The Department of Defense's Advanced Research Projects Agency (ARPA) has also recognized the importance of networks with regards to VEs in one of its seven science and technology thrusts. Since 1984 ARPA has funded the simulation network (SIMNET) system, which enables simultaneous participation of approximately 300 players in simulated aircraft and ground combat vehicles located in Europe and the United States on the same virtual battlefield. ARPA is currently working on a much larger technology demonstration to "create a synthetic theater of war" using the Defense Simulation Internet (DSI). This network will link thousands of real and simulated forces all across the United States (Reddy, 1992).

It is anticipated that, in the future, high-speed networks will allow VE systems to take advantage of distributed resources, including shared databases, multimedia sources, and processors, by providing the required computational power for building the most demanding applications. High-speed networks will provide VE applications with access to huge datasets generated by space probes, dynamic climatic information from weather models, and real-time imaging systems such as ultrasound.

One less serious but rather lucrative combination of VE and networking that is currently in use is multi-user interactive VE games offered by Genie and Sierra Online services. These games are provided over slow telephone lines with limited graphics. Other upcoming cooperative arrangements are planned between VE and cable television. Some examples of these are the promise of multi-user games by Sega and Nintendo and the development of virtual Walmart department stores by the Home Shopping Network.

STATUS OF THE TECHNOLOGY

Distributed VE will require enormous bandwidth to support multiple users, video, audio, and possibly the exchange of three-dimensional graphic primitives and models in real time. Moreover, new protocols and techniques are required to appropriately handle the mix of data over a network link. The technologies providing these gains in performance blur the traditional distinction between local-area and wide-area networks (LANs and WANs). There is also a convergence between networks that have traditionally carried only voice and video over point-to-point links (circuit-switching) and those that have handled only data such as electronic mail and file transfers (packet-switching).

Wide-Area Networks

The fabric of our national telecommunications infrastructure is being radically altered by the rapid installation of fiber optic cabling capable of



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