The Distributed Interactive Simulation (DIS) Protocols are the best attempt at providing simulation connectivity available today. These allow heterogeneous simulations to operate together based on a common understanding of a few message types and their contents. Integrated DIS simulations have typically been single-object, simple-engagement simulations. Tank simulators engage in combat activities which involve seeing and shooting enemy ground objects. Helicopter simulators see and shoot other airborne and ground-based objects. As long as object interactions remain this simple, heterogeneous simulations will operate together harmoniously.

The Aggregate Level Simulation Protocol (ALSP) is an ARPA generated project designed to join constructive simulations in much the same way that DIS does virtual simulations. Simulated entities engage in a wide variety of activities, including seeing, sensing, moving, shooting, jamming, communicating, and reorganizing themselves. Creating an interface protocol to accommodate all of these events in a consistent manner has proven very difficult, and has not been completely accomplished. The original design and modeling frameworks within each existing simulation often make it impossible to share an event between the simulations and still calculate a fair and consistent outcome.

This paper will explore some of the difficulties involved in integrating a very diverse and complicated set of simulation events. Many of the problems encountered in the constructive world over the past 15 years foreshadow those that will be uncovered in the virtual world. The first step in achieving Interoperability is to allow communications, a challenge the DIS protocols are addressing well. But, once this is accomplished the dissimilarities of the integrated simulations will become apparent, and obtrusive. The paper uses analogies and actual interoperability examples to illustrate these problems. It then proposes the need for a common modeling framework and transformation algorithms which must be shared by the simulations that are to become interoperable.