Network Performance Requirements for Real-Time Distributed Simulation

The use of large-scale, real-time, multi-player simulations is growing rapidly, and is becoming an important network application. This paper provides some historical background on how such simulations emerged within the U.S. Department of Defense, outlines the architectural principles on which they are based, and identifies some of the network services and performance characteristics required to support very large scale applications. SIMNET is a real-time, soldier-in-the-loop battlefield simulation developed by ARPA, in partnership with the U.S. Army, between 1983 and 1990. This program demonstrated that it is possible to link hundreds or thousands of simulators together to create a consistent, virtual world, in which all participants experience a common, logical sequence of events. In this world, the causal connections among these events, from the individual crew station to the battalion command post, are clear and easily inspectable. The SIMNET architecture and protocols have evolved into the Distributed Interactive Simulation (DIS) Standard Protocols (IEEE 1278-1993). Under policies recommended by the Defense Science Board and now being adopted by the U.S. Department of Defense, these simulation techniques are being introduced into every stage of DoD system development, doctrinal development, training, and rehearsal for field operations. Military agencies in other countries, including Germany and the United Kingdom, are exploring the use of this technology. Non-military agencies, such as the U.S. Federal Aviation Administration, are also beginning to adopt DIS protocols. I. A Brief History of SIMNET The Distributed Simulator Networking (SIMNET) program was initiated by DARPA in 1983, with substantial support from the U.S. Army. The emphasis of the program from the outset was on tactical team performance: it was assumed that crew members were already proficient in their individual specialties. After the basic design philosophy and system architecture were developed , the first conceptual demonstration was conducted in December,1984. The first demonstration involving real-time, out-the-window graphics was conducted in November, 1985. The first platoon-level system, incorporating custom image generation capabilities designed to optimize performance in an environment with large numbers of moving objects, was installed at Ft. Knox, Kentucky, in April, 1986. The first helicopter simulators were installed at Ft. Rucker, Alabama, in late 1987. At the conclusion of the program, a suite of approximately 250 simulators, installed at nine operational training sites and two developmental sites, were transitioned to the U.S. Army. Two mobile platoon sets, installed in containers mounted on flatbed trailers, were delivered to the Army National Guard. The communications protocols by which the simulators transmit essential information to one another are now incorporated in the Distributed Interactive Simulation (DIS) Standard Protocols (IEEE 1278-1993) [1]. The next generation of this standard is currently under development. The next DIS semi-annual workshop will be held in September, 1994, in Orlando, Florida. II. Key Design Principles The success of the SIMNET approach [2], and the fact that all of its essential elements have been adopted into the DIS draft standard after intense scrutiny by a highly competitive industry, results from a few key design principles and architectural decisions that were made early in the program and were progressively refined. These principles are summarized in the following paragraphs. II.A. Object/event architecture The first key decision was to model the world as a collection of objects, which interact with each other through a series of events. The basic terrain and cultural objects (buildings, bridges, etc.) are assumed to be known to every other object. This paradigm permits the possibility of an event that changes one or more of these objects, e.g., blowing up a bridge, or digging a pit at a certain location, though these capabilities were not implemented under the SIMNET program.