PBX private networking has evolved dramatically during the past 25 years. The earliest PBX networking arrangements consisted of two switch nodes linked by a dedicated, private line facility, such as an E&M tie trunk, to save on long distance toll charges. The primary benefit was cost savings. During the late 1970s more complex private tandem network configurations were made available, consisting of a meshed network of private line facilities linking tandem switch PBX nodes, main PBX nodes, and satellite systems. AT&T s first modern private PBX networking option was called Enhanced Private Switched Communications Service (EPSCS). First tariffed in the mid-1970s, it was quickly replaced by the better known and higher-performance ETN offering in the late 1970s. AT&T s innovative PBX private networking option was initially proprietary, but other leading PBX manufacturers at the time, including Northern Telecom (ESN), NEC (EPN), and Rolm (RolmNet), soon offered similar PBX options. After a few years, the competitive PBX offerings became compatible with ETN. By the mid-1980s, a customer could configure a network with a mix of PBX tandem switch nodes from multiple manufacturers.

The original ETN options were based on in-band signaling techniques supporting a network dialing plan and automatic alternate routing between nodes within the network. In addition to cost savings benefits using fixed tariff private line carrier facilities, customers enjoyed greater control over network operation and use. All of this was done initially with narrowband analog trunking facilities. The availability of digital T1-carrier trunk services in the mid-1980s...