1310 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



limited to relatively short distances and to line facilities having dc paths 

 available to them. In planning for natiomnde dialing of long-distance 

 calls the need for an ac signaling system for dial telephone trunks be- 

 came 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 sig- 

 naling 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) Nl 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. 



Both in-band and out-of-band signaling have advantages and dis- 

 advantages. The in-band single-frequency signaling system uses ac in 

 the voice frequency range to pass full supervision and dial pulsing sig- 

 nals over the same paths that are furnished for voice transmission in 

 telephone trunks. This is accomplished without any loss in band width, 

 change in line facility or addition of intermediate signaling eciuipment. 

 On most calls voice and signal transmission are not required at the 

 same time. On the few calls going to intercept operators voice trans- 

 mission is impaired slightly by the effect of signal tone being on in one 

 direction. On calls encountering busies, it is desirable to return both 

 flashing supervision and interrupted audible tones. This can be done 

 with out-of-band signaling but in-band signaling can return either but 

 not both. The signaling system allows remote build-up and breakdown 

 and provides for supervision of the temporary connections ordinarily 

 used. Control and supervision of distant ends of trunks is required con- 

 tinuously whereas dial pulsing is required only at the start of calls and 

 speech transmission is required only when connections are established. 



TRUNK SIGNALS 



Before going into the details of the signaling system itself it seems 

 appropriate to review the trunk signals it is called upon to transmit. 

 Most intertoll trunks are arranged for two-way operation, which means 

 that a connection can originate at either end. To permit this operation, 

 the signaling in each direction must be symmetrical and the trunk must 

 allow the direction in which the connection is established to determine 



