Numerous customer configurations require a distributed PBX common equipment design that supports a single system image for all features, functions, and operations. These configurations include a campus environment covering a large geographic area, with loop length runs exceeding supported parameters, and customers with communications requirements across two or more locations with right-of-way cabling options. A distributed common equipment design may consist of any combination of the following design elements:

1. Centralized, dispersed, or distributed call processing

2. Centralized, distributed, or dispersed switch network design

3. Distributed port equipment cabinets/carriers (main and remote equipment rooms)

A distributed call processing design is preferable because local call processing reduces the probability of down time for each port cabinet. Distributed port cabinets linked to a centrally located common control complex (centralized or dispersed design) depend on intercabinet links outside a secure equipment room for all call processing functions, and link failure or problems increase as distances between cabinets increase.

For switching functions a distributed or dispersed design is preferable to a centralized stage because local switching requirements do not depend on intercabinet links for all switched call connections, as the centralized design does. Localized switching reduces intercabinet link communications channel requirements.

The first fully distributed common equipment design was the Intecom IBX S/40, introduced in 1980, which was an immediate success with universities with large campuses. The first IBX installation was at the University of Chicago. Another PBX system based on a distributed cabinet design, which has been popular...