In-Band Single-Frequency Signaling

The signaling requirements for dial telephone operation are naturally more exacting than those for manual switching methods. This means a high order of signaling system is needed to satisfy the requirements for the toll telephone plant and for automatic toll switching systems described in recent papers in this JOURNAL.1, 2 Indeed the advantages in speed and economy of dial telephone systems depend to a large extent upon the type of signaling provided for them. The signaling arrangements for intertoll telephone trunks which are the links between telephone switching systems, therefore, become most important. Dial operation in the past has been based upon dc signaling which is 1309 1310 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1 9 5 4 limited to relatively short distances and to line facilities having dc paths available to them. In planning for nationwide dialing of long-distance calls the need for an ac signaling system for dial telephone trunks became apparent. The length of intertoll trunks and the extensive growth in carrier line facilities which do not have associated dc paths made it necessary to develop ac dial signaling systems. The single-frequency signaling plan was developed for this purpose and is the first of its kind to satisfy the conditions associated with long distance intertoll dialing in the Bell System. There are now several trunk signaling means that may be grouped as using the SF signaling plan. These are (1) the adaptation of VF carrier telegraph requiring an additional line channel independent of the voice transmission line facilities, (2) N1 and 01 carrier signaling furnished as part of these carrier terminals using 3,700 cycles outside but adjacent to the voice paths, and (3) 1600-cycle and 2400-cycle signaling systems, the in-band systems that use the voice paths.