Legacy PBX Common Equipment

PBX common equipment can be divided into two main hardware categories: cabinets and printed circuit cards. There are many cabinet and card category types, and each PBX system model is designed and configured with the use of unique and proprietary hardware equipment. Neither cabinets nor cards are interchangeable between different PBX systems from different manufacturers. Proprietary common equipment hardware is currently the major reason the open design platform of new IP-PBX client/server systems is attracting the attention of many customers. Although the industry trend is toward reduced dependence on proprietary common equipment, the very large installed base of traditional PBX systems will guarantee the continued requirement for new equipment cabinets and port circuit interface cards for many years to come.

The PBX starter package is usually called the main system assembly and includes the necessary hardware elements and software for basic system operation. Small port size customers may require only a single cabinet equipped with all of the necessary equipment to support their station and trunk interface needs, but very large port size customers may require a dozen or more colocated or distributed cabinets. For any specific customer port size requirement, common equipment costs are relatively fixed regardless of feature or function requirements because the same core hardware components that support plain old telphone system (POTS) applications are required for advanced call center or networking applications. The advanced application configurations may include adjunct application servers, but the common equipment hardware supporting port station and trunk requirements is usually application independent. For example, desktop telephone instrument prices can differ by hundreds of dollars across a family portfolio, but cabinet and port interface circuit card costs supporting an inexpensive analog telephone are about the same as compared with an expensive ACD-based displayphone model with a headset interface. Figure 1 shows the basic PBX common equipment components and their relation to one another.

Figure 1: Basic PBX architecture.

PBX common equipment installations can range from a wall-mounted small cabinet design supporting fewer than 100 stations to floor-based multiple cabinet designs supporting 20,000 station users, but each configuration provides the same fundamental call processing, switching, port interface, and applications support capabilities. Single-carrier cabinet designs are usually based on a common control and switching complex consisting of a limited number of multifunction printed circuit boards to conserve cabinet real estate and a few port interface card slots. The large system models may have one or more carriers dedicated to common control functions, dedicated switch network carriers, and numerous expansion cabinets primarily supporting port interface circuit cards. The basic PBX operational elements are the same in the small and large models, although the common equipment design is significantly different.