Simulation and Modelling - Old Questions

9.  Draw and describe different types of GPSS blocks that are used to deal with queues?

GPSS (General Purpose Simulation System) is a highly structured and special purpose simulation language based on process interaction approach and oriented toward queuing systems.
- The system being simulated is described by the block diagram using various GPSS blocks.
- Each block represents events, delays or other actions that affect the transaction flow.

The two main blocks that are used  with queues are QUEUE and DEPART.

QUEUE BLOCK:

This block will instruct GPSS to start gathering queuing statistics on the queue named in its attribute value. SYNTAX: QUEUE A

ATTRIBUTES:  A = name of queue (for example: garage)

DEPART BLOCK:

This block instructs GPSS that a transaction is leaving the queue named in it‟s attribute value. This is necessary in order to compile the statistics on the queue. 

SYNTAX: DEPART A

ATTRIBUTES: A = name of the queue (for example: checkout)

Some other blocks that are used with above blocks are:

GENERATE BLOCK:

This block will produce a flow of transactions with inter-arrival times determined by the attribute values. The distribution of inter-arrival times follows a uniform probability distribution.

SYNTAX: GENERATE A,B,C,D,E

ATTRIBUTES:  A = average value of uniform distribution, B = half-width of uniform distribution, C = time delay before first transaction is generated, D = maximum number of transactions generated, E = priority allocated to transactions

ADVANCE BLOCK:

This block represents the servicing of a transaction. The servicing times follow a uniform probability distribution. 

SYNTAX: ADVANCE A,B

ATTRIBUTES: A = average value of uniform distribution, B = half-width of uniform distribution

TERMINATE BLOCK:

This block destroys any transaction entering it and removes it from computer memory. Each time a transaction enters this block it decrements a counter by an amount equal to its attribute value. The counter is set by the user upon starting the simulation.

SYNTAX: TERMINATE A

ATTRIBUTES:  A = decrements simulation counter by this amount

etc.