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locations further extends the capabilities of human-machine interaction in synthetic computer-generated environments. The variety of desired connections between people, artificial entities, and information can be summarized by the slogan "connecting everything to everything." The scope of virtual environment development is so broad that it can be seen as an inclusive superset of all other global information infrastructure applications. As the diversity and detail of virtual environments increase without bound, network requirements become the primary bottleneck.

The most noticeable characteristic of virtual environments is interactive 3D graphics, which are ordinarily concerned with coordinating a handful of input devices while placing realistic renderings at fast frame rates on a single screen. Networking permits connecting virtual worlds with realistic distributed models and diverse inputs/outputs on a truly global scale. Graphics and virtual world designers interested in large-scale interactions can now consider the worldwide Internet as a direct extension of their computer. We show that a variety of networking techniques can be combined with traditional interactive 3D graphics to collectively provide almost unlimited connectivity. In particular, the following services are essential for virtual world communications: reliable point-to-point communications, interaction protocols such as the IEEE standard distributed interactive simulation (DIS) protocol, WWW connectivity, and multicast communications.

Existing Infrastructure Technologies

Layered Models

The integration of networks with large-scale virtual environments occurs by invoking underlying network functions from within applications. Figure 1 shows how the seven layers of the well-known open systems interconnection (OSI) standard network model generally correspond to the effective layers of the IP standard. Functional characteristic definitions of the IP layers follow in Box 1.

Figure
Correspondence between OSI and IP protocol layer models, and
objects passed between corresponding layers on separate hosts.

These diagrams and definitions are merely an overview but help illustrate the logical relationship and relative expense of different network interactions. In general, network operations consume proportionately more processor cycles at the higher layers. Minimizing this computational burden is important for minimizing latency and maintaining virtual world responsiveness.