Proceedings of the 2004 Winter Simulation Conference R .G. Ingalls, M. D. Rossetti, J. S. Smith, and B. A. Peters, eds ABSTRACT Ontologies represent the next important phase of the World Wide Web, creating a semantic web which links together disparate pieces of information and knowledge. Creating ontologies within computer simulation can be seen as a logical next phase of the web-based modeling and simula- tion thrust, where the emphasis is on knowledge and its representation rather than on run-time network characteris- tics. We introduce the concept of an ontology and then survey two groups performing research in this area at the Universities of Florida and Georgia, respectively. 1 INTRODUCTION Web content has traditionally been created through the HyperText Markup Language (HTML). For example, most web pages, whether generated on the fly or created with a web page authoring tool, are structured in HTML. HTML tags define how text looks—the presentation of the docu- ment, but not its underlying semantic content. Even though presentation languages do provide a layer of semantics, the semantics are focused on whether to create a bold-faced font or how to indent a paragraph. The knowledge con- tained within the paragraph must be gleaned by the human reader of the web page, and so it becomes difficult to automate knowledge acquisition without a complex proce- dure involving natural language parsing. Suppose, for in- stance, that the web page had the following text phrase: “…the model contains a Markov chain of 4 states labeled M1, M2, M3 and M4…” It might be more productive to specify the structure of the Markov chain and then generate the text on the fly. That way, the knowledge takes center stage, and can be used to link to other knowledge about Markov chains, states, and models. This more productive approach is basically the goal of the Semantic Web (Berners-Lee et al. 2001)—to surface the underlying knowledge about domains and objects, rather than focus solely on how this knowledge is pre- sented in the way of text or diagrams. An ontology represents a knowledge representation used to capture information and knowledge about a sub- ject, generally within the structure of a semantic network, consisting of a diagram composed of nodes and arcs. An ontology may be expressed in the Extensible Markup Lan- guage (XML), the relatively new lingua franca of the web. Ontologies establish a semantic foundation for models, systems, results, applications and markup languages that enhances discovery, use and interoperation. The evolving web ontology initiative is developing a suite of languages to facilitate this: RDF, RDF-S, OWL, and SWRL. Figure 1 illustrates a simple ontology, which relates various Petri Net (Fishwick 1995) models together. Figure 1: An Ontology for Petri Nets It specifies the following knowledge: (1) A Petri net is a kind of (ako) Timed Petri Net, which in turn is a kind of Stochastic Petri Net; (2) Each Petri Net is composed of Places and Transitions; and (3) One particular Petri Net is labeled Pnet1. While simple in structure, this knowledge can be used in various ways. The ontology has archival or educational taxonomic properties, providing categorical information (i.e., that Timed Petri Nets are types of Sto- chastic Petri Nets). Also, the instance Pnet1 defines one specific Petri Net that may be associated with a manufac- turing process, for example. In this latter sense, the ontol- ONTOLOGIES FOR MODELING AND SIMULATION: ISSUES AND APPROACHES Paul A. Fishwick Computer and Information Science & Engineering Bldg CSE, Room 301 University of Florida Gainesville, FL 32611, U.S.A. John A. Miller Department of Computer Science 415 Boyd Graduate Studies Research Center University of Georgia Athens, GA 30602, U.S.A. 259