Scalable Coherent Interconnect
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SCI, for Scalable Coherent Interface, is a high-speed computer bus that supports a variety of topologies, speeds and connection systems. It is used primarily in the high-performance computing market, where it is used to build NUMA computer clusters and other parallel architectures. In this role most systems use high speed networking devices, typically Gigabit Ethernet, but SCI is much "lighter" in terms of the protocol overhead, and can offer considerably higher performance regardless of the actual link speed.
SCI originally developed from the fabled Futurebus (IEEE 896) program that started in 1987. Soon after the project started, members of the engineering teams predicted it would already be too slow for the high-end marketplace by the time it would be released in the early 1990s. In response, a group spun off to form the SCI standard targeted at this market. SCI was essentially a subset of Futurebus features that could be easily implemented at high speed, along with a few minor additions to make it easier to connect to other systems, such as VMEbus. SCI became IEEE standard (IEEE 1596-1992) in 1992.
The original intent was to create a single standard that could be used for all buses in the computer. To quote from the standards website, SCI is a: combination computer backplane bus, processor memory bus, I/O bus, high performance switch, packet switch, ring, mesh, local area network, optical network, parallel bus, serial bus, information sharing and information communication system that provides distributed directory based cache coherency for a global shared memory model and uses electrical or fiber optic point-to-point unidirectional cables of various widths.
SCI has seen considerably more limited use in the "real world". It is typically installed as an adaptor in a PCI slot, connecting machines to a central switch. It was implemented in this form, although with several minor incompatibilities, by a number of vendors soon after it was standardized in 1992, including Sun Microsystems, who standardized on SCI for all of their high-performance deployments, mainly using cards manufactured by Dolphin Interconnect Solutions Inc. It was also used by Sequent Computer Systems as the processor memory bus in their NUMA-Q systems.
Meanwhile the Futurebus standard dragged on, but eventually reformed in the InfiniBand system, which is similar to SCI in many ways.
