Importance

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Abstract

A telephone switching system comprising a central office switching unit and a remote field switching unit controlled by the central office unit. The field unit provides controlled connections between a number of trunks extending from the central office unit and a greater number of subscribers connected to the field unit. The field unit additionally contains a number of intracall lines for establishing a connection between two subscribers without the need for sustained trunk connection after the call is established. The central office unit includes a system controller comprising a microprocessor and associated memory units. The system controller receives information interrupts from both the central office unit and the remote field unit as to subscriber calls, subscriber identity, trunk availability and overall system condition alarms. Appropriate control signals are sent from the central office unit to the remote unit to establish the appropriate connections between the units for subscriber connections. Control and data words and interrupts travel between the central office unit and the remote field unit over a voice grade, duplex data channel. Control and data words are in binary and serial encoded on PSK carriers transmitted through the data channel. Each unit transmits a distinct carrier, and each unit contains a filter network to separate the incoming and outgoing carriers. The control and data words include parity lists and error correction bits for correction of errors in the transmitted words by the system controller. A phase lock loop in each unit demodulates the carrier to remove the serial words.

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First Claim