COMNET III CACI

only those details which are significant for answering the problem at hand. This implies
that you are aware of the problem at hand, i.e. that you have a clear understanding about
what statistics you are looking for. In other words: you need to define a goal for the
simulation, and this goal will define the scope of the details which should be modeled.
Typical goals which can be modeled with COMNET III are:
• Performance modeling: obtaining utilization statistics for the nodes, buffers, links,
ports in the network.
• Resilience analysis: obtaining utilization or delay statistics about the impacts of
network device failures.
• Network design: obtaining delay and utilization statistics about alternative network
designs and making sure that the design supports the network services requirements.
• Network planning: obtaining performance statistics for future network changes, such
as the addition of new users, applications or network devices.
This requirement for network modeling also implies that the same network might result in
different simulation models due to differences in these goals. For example, a study might
focus on the signaling side of the network. The links might only represent the available
bandwidth for signaling. Only the signaling traffic might be represented in such a model.
Another study of the same network might focus on the performance under peak traffic
loads. In this case, the traffic profiles correspond to the peak loads, the signaling traffic
might be omitted in the model altogether.
An even more fundamental question is whether you want to model the internal
architecture of an ATM switch or whether you want to model an ATM network. In the
first case, you would enter details about the internal architecture of the switch, such as
possible bus speeds, buffering characteristics and functions, as well as the interactions
between the different components in the switch. If you decide to model an ATM
network, you would typically not model the switches at such a low level of detail. The
argument here is that the detailed functions performed by the switch do not significantly
contribute to the simulation results when looking at an entire ATM network. Adding the
detail to model delays in the order of magnitude of nanoseconds - for example by
modeling individual processing delays within an ATM switch - is not going to contribute
significantly to delay result in the order of magnitude of possibly seconds. The additional
accuracy gained from modeling at a high level of detail is far outweighed by the cost and
effort required to incorporate the details. What is more, the computer running the
simulation must work much harder to keep track of the insignificant events.
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