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appropriate security clearance, could join the system from anywhere and its user could participate in group and
individual work as desired.
The advantages of the Praxis Java-based framework are many, but the main ones are those attributable to the
nature of the Java language itself, including a profoundly object-oriented design. These advantages are:
simplicity, easy (i.e. piece by piece) expansion/modification, ease of maintenance, and relatively low
development costs. Cross platform connectivity, centralized automatic upgrades, no need for any special client
software, and that it supports multi-user, multi-level, multi-role distributed cooperative workgroups are further
advantages of the framework.
The concept behind the Praxis framework is distributed communities including communities of communities,
their needs for information retrieval and processing, and especially the communal workgroup environment,
including different levels of security clearance and different roles. Different roles allow different individuals
with different requirements to see and deal with different subsets of information in different ways as defined by
the role type attached to their authorization.
Since the entire framework is platform independent and completely modular, new components, including new
sources, are simply added as plug-ins. Clients do not normally require any special software, and server
software may be added to the system from a central location during ordinary operation. The existing
framework has been designed with an emphasis on distributed servers and distributed working groups as a
platform for complex multivariate simulations.
The Simulation System
The simulation system is built around certain devices and concepts:
• Interest Groups - groups of people who are interested in a certain aspect of a simulation or knowledge
base.
• Roles - individuals within an interest group may interact with others in a specific way, e.g. an instructor
has different interaction requirements from a trainee.
• Scenarios - a simulator ought to be able to emulate certain incidents or accidents - we term each emulation
sequence a scenario
• Scripts - a script is a certain kind of code which controls the simulation engine in such a way as to produce
a scenario. Scripts are written in a language which is both conditional and massively parallel.
• Communications - a communication system is provided so that individual trainees can communicate with
other trainees or with virtual people as a part of the training exercise.
• Virtual People - are simulated people who take certain roles when there are no real people to fill them.
Virtual People are controlled by Scripts, but can be replaced on the fly by real people.
An example:
In typical operation a number of people may be involved in a simulation exercise with many dynamically
interacting components. For example, consider the simulation of several ships in a Navy exercise. In the
simplest form, a number of trainees would assume different shipboard roles as they interact with each other
and with the simulation engine. Each trainee would be served information, including the correct visual
information, according to his or her role. The steersman would get a compass and a view out the wheelhouse
window (using VRML 3 ), for example, while another member of the bridge team might get a radar screen and
a view through the main windows. Engine room personnel would get engine operating information and
visuals, while sonar personnel would get sonar screens, etc.
Certain aspects of the simulation are controlled by a script. For example, mechanical faults may occur in the
ship, other (enemy) ships or submarines may arrive on the scene (each with its own crew of real or virtual
people). While scripts control certain aspects of the simulation, they also control “observations/evaluations” the
simulation system makes of the trainees moves – how they exercise their roles, and so forth. Logs of the
activities of the trainees are also kept.
3 Virtual Reality Markup Language – allowing user-controlled 3D virtual worlds.