Line Circuit Interfaces for a Digital Switching System

Crucial to the design of digital switching systems for local switching applications is the line circuit interface. It forms the boundary between the analog, 2-wire, subscriber loop, with its attendant environment hostile to low-voltage electronics, and the digital logic environment. ITT North's DSS uses a Quad Line Circuit (QLC) that provides the line circuit interface between four subscriber loops and two PCM multiplexed highways. The QLC contains per-line elements consisting of an electronic 2W/4W hybrid, transmit and receive PCM filters, a codec, two card-mounted relays, and distributed logic for various gating and clocking functions. In addition, there is digital logic common to all four lines for interfacing with redundant microprocessors common to 320 lines. Design of the QLC in a dynamic technology evolution is discussed, and several evolutions of the initial design are presented.