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5. GPSS/H Control Concepts

So far, GPSS/H has been introduced and described as a system for modeling queueing systems and nets. The incoming requests were embodied by Transactions proceeding through the model. Their route was determined by the Blocks they had to pass. Their interaction was limited to being in each others way or rivaling in order to accesslimited resources.
The following chapter will now be focused on the interaction of Transactions, the interaction of simulation- and animation model and the internal control as well as the one defined by the user.Therefore we will no longer be considering the simulation of queueing systems, butintroduce GPSS/H's universal modeling philosophy, which is suitable for any system.

Table of Contents

5.1. GPSS/H Macros and Subroutines

In GPSS/H like in other programming languages, Macros are used for generating similar parts of source code. Thus the source code for large models can be simplified, shortened and is much easier to understand. As an example we will introduceand describe animation macros which are useful to generate animation trace files.
Subroutines have one advantage compared to macros: They don't expand the source code. Subroutines are a series of Blocks being passed by Transactions of different origin. After proceeding through these Blocks, the Transactions can return to their starting point.

5.2. GPSS/H Internal Control

So far GPSS/H's internal control has been mentioned several times as a main means for controling the movement of Transactions and for determining sequences to execute operations.Here it will again be described step by step.
Data structures used by the internal control will also be described. In addition to simulation clocks, these are different chains of Transactions, which will be discussed here one more time.

5.3. Preemption and User Chains

In order to reconstruct real processes, there is a possiblity to preempt facilities as well as temporary or final replacement of Transactions currently occupying a Facility.
GPSS/H's internal control controls the proceeding Transactions following a fixed algorithm. In order to reconstruct specific processes instead of using those binding sequences, selected Transactions can escape the internal control if moved to User Chains. They will stay there until they are freed by another Transaction. By this means it is possible to model interactive processes of the real world.

5.4. Assembly sets and Groups

Transactions can clone themselve. Each Transaction, together with its clones, forms an assembly set. Transactions of the same assembly set can wait for each other. Their proceeding through the model can be sychronized. After being separated, they can also be gathered and put together again.
With the means reviewed so far, a direct exchange of information between Transactions is impossible. This problem can be solved by inserting a Transaction in a Group or Set. The parameters of these Transactions are then accessable by other Transactions. Others can then read the former private parameters or assign them new values. In addition to groups of Transactions there are groups of values. These are sets, for which several set operations are available.

5.5. Model Experiments and Output AnalysisAt Work!

In general, a single simulation is not enough to answer the questions raised in a simulation study. Series of experiments have to be executed, using GPSS/H has several statements supporting experiments. They will be introduced and described there.
5. GPSS/H Controling Concepts
5.1.GPSS/H Macros and Subroutines
5.2.GPSS/H's Internal Control
5.3.Preemption and User Chains
5.4.Assembly Sets and Groups
5.5.Controling Experiments
Global Outline
1.Introduction to Modeling, Simulation and Animation.
2.Proof Animation™.
3.GPSS/H as a Programming language.
4.GPSS/H Queueing Models.
5.GPSS/H Controling Concepts.
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