Some Design Features of the N-1 Carrier Telephone System

The economies which result from sharing the cost of line facilities among a number of channels, and the transmission advantages of carrier circuits (in the form of high speed transmission which minimizes delay and echo effects, low net loss and high quality), have combined to bring about a revolution in long distance telephony. Whereas fifteen years ago only 8% of the toll circuit mileage of the Bell System was furnished by carrier, today carrier circuits comprise about two-thirds of the total mileage. The minimum distance, however, for which carrier can economically replace voice frequency transmission has been limited by the cost of the carrier equipment, the cost of line treatment, the expense of installation and associated job engineering, and the maintenance effort required. As a result, the shorter toll circuits, relatively large in number though not in circuit mileage, have continued to operate at voice frequency. The newly developed Type N - l Carrier Telephone System is aimed primarily at expanding the application for carrier into this field of short haul service. As explained elsewhere1, it is designed to obtain the advantages of carrier for toll and exchange cable circuits for lengths a small fraction of the previous economic minimum. Many system and circuit features contribute to this end. There are 12 channels per system with 8 kc spacing between carriers. The carrier and both sidebands are transmitted. All of the pairs in a single cable can be used for Type N without special cable treatment.