Computer Stored Program Control

Until PBX systems incorporated computer technology into its call processing system design, features and functions were extremely limited. Station user features were restricted to those operations that could be handled by mechanical means. The general availability of computer SPC meant that features could be based on software programming tools, and feature development was limited only by a programmer’s imagination. Many PBX functions that are currently viewed as basic telephony capabilities, such as call forwarding and station activated conferencing, were first implemented through computer SPC. Network routing tables and CDR would not be available without computer programming capabilities.

The first SPC PBX system was introduced by Northern Telecom in the early 1970s. Known by a variety of names, including the Pulse and the SG-1, the Northern Telecom system was the first PBX to use a computer software program to perform basic call processing functions, such as provisioning of dial tone, and implement simple station user features, such as hold and transfer. In the United States, Northern Telecom distributed the system through the Pacific Telephone and Telegraph local telephony company, but sales of the new PBX design were limited. It was not until AT&T introduced the Dimension PBX system in 1974 that an SPC communications system was distributed on a large scale through each of the Bell System’s local operating companies. Dimension became one of the best-selling PBXs of all time, although AT&T’s market share declined throughout the life cycle of the product. After the Dimension PBX announcement, there was a flood of SPC communications systems from AT&T’s competitors. Between 1974 and 1980 SPC PBXs went from a 1 to a 95 percent market saturation level for new system installations.

The first computer-based PBXs were based on a centralized processing design. A single computer-based call processing element was used for all system call processing and switching operations. PBX manufacturers of the early digital SPC systems designed and manufactured their own processing hardware and were the designers and developers of the operating system used as a platform for software feature applications. The first-generation digital PBXs were based on call processing designs that closely resembled the minicomputers of the 1970s. Many computer manufacturers became interested in the PBX industry as a new potential market for their products, and a few actually attempted to design a telephony system. Rolm was a manufacturer of military specification computers who successfully entered the PBX market, but most failed. IBM designed, manufactured, and marketed PBXs for the European market but was unable to compete in North America. Digital Equipment Corporation (DEC) was rumored to be developing a PBX based on its VAX minicomputer design, but no product was ever officially announced.

Computer technology in the 1970s was relatively expensive as compared with current prices, and the high cost to design and manufacture a digital PBX was reflected in the enduser price at the time. Common control equipment hardware was priced several times the current cost to customers, even though the features in the 1970s were minimal compared with those of today, and the call processing power of the system was a fraction of today’s capacity limits. PBX call processing design evolved significantly during the 1980s when third-party microprocessors were generally available, and prices began their exponential decline. Dispersed and/or distributed call processing designs became the standard architecture platform for PBX systems. The single, centralized, common control element gave way to dedicated processing elements for diagnostics and maintenance operations, localized call processing and switching functions, and systems administration. Basic function electronic telephones with internal processor chips evolved into intelligent digital telephones. Adjunct applications processors provided enhanced functionality behind the core PBX system.

During the 1980s PBXs could be classified into one of three call processing system designs: centralized, distributed, and dispersed. System processor elements expanded from the common control complex to expansion port cabinets and even to individual port circuit cards. The focus of PBX system design was shifting from hardware to software. From the 1970s through the mid-1980s more research and development dollars were spent on hardware upgrades and enhancements, with a focus on digital switching and SPC functions. By the late 1980s most research dollars were being spent on software programming. The emergence in the 1990s of third-party CTI software applications programs running on adjunct servers linked to the PBX officially signaled the beginning of the end of proprietary common control and call processing designs. At the beginning of the twenty-first century, almost 90 percent of PBX research and development dollars were devoted to software applications programming. Little money is spent on core call processing hardware because third-party microprocessors, digital signal processors, and servers, instead of the original self-designed and manufactured computer system, are used.

Today’s PBX call processing design is as likely to be based on a customer-provided Windows NT server from Compaq, IBM, or Dell rather than a proprietary common control cabinet from the PBX supplier. Customers may experience lower system reliability levels using third-party servers not designed by their manufacturer for the heavy-duty real-time call processing demands of telephony communications, but the lower price alleviates the risk factor to some extent.