Signal-Frequency Signaling System for Supervision and Dialing over Long-Distance Telephone Trunks

The single-frequency signaling system for long-distance telephone trunks frees dial calls from the range and other limitations imposed by d-c signaling methods. It uses alternating currents in the voice range as the signaling medium and so can be used with any trunk of any length or type of line facility which meets voice-transmission requirements. The signaling requirements, design problems, main features of the circuit and equipment arrangements, and the operation of this system are outlined in this paper. The system described is the first practical arrangement of its type satisfactorily to meet all the conditions of telephone service in the Bell Telephone System.