Evolution of the Digital PBX, 1975–2000

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 PBX system from a voice-only communications system to the mixed-media communications system currently being marketed and sold. Other significant PBX system design changes that have occurred during the past quarter century include computer-stored program control, evolution of a modular, distributed system design for processing, switching, and port interface operations, and digital transmission between the station user desktop and the common equipment. The same basic design elements of a PBX system remain the same—call processing, switching, port interfacing, and transmission—but the technology and architecture of the system have certainly changed (Figure 1).


Figure 1: PBX evolution timeline: major design developments.

PBX system features and functions have also evolved since the first digital, SPC systems were introduced. The early digital PBX systems had fewer than 100 total features in support of station user, attendant, and system call processing requirements. Slowly, with each new software feature release, PBX system software options expanded to include support for multiple system networks, ACD-based call center applications, and integrated voice/data communications. Enhanced system options, such as video communications, computer telephony, mobile communications, and messaging, were continually added to total PBX system offering. Some of the features and functions were based solely on software programming, but many required hardware elements, such as adjunct servers or special signaling interface cards, to implement and operate. Most current communications users are not aware of the significant evolution in system performance capabilities because few station users take advantage of the wide range of features and functions available on their PBX systems.

Herewith is a review and discussion of the major digital PBX system design, feature, and functional changes and enhancements leading to the development of the next generation of IP telephony enterprise communications systems.