D4 Digital Channel Bank Family: Overview

Overview By J. CHERNAK and J. J. LANG (Manuscript received June 25, 1981) This article presents an overview of the D4 Digital Bank Family and introduces a series of articles on the subject. The D4 family represents a collection of features and network applications derived from a single design, the basic D4 digital terminal. In 1962, the first digital carrier system was introduced into the telephone plant as an alternative to metallic interoffice trunks. On this system 24 voice signals are amplitude sampled, time shared, and digitally encoded; each direction of transmission is on a single pair of copper wires. This results in a twelve-to-one improvement in efficiency in the use of copper pairs as compared with the two-wire metallic trunks. The first-generation channel bank is designated the D1 bank, the D1 standing for Digital, first generation. The D1 bank performs the time-shared sampling and digital encoding of the 24 voice signals into a 1.544 million pulses per second signal. Since 1962 four generations of digital banks have been developed. The present generation--the subject of this special issue--is designated the D4 bank, and was introduced into the field at the end of 1976. The success of digital banks and associated transmission facilities can be expressed by their penetration into the metropolitan interoffice network. Today, the digital facilities account for about 35 percent of the total metropolitan interoffice trunks. In terms of voice-circuit miles, the digital facilities provide 80 percent of the total, that is, 100 million out of a total of 125 million voice-circuit miles.