The digital time-division switching network of No. 4 ess imposes new requirements on the transmission/switching interface, while at the same time presenting opportunities for major improvements and economies. Two new terminals–the Digroup Terminal (dt) and the Voiceband Interface (vif)–perform the interfacing for digital and analog transmission systems, respectively. Access to the switching network itself is via serial pcm links, each accommodating 120 voice-frequency channels. The dt terminates digital facilities and performs the multiplexing/demultiplexing necessary to interface the pcm links at the switch. The vif terminates four-wire, voice-frequency analog trunks and performs analog-to-digital and digital-to-analog conversion and multiplexing/demultiplexing to interface with the switch. Both terminals incorporate extensive maintenance hardware and reconfiguration capability. Unitized Facility Terminals (ufts) are used for a variety of functions–attenuation, signaling conversion, etc.–that must be performed on analog trunks between the basic facility distributing frame on the one side and the vif and signal processor on the other. The use of ufts, a standard switching/transmission interface, and modular equipment arrangements and floor plans result in substantial savings in floor space, cabling, and distributing frame requirements, when compared with traditional switching/transmission systems.
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Boyle et al. (1977) studied this question.
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