Local Processors
The first generation of stored program control PBX systems was based on centralized call processing system designs. Call processing system designs evolved to include a variety of processing elements outside the common control complex but under the control of the Main System Processor. These processing elements are sometimes referred to as slave controllers or local processors. These processing elements may be used for a variety of functions, such as systems administration and maintenance; function-specific applications, such as messaging, or ACD routing, queuing, and reporting; local switch network access; and diagnostics. Small PBX system models usually centralize all call processing, administration, and maintenance functions within the common control complex, but intermediate/large system models may use dedicated processor elements to offload some call processing operations from the Main System Processor or dedicated processors to handle systems administration and maintenance services.
Port Circuit Card Microcontrollers
The most common local processor element is a microprocessor controller resident on a port circuit card. The primary functions of the on-board microcontroller are to pass control signals originating from the Main System Processor to the individual station/trunk circuits and provide a signaling link between the peripherals and the common control complex. The port circuit board microcontrollers function independently of one another and are responsible only for the port circuit terminations on the printed circuit card. The microcontroller has the primary responsibility for monitoring the status of its colocated port circuit terminations and peripherals. It also provides the processing intelligence for the physical link connections to the local TDM bus under the command of the main control complex and/or cabinet/carrier shelf processors. The very localized processing functions performed by the microcontrollers are generally considered mundane and repetitive but are necessary to support the call processing functions of the main control complex. Localizing some processing operations at the level of the port circuit card reduces the processing load of the main system processor and increases the overall system call processing capacity potential.
The AT&T System 75 integrated the first port circuit card microcontroller into a PBX system call processing design in 1984. Today it is a standard port circuit card design element. The current microcontrollers are not based on the latest processor platforms but on older 8-bit, 16-bit, or 32-bit microprocessors. All port terminations on the printed circuit card depend on the local microprocessor, and processor failure or error will result in loss of service. No PBX system design offers a redundant microcontroller design option because service loss is limited to a small percentage of total system ports. For this reason, it is sometimes prudent for a customers to distribute vital peripheral resources across two or more port circuit cards.
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