Overview

There are two distinct generations of PBX systems based on the fundamental transmission and switching platform used to support signaling, control, and communications to and from the station user desktop and the common equipment. The first generation was known collectively as analog PBXs and included systems with a variety of internal switching network designs, such as step-by-step and crossbar, for port-to-port connections. The second generation, known as digital PBXs, converted analog voice signals into digital bit format using a codec in the desktop telephone terminal or at the port interface circuit card. Time division multiplexed (TDM) transmission buses were used as the core switching network for internal connections between peripheral port interfaces. Second-generation PBX systems used circuit-switched connections based on Pulse Code Modulation (PCM) techniques to establish communications channels between stations and/or trunk ports. The emerging generation of PBXs is based on IP signaling and communications protocols and interface standards commonly used for LAN and Wide Area Network (WAN) data communications but adapted for voice communications applications.

The digital circuit-switched PBX systems being marketed and installed today evolved directly from the first PBXs to use a digital transmission format across the internal switching network introduced in the mid-1970s by several manufacturers within a very short period. Before 1975, the earlier generation of premises communications systems was based entirely on an analog transmission and switching platform for communications between station users and/or trunk circuits. Using a digital transmission format was the first step toward the evolution of the...