Distributive Interactive Simulation
Distributive Interactive Simulation (DIS) was initially a standards initiative by the U.S. Department of Defense/Modeling & Simulation) to address the needs of sustainability training (the maintenance of skill levels), given the large number of complex weapon systems being fielded, and the need to interoperate in large group environments.
DIS was a distributed concept. Individual simulators (aircraft, tank, and so forth) would be networked together using the internet. They would issue predefined enumerated message types (called Protocol Data Units or PDUs) that would report their positions, movements, actions (such as firing their weapons, and effects (detonations). Each participant was responsible for rendering locally the effects of what was being reported over the wire.
For example: a tank simulator firing its main gun would issue a Fire PDU. Periodically, the simulator would issue position updates (Movement PDUs) on the position of the shell, and terminate with a Detonation PDU. The target of the incoming fire could attempt to move out of position (evasion), or if hit, determine the battle damage and either report itself as destroyed, damaged, or no effect (via State PDUs). This tended to generate a lot of network traffic, and in widely dispersed sites: loss of PDUs which made for jerky effects or even jitter when the PDUs arrived out of order from the how they were sent. Fidelity was another issue, as well as time synchronization across the network (for example, vehicles "floating" in air, because the vehicle was using different DEM interpolations from the renderer drawing the vehicle's position in its field of view).
DIS was later replaced by the High Level Architecture (HLA) initiative. HLA addressed the need for large federations of simulators that could participate together using a simplified common interface (the Run-Time Infrastructure or RTI). The RTI handled conversions of the PDUs, and since it was working off the Publish and Subscribe principle, filtering messages so as not to overwhelm the host simulator with traffic that didn't impact it (for example, a simulator without radar capability wouldn't subscribe to Movement PDUs).
References
IEEE 1516
RTI Interoperability Study Group Final Report - Michael D. Myjak, Duncan Clark, Tom Lake
C4ISR/Sim Technical Reference Model Study Group Final Report - SISO-REF-008-2002 (September 2002)