Distributed Control in a Key Telephone System

This paper describes the distributed microprocessor architecture used in the Electra IMS digital communication system. The system has been in service in the U.S. as of 1985. The system\´s maximum line size is 160, its maximum trunk size is 80, and it can accommodate both normal single-line telephones and proprietary digital telephones. Data switching can also be performed because of the PCM/ TDM architecture. The distributed architecture is composed of a system control (SCU) and six module control units (MCU), each MCU handles 32 ports of peripheral equipment. The SCU and MCU\´s have a simplified common OS designed to handle the basic call processes. The SCU executes line-state control, time-slot control, and systemfeature application. The MCU executes peripheral-operation control and peripheral-status control. Software architecture is divided into small modules which represent each system function at the SCU, or each operating sequence at the MCU, so that addition or deletion of system features is very easy to implement. Hand-shaking belwccn the SCU and MCU\´s is controlled using a "message" which consists of several words. This architecture enables a nonsquare key telephone configuration, the building-block concept, and future system enhancement.