Carriers, high-end service providers and multinational enterprises dealing in real-time convergence applications have demanded up to "six nines" (99.9999%) reliability from their switching and computing systems. This is why early PCs, with their microprocessor, non-redundant circuitry and mass-produced ambiance, took so long to become a player in the stringent operating environments found at the top of the convergence chain.

The intense reliability requirements for high-end NEBS certified, carrier-class equipment led to a "big iron" view of what could and couldn't work in a central office or multinational corporation, and for many years high-end equipment consisted of dedicated, proprietary switching devices that took years to pass regression testing. Such equipment was inevitably late to market, inefficient, and generally didn't offer many enhanced features.

The appearance of high availability rackmount PCs caused some concern to those mainframe or minicomputer-trained technicians who tended to look down condescendingly at the PC to begin with. Still, the high availability PC makers persevered.

It was felt that fault resilient PCs still needed to run well-known software, but should somehow be running with "tougher" hardware.