In the 1980s, the SIMNET program proved that local area network (and later wide area network) technology could be used to connect significant numbers of simulators, allowing operators to take part in joint simulations through a common synthetic environment. A series of semi-annual workshops began in 1989 to expand on the concept and explore standards to support the interoperability of defense simulations. These were later known as the DIS workshops. Developed by a group of volunteers from the government, industry, and academia, the first DIS draft standard was published in June 1990, becoming the starting point for simulation interoperability. Addressing the format and semantics for the exchange of simulation data between simulations, the draft standard defined the data exchanged as Protocol Data Units (PDUs). It was approved as an Institute of Electrical and Electronics Engineers (IEEE) standard in 1993. By then, an improved second version of the PDU standard was already in development. An additional standard, defining the underlying communications services required by the PDUs, was also taking shape. This Communications Services standard was approved by the IEEE alongside the second version of the PDU standard in 1995. Since then, three DIS recommended practices have also been approved by the IEEE.

Meanwhile, a 1989 experiment in distributed wargaming using aggregate-level discrete event simulations, or "constructive" simulations, led to MITRE being asked to investigate the possibility of applying the basic SIMNET principles to the development of a general, systematic approach to constructive simulation interoperability. MITRE determined that the underlying principles of SIMNET could be applied to constructive simulations, but that additional requirements were necessary to overcome the technical problems inherent in connecting simulators of different types and platforms. First, the simulations used a variety of different approaches to evolve time. Time management would be needed so that the passage of time would appear the same to all users, and the causality of events would be preserved. Second, each simulation had its own method of representing data. Data management was necessary so that they could each understand the data being shared. Finally, each simulation had its own architecture, which any approach to creating a shared environment must allow it to continue to use.

Some members of MITRE's HLA team: Richard Weatherly and Susan Symington (back), Fredrick Kuhl and Annette wilson (front)

Based on the SIMNET principles (no central node, geographic distribution, object ownership, and message-based protocol) and the additional requirements, the ALSP concept was formed. Once again, members of government, industry, and academia worked together on the project. For exercises held in 1992, MITRE developed the ALSP infrastructure software that provided common software services to the simulations involved. Using ALSP, new groups of interacting constructive simulations, or confederations, were fielded every year through 1998. MITRE's experience with ALSP would soon put it in a position to make a major contribution to the DMSO's next major M&S project, the HLA.

Development of the HLA

The DMSO was established in 1991, fueled by the desire to bring together the DOD's simulation technologies to support a broad range of M&S activities. Extending beyond the initial application of training, these include research and development, acquisition, test and evaluation, concept demonstration, and other areas. Seeking a unified ADS architecture to provide a framework for all M&S interoperability, the DOD funded the development of the HLA through the DMSO.

In 1996, a number of prototyping activities were undertaken to refine the architecture and demonstrate the HLA's usefulness. These efforts, known as protofederation experiments, were successful to the point that the DOD adopted the HLA as the standard for all of its future M&S activities. Since 1996, the DMSO has focused its efforts on extending and exploring the HLA's capabilities. It has also been working to move existing DIS-based systems toward HLA compliance.

As of the first day of fiscal year 1999, the DOD stopped providing funds for the development of simulations that were not HLA compliant. By fiscal year 2001, the DMSO will support only HLA compliant simulations. To ensure that private industry can design and develop M&S products that are HLA compliant, the DMSO has made the HLA specifications and supporting software freely available to the broader simulation development and user community.