Simulation and Modelling - Old Questions

11.  What is absolute clock and relative clock time in GPSS? How do you update the clock time in simulation? Explain.

Absolute clock measures the simulated time that has elapsed since the simulation began.

Relative clock tells how much simulated time has elapsed since a RESET statement was most recently executed. The relative clock has no special meaning unless one or more RESET run-control statements are used in the model file.

When there are no RESET statements in a model file, the RELATIVE and ABSOLUTE CLOCKS have identical values.

Clock time is updated based on the following two models:

1. Fixed time-step model

In this the timer simulated by the computer is updated at a fixed time interval. The system is checked to see if any event has taken place during that interval. All the events which take place during the time interval are considered to have occurred simultaneously at the end of the interval.

2. Event-to-event model

It is also known as the next-event model. In this the computer advances the time to the occurrence of the next event. So it shifts from one event to the next event and the system state does not change in between. A track of the current time is kept when something interesting happens to the system.

The flowcharts for both models is shown below.