Distributed PBX Common Equipment Design

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 on university campuses, is the Ericsson MD-110. University systems have been the largest single customer market sector for the Intecom and Ericsson systems during the past 20 years. Intecom PBX systems can support distributed cabinets several miles from the main equipment room housing the central control complex. Since its introduction, the Intecom PBXs have used a fiber optic cabling infrastructure in support of call processing signaling and switch network transmission links. The MD-110 can support more than 200 distributed port cabinets, each with a local common control complex, across virtually unlimited distances over PCM-based transmission links, with the use of copper, broadband fiber, or microwave transmission media.

Several new IP-PBX systems designs using a LAN/WAN infrastructure to support dispersed station users are ideal single system solutions in support of large coverage communications requirements. Systems from recent market entrants such as Cisco Systems and Sphere Communications fall into this design category. The traditional Siemens 300H architecture design is not based on an IP-based LAN/WAN cabling infrastructure, but a recently introduced IP-based remote carrier option is supported over a dark optical fiber cable between the main equipment room and the remote building location. The Alcatel OmniPCX 4400 can use a variety of intercabinet transmission links between distributed cabinet clusters, including PCM, IP, or ATM formats and copper or optical fiber cabling. Avaya, too, can use an ATM LAN/WAN infrastructure to support its EPN equipment cabinets for on-site or off-site requirements. Each of these systems also supports redundant common control capabilities.

Among the new IP-PBX system designs, the Cisco offering stands out. The Cisco AVVID IP telephony system can be configured with up to five active call servers. The call servers can be in the same equipment room, dispersed across the same customer premises, or across multiple customer premises. IP telephones can be supported by one of three servers for redundant call processing control. In case of WAN link failure, Cisco was the first IP-PBX supplier to announce support of basic telephony functions at remote locations with the use of a call processor blade embedded in one of its IP routers. The remote survivable telephony system option is an emergency backup system with limited call processing and feature management capabilities when desktop telephones/soft-phones do not have access to a centralized call telephony server.