Conceptual Framework
A conceptual framework is an organizing structure of concepts that facilitates simulation model development. Common conceptual frameworks include: event scheduling, activity scanning and process interaction.
The ALSP conceptual framework is object-based where a model is composed of objects that are characterized by attributes to which values are assigned. Object classes are organized hierarchically in much the same manner as with object-oriented programming languages. ALSP supports a confederation of simulations that coordinate using a common model.
To design a mechanism that permits existing simulations to interact, two strategies are possible: (1) define an infrastructure that translates between the representations in each simulation, or (2) define a common representational scheme and require all simulations to map to that scheme.
The first strategy requires few perturbations to existing simulations; interaction is facilitated entirely through the interconnection infrastructure. However, this solution does not scale well. Because of an underlying requirement for scalability, the ALSP design adopted the second strategy. ALSP prescribes that each simulation maps between the representational scheme of the confederation and its own representational scheme. This mapping represents one of the three ways in which a simulation must be altered to participate in an ALSP confederation. The remaining modifications are:
- Recognizing that the simulation doesn’t own all of the objects that it perceives.
- Modifying the simulation’s internal time advance mechanism so that it works cooperatively with the other simulations within the confederation.
In stand-alone simulations, objects come into (and go out of) existence with the passage of simulation time and the disposition of these objects is solely the purview of the simulation. When acting within a confederation, the simulation-object relationship is more complicated.
The simulation-object ownership property is dynamic, i.e. during its lifetime an object may be owned by more than one simulation. In fact, for any value of simulation time, several simulations may own different attributes of a given object. By convention, a simulation owns an object if it owns the "identifying" attribute of the object. Owning an object’s attribute means that a simulation is responsible for calculating and reporting changes to the value of the attribute. Objects not owned by a particular simulation but within the area of perception for the simulation are known as ghosts. Ghosts are local copies of objects owned by other simulations.
When a simulation creates an object, it reports this fact to the confederation to let other simulations create ghosts. Likewise, when a simulation deletes an object, it reports this fact to enable ghost deletion. Whenever a simulation takes an action between one of its objects and a ghost, the simulation must report this to the confederation. In the parlance of ALSP, this is an interaction. These fundamental concepts provide the basis for the remainder of the presentation. The term confederation model describes the object hierarchy, attributes and interactions supported by a confederation.
Read more about this topic: Aggregate Level Simulation Protocol
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