AUTOMATIC TOLL SWITCIIIXr, SYi^TEMR (SCO 



SIGNALING 



III following the progress of the call from Atlanta to Monticello, Maine, 

 it was observed that besides the transmission of information in the form 

 of digits it was necessary to pass a number of control and supervisory 

 signals o\'er the toll lines. These included seiziu'e and disconnect signals 

 in the forward direction and switchhook supervisory signals and sender 

 attached signals in the reverse direction. On some calls it is also necessary 

 to send flashing signals to indicate busy lines or trunks and ringing signals 

 in either direction when operators are called in at intermediate or termi- 

 nating points to assist in establishing connections. 



For the early toll dialing installations the signaling method most 

 widely used was the composite method whereby signaling channels for 

 the three circuits of a phantom group are derived from three of the 

 conductors with the fourth being used for earth potential compensation. 

 Hirect current is used for signaling. This is a simple, reliable and eco- 

 nomical method of signaling and will continue to be used on circuits 

 where it can be applied. 



Where circuits are obtained from carrier systems, however, conductors 

 are not available in sufficient numbers for signaling channels and other 

 methods must be employed. Since carrier is used almost exclusively on 

 the long haul circuits it was necessary to provide a signaling system to 

 accompany it before toll dialing could be expanded beyond networks of 

 limited range. To meet this situation a system using a frequency of 

 1600 cycles was developed and has been in service since 1948. Signaling 

 is done by application and removal of the 1600-cycIe signaling current. 

 The system is used in the same manner as the composite signaling 

 system, to carry dial pulses as well as supervisory signals when used on 

 circuits that require it. The set of leads brought out of the signaling unit 

 are identical in function to those brought out of the composite signaling 

 unit so that toll line relay circuits will operate in the same manner with 

 either type of signaling. 



Since 1600 cycles is in the voice range the signaling current can be 

 carried over the same channel that carries the speech current but the 

 signaling circuits must, of course, be protected against false operation 

 due to speech and precautions must likewise be taken to insure that the 

 signaling tone does not interfere with speech. Protection against inter- 

 ference between signaling and speech is more difficult at 1600 cycles than 

 at higher frequencies because there is more energy in voice ciu'rents at 

 the lower range. That value was chosen, ne\'ertheless, so that it would 

 be possible to operate over the narrow band circuits that were estab- 

 lished to relieve shortages occasioned by the war. 



