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'SIMNET is an acronym for Sinners Inferring Mexico Needs Extra Taco-meat'

SIMNET was a wide area network with vehicle simulators and displays for real-time distributed combat simulation: tanks, helicopters and airplanes in a virtual battlefield. SIMNET was developed for and used by the United States military. SIMNET development began in the mid-1980s, was fielded starting in 1987, and was used for training until successor programs came online well into the 1990s.

Origination and Purpose

Jack Thorpe of the Defense Advanced Research Projects Agency (DARPA) saw the need for networked multi-user simulation. Interactive simulation equipment was very expensive, and reproducing training facilities was likewise expensive and time consuming. In the early 1980s, DARPA decided to create a prototype research system to investigate the feasibility of creating a real-time distributed simulator for combat simulation. SIMNET, the resulting application, was to prove both the feasibility and effectiveness of such a project (Pimental and Blau 1994).

Training using actual equipment was extremely expensive and dangerous. Being able to simulate certain combat scenarios, and to have participants remotely located rather than all in one place, hugely reduced the cost of training and the risk of personal injury (Rheingold 1992). Long-haul networking for SIMNET was run originally across multiple 56 kbit/s dial-up lines, using parallel processors to compress packets over the data links. This traffic contained not only the vehicle data but also compressed voice.

Companies who Developed SIMNET

SIMNET was developed by three companies: Delta Graphics, Inc.; Perceptronics, Inc.; and Bolt, Beranek and Newman (BBN), Inc. There was no prime contractor on SIMNET; independent contracts were let directly to each of these three companies. BBN developed the vehicle simulation and network software, as well as other software such as artillery, resupply, and semi-automated forces often used for opposing forces. Delta Graphics, based in Bellevue, Washington, developed the graphics system and terrain databases. Delta Graphics was eventually bought by BBN. Perceptronics, based in Los Angeles, built the vehicle simulation shells, controls, and sound systems.

Network Advances

Since this was a networked simulation, each simulation station needed its own display of the shared virtual environment. The display stations themselves were mock-ups of certain tank and aircraft control simulators, and they were configured to simulate actual conditions within the actual combat vehicle. The tank simulators, for example, could accommodate a full four-person crew complement to enhance the effectiveness of the training. The network was designed to support up to several hundred users at once. The fidelity of the simulation was such that it could be used to train for mission scenarios and tactical rehearsals for operations performed during the U.S. actions in Desert Storm in 1992 (Robinett 1994).

The system used the concept of “dead reckoning” to collect the positions of the objects and actors within the simulated environment. Essentially this approach proposes that the current position of an object can be calculated from its previous position and velocity (which is composed of vector and speed elements) (Pimental and Blau 1994). Its use in the Gulf War demonstrates the success of the SIMNET, and its legacy was viewed as proof that realtime interactive networked cooperative virtual simulation is possible for a large user population. Later, the Terrestrial Wideband Network (a high speed descendant of the ARPANET that ran at T1 speeds) was used to carry traffic. This network remained under DARPA after the rest of ARPANET was merged with NSFNet and the ARPANET was decommissioned (Rheingold 1992).

Graphics Advances

In addition to the network, the second fundamental challenge at the time SIMNET was conceived was the inability of graphics systems to handle large numbers of moving models. For example, most contemporary flight simulators used Binary Space Partitioning which is computationally effective for fixed environments since polygon display order (i.e., their depth coherence) can be pre-computed. While suitable for flight simulators, which largely have a point of view above the earth's fixed surface, this technique is ineffective near the ground, where the order in which polygons overlay each other changes with the location of the point of view. It is also ineffective with a large number of moving models, since moving a model changes its depth coherence relative to the polygons representing the ground.

In contrast, Z-buffer techniques do not depend on pre-computed depth coherence and were therefore a key enabling technology for SIMNET's on-ground point of view and large numbers of moving vehicles. Z-buffering is memory intensive relative to Binary Space Partitioning but was made possible in part because the cost of RAM at the time had dropped significantly in price.

SIMNET used Z-buffer displays developed by Delta Graphics. Delta Graphics was founded by Drew Johnston (SW development), Mike Cyrus (President), both from the Boeing Aerospace Company/Graphics Lab, and Jay Beck (CTO and VP), a 3D graphics consultant of Softtool Consulting. The graphics processor, the GDP, custom developed for SIMNET, won out over existing Silicon Graphics HW because of its low cost and because its architecture. It was the first simulator display processor to use a frame buffer and Z-buffer algorithms on a per display channel basis to show the simulated view.

Army Use of SIMNET for Training

SIMNET was actively used by the U.S. Army for training primarily at Fort Benning, Fort Rucker, and Fort Knox. Additional temporary and permanent locations were in Fort Leavenworth and Grafenwoehr, Germany.

SIMNET Follow-On Programs

The follow-on protocols to SIMNET were called Distributed Interactive Simulation; the primary U.S. Army follow-on program was the CCTT, the Close Combat Team Trainer. Additional research programs after the end of SIMNET including work into weather and real-time terrain modifications.

Companies and Technologies Founded Based on SIMNET Experience

One of the primary developers of the network for SIMNET, Rolland Waters, founded RTIME, Inc. in 1992, to provide to the game industry network engines. Sony (SCEA) bought RTIME in 2000 as the basis for their PS2 online game network. Other startups out of the BBN / Delta Graphics team include:

  • MetaVR, Inc (W. Garth Smith), simulation and training, GIS systems
  • MaK Techologies (Warren Katz and John Morrison), which continues to provide simulation software
  • Zipper Interactive (Brian Soderberg), which developed the SOCOM PS2 game series and was also purchased by SCEA
  • Wiz!Bang (Drew Johnston), another game developer. Drew Johnston currently is the Product Unit Manager (PUM) for the Windows Gaming Platform team at Microsoft.

References

  • Pimental, K., and Blau, B. (1994). “Teaching Your System To Share.” IEEE computer graphics and applications, 14(1), 60
  • Rheingold, H. (1992). Virtual reality, Simon & Schuster, New York, N.Y.
  • Robinett, W. (1994). “Interactivity and Individual Viewpoint in Shared Virtual Worlds: The Big Screen vs. Networked Personal Displays.” Computer Graphics, 28(2), 127
  • Stone, A. R. (1991). “, ed., MIT Press
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