In general, a supercomputer center is a site with a supercomputer that is shared by a number of other sites, usually research sites. Formerly, in the United States, there were five supercomputer centers interconnected on the vBNS, a special backbone network financed by the National Science Foundation (NSF) and operated by MCI for the exclusive use of designated research centers. On October 1, 1997, the five supercomputer centers were replaced by two new NSF-funded programs. The new program, Partnerships for Advanced Computational Infrastructure (PACI), named two of the former supercomputer center sites as focuses for a new scientific technology infrastructure (known as the National Technology Grid) that would interconnect 50 university and scientific sites.
The first of the two NSF-funded programs is the National Computational Science Alliance (Alliance), being led by the National Center for Supercomputing Applications (NCSA) in Urbana, Illinois. The second program, the National Partnership for an Advanced Computational Infrastructure (NPACI) is led by the San Diego Supercomputer Center at the University of California at San Diego. The earlier supercomputer center program emphasized supercomputer access from researchers' desktops. The new program aims to exploit the entire World Wide Web as a new "computer-mediated center of knowledge." Part of that program is to develop a wired superstructure, referred to as the National Technology Grid, that will enable very high-bandwidth applications such as virtual reality environments.