GERT Simulation Program simulates GERT networks to obtain statistics on specified nodes of the network. It performs sampling experiments to determine which branches of the network are taken and how long it takes to traverse a branch of the network

Alan, A.

Pritsker, B.

English

NASA Technical Reports Server

December 1968
	 Brief 68-10457 
NASA TECH BRIEF 
NASA Teèh Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
GERT Simulation Program for GERT Network Analysis 
The problem: 
To develop a method to obtain statistics on speci-
fied nodes of GERT networks in determining which 
branch of the network should be taken and how long 
it would take. 
The solution: 
The GERT Simulation Program simulates GERT 
networks to obtain statistics on specified nodes of the 
network. It performs sampling experiments to deter-
mine which branches of the network are taken and 
how long it takes to traverse a branch of the net-
work. 
How it's done: 
The program for simulating GERT networks in-
volves the use of the GASP IIA simulation knowledge. 
Knowledge of simulation techniques and GASP IIA 
is not necessary in order to use the GERT Simulation 
Program. The input to the program is a description 
of the network in terms of its nodes and branches 
along with control information for setting up the 
simulation conditions. After receiving the network to 
be simulated from the input data and the number of 
simulations of the network to be performed, the pro-
gram automatically performs a simulation by selec-
ting branches to be traversed based on random num-
bers, and the time it takes to perform the activities 
represented by the branches which are based on sam-
ples from the distributions inserted as input informa-
tion. Statistics are automatically collected on nodes 
as specified by the user of the program. When all 
simulations requested by the user have been completed, 
a summary report which describes the network 
simulated and the final results are printed. The final 
results include the following statistics: 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
1. The probability that a specified node is realized 
2. The average time to realize the specified node 
3. An estimate of standard deviation of the time to 
realize the specified node 
4. The minimum time observed to realize the speci-
fied node 
5. The maximum time observed to realize the speci-
fied node 
6. A histogram of the times to realize the specified 
node' 
A user of the GERT Simulation Program need only 
be familiar with the input specifications in order to 
simulate a GERT network. 
The program performs a simulation by advancing 
time from event to event. In simulation parlance this 
is termed a next event simulation. The events as-
sociated with a GERT network are: 
1. Start of the simulation, and 
2. End of an activity. 
The start event causes all source nodes to be real-
ized and schedules the activities emanating from the 
source nodes according to the output type of the 
source node. The output type for all nodes is either 
deterministic or probabilistic. In the former case, 
all activities emanating from the node are scheduled 
and, in the latter case, only one of the activities ema-
nating from the node is scheduled. By scheduling an 
activity is meant that an event "end of activity" is 
caused to occur at some future point in time. A next 
event simulation proceeds from event to event until 
the conditions which indicate that the simulation is 
completed are obtained. For the simulation of GERT 
networks, all events involve end of activity times and 
the simulation of the network proceeds from one end 
of activity
 time to another.
(continued overleaf) 
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19680000455 2020-03-12T06:03:58+00:00Z
Notes: 
1. This program is written in the GASP ha program-
ming language which in turn is written in Fortran 
IV as specified by the IBM 1130 computer. 
2. The number of nodes that can be accommodated 
by the program is dependent upon the storage 
capacity of the machine being used. The present 
version of the program is dimensioned to permit 
a network with 30 nodes and 40 branches. This 
assumes that the machine being used has only 
8000 words of core storage.
3. Inquiries concerning this innovation may be di-
rected to:
COSMIC 
Computer Center 
University of Georgia 
Athens, Georgia 30601 
Reference: B68-10457 
Patent status: 
No patent action is contemplated by NASA. 
Source: A. Alan B. Pritsker 
of Arizona State University 
under contract to 
Electronics Research Center 
(ERC- 10209) 
Brief 68-10457	 1	 Category 06


GERT simulation program for GERT network analysis

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