D2 Channel Bank: System Aspects

The first digital transmission system used for commercial telephone service was introduced by the Bell System in 1962. This system consists 1641 1642 T H E BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1972 D2 Channel Bank of the D1 Channel Bank and the T1 Repeatered Line.1 The D1 Bank uses pulse code modulation to convert 24 voice-frequency signals and the associated signaling information into a 1.544 megabits per second digital stream for transmission over the T1 Repeatered Line. Because it has met its performance and cost expectations, the D l / T l system, which is also known as "the T-Carrier," has been favorably received SYSTEM ASPECTS 1643 by the telephone companies. Today it is the fastest growing carrier facility in the Bell System supplying nearly a million voice channels throughout the country. Its success is a direct result of one of the characteristics of digital systems-low terminal cost. The success of D l / T l has stimulated planning of a digital communications network. A fully digital network includes both digital switching and digital transmission. Most of the effort since 1962, however, has been directed towards the development of transmission systems for this network. 2,3 In this network there will be a hierarchy of digital transmission facilities that provide for long-haul transmission at high bit rates, 4 digital terminals that convert a variety of signals into a suitable digital form, and digital multiplexers that can derive several smaller capacity digital facilities from a large capacity facility.