AT&T s original ETN offering evolved into DCS in 1982 when the U.S. Navy required a single communications system for its San Diego naval base operations, with a port capacity far greater than the parameter limitations of any single PBX system model available at the time. The AT&T solution was not to design an extremely large PBX system but to intelligently network multiple systems to provide the appearance of one system for most common internal station-to-station user operations. Originally known as the Defense Metropolitan Area Telecommunications System (DMATS), the AT&T Dimension PBX option was renamed the Distributed Communications System (DCS). The AT&T Dimension DCS option became very popular in a short period and forced competitors to develop IFTN offerings of their own. Among the other wellknown IFTN brand name options developed almost 20 years ago and still marketed today are Siemens CorNet, NEC CCIS (since enhanced to Fusion CCS), and Fujitsu FIPN.

An IFTN has the property of transparency with respect to all on-net calling, and many feature operations. Transparency is the ability of the system, from a station user s perspective, to operate across multiple network nodes in the same way it does at the local node. This allows for a limited digit dialing plan for all on-net calls, thereby eliminating the need for PBX location codes and network extension numbers. All intercom calls are dialed with extension numbers corresponding directly to station user directory numbers.

An IFTN design is based on the ETN model,...