The distributed cabinet design was dictated by distance limitations imposed by digital signal links to the digital desktop. Intecom was forced to bring the port cabinet closer to the station user. Analog telephones could support cabling loop lengths of 1 or 2 miles, but the Intecom ITE digital telephones were limited to 1,000 feet between wall jack and port circuit card.
The next logical step in a modular system design was to remote port cabinet miles away from the PBX common control complex using telephone company trunk carrier circuits. Northern Telecom was the first to accomplish this when it designed a remote peripheral cabinet for its SL-1 PBX in 1982. Using analog trunk circuits, the remote cabinet depended on the main PBX location for all call processing and switching functions, but at least a customer could support two or more distributed locations with a single PBX system. If the trunk circuit link to the remote location failed, however, the remote location was left without communications service. A spare processing option at the remote location would solve the link failure problem, so Intecom announced such an option about one year after Northern Telecom introduced the first remote cabinet option.
By the mid-1980s several PBXs offered remote cabinet options, but only Intecom has a remote survivable processor option. Other manufacturers offered an alternative solution to the remote cabinet option and in some ways a better PBX system design. A PBX system first announced in the early 1980s, and still working today after many upgrades and enhancements, was the Ericsson MD-110 PBX. Based on its own central office switching system, Ericsson’s MD-110 was a fully distributed communications system from a call processing, switching, and cabinet architecture perspective. Each MD-110 Line Interface Module (LIM) contained a common control complex that operated independently yet in coordination with every other LIM cabinet in the system. The LIMs could be geographically dispersed on a campus location or across a telephone network (analog or digital trunks, copper or fiber optic cabling, microwave or satellite transmission). Each LIM had its own switching system backplane and communicated with other LIMs via a centralized group switch complex. PCM links between the LIMs and group switch could be duplicated, as could the group switch (Figure 1).
In the mid-1980s Rolm introduced a PBX design similar to the Ericsson offering. The Rolm CBX II 9000 did not have a centralized group switch but it did have functionally independent control cabinet clusters. The Northern Telecom SL-100, a modified version of the manufacturer’s DMS-100 central office switching system, became a popular PBX system for very large (thousands to tens of thousands of user stations) distributed communications configurations requiring an extremely high level of reliability and redundancy. The SL-100 Remote Switch Center (RSC) option could be located hundreds of miles from the main PBX location, support thousands of stations users, and function as a standalone system, if necessary, with minimal loss of features if the control link to the main common control complex failed. The growing availability of PBXs capable of supporting multiple common control complexes and port cabinets geographically dispersed across great distances marked a distinct change from the old, monolithic design platform of PBXs before 1980.
For customers with single-location requirements and not interested in remote port cabinet options, the most important PBX cabinet innovation of the early 1980s was the introduction of the stackable cabinet design. PBX control and port cabinets were traditionally based on large, multiple carrier steel frames. Customers would be forced to buy and install an expensive large cabinet capable of supporting several equipment shelves, even if they required expansion for a few stations. The incremental cost to add a few stations was very expensive. When the NEC NEAX2400 was introduced in 1983, it was the first PBX based on a stackable cabinet design, with dedicated single-shelf cabinets for call processing functions and stackable port cabinet shelves. Up to four Port Interface Module (PIM) single-shelf cabinets could be stacked on top of each other, sharing a common switching and processing backplane. Each PIM had a dedicated Port Processor Interface and a dedicated Time Slot Interexchange (TSI). The NEAX2400 offered customers a cost-effective solution for modest growth requirements as compared with PBX systems based solely on large expansion port cabinets costing tens of thousands of dollars even if only a few expansion ports were required.
By the early 1990s almost all PBX systems targeted to customers with small and/or intermediate port requirements were based on modular, stackable port cabinet designs. Many PBX manufacturers offered a remote port cabinet option to customers desiring a single communication system for multiple-location configuration requirements. Distributed processing and switching designs were becoming commonplace. The emergence of CTI in the 1990s allowed manufacturers to offload advanced software options, particularly for call center management, onto adjunct servers dedicated for a specific application. Optional software application programs run on proprietary or customer-provided server equipment reduced the call processing load on the main control complex and offered a more flexible migration and upgrade path to enhance older PBX system platforms that still performed the basic communications functions with little problem. The early CTI hardware solutions required proprietary hardware links between PBX and server, but evolving PBX architecture design led to standardized TCP/IP links over Ethernet LANs.
The development of call processor control signaling over LAN infrastructures simplified the installation of third-party hardware and software solutions behind the core PBX system and kickstarted development activity for IP telephony and the emerging client/server IP-PBX system design. Using the LAN infrastructure (Ethernet switches, multiservice routers) for voice transport and switching between LAN-connected PC client softphones and LAN-connected servers for call processing is the ultimate modular system design because the processing and switching functions are totally distributed across the entire network.
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