The N3 Carrier System: Objectives and Transmission Featurespages 776 - 777

The Bell System has been engaged continuously in a vigorous program of carrier telephone system development since the now obsolete Type A system was introduced in 1918. This development effort has been aimed at increasing the utilization of available bandwidth, extending the operational distance, improving transmission performance, 767 2 TIIE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 19fifi reducing transmission facility costs (initial and operating), and investigating new methods of modulation. Prior to the end of World War II, the expense of carrier terminals and repeaters limited their application to the longer toll trunks. The type N1 carrier system, 1 which was introduced in 1950, had the design objective of providing economical telephone trunks in the 15- to 200mile range. The N1 carrier system derived 12 voice channels and utilized double-sideband transmission over two repeatered cable pairs. A second short-haul carrier system, the type O, was developed subsequently for open wire lines. This system derived a maximum of 16 single-sideband voice channels in four groups of four channels over a single open wire pair. Terminals of the type 0 carrier system and the N1 carrier line facilities were then combined to form the ON carrier system. 2 The ON2 system derived 24 single-sideband carrier channels using the same line frequency space utilized by the 12 channel N1 carrier system. In the early 1960's, a broad program was begun to redesign these short-haul carrier systems. A comprehensive description of this program has been published, 3 so an outline of the pertinent aspects will suffice here.