to occur after T2.

When simulation execution time is paced by wall clock time, then it is known as real time simulation.