In-Band Single-Frequency Signaling 



By A. WEAVER and N. A. NEWELL 



(]Manuscri])t recoivcd June 7, 195-1) 



Single-frequency signaling liberates dial systems from the restrictions of 

 dc signaling methods. This freedom^ as might be expected, is most important 

 in the long distance telephone plant where trunks are frequently too long or 

 have no conductors for dc signaling. The general plan of signal frequency 

 (SF) sigrialing is based upon contiyiuous signaling because of it's speed 

 and reliability. In this respect it is like the usual dc trunk signaling schemes. 

 SF uses steady current in the trunk signaling path for the normal idle trunk 

 condition and no current in the signaling path for the other and alternate 

 busy (talking) trunk condition. This choice of signal conditions is essential 

 for SF signaling in-band systems, which as the name implies operate within 

 the standard voice channel, to avoid conflict between signal and voice trans- 

 mission. The same conditions are also used in SF out-of-band and separate 

 line systems. 



The in-band SF system can be used with any type or length of line facility 

 that meets normal voice transmission requirements and is therefore the pre- 

 ferred 7nethod used by the Bell System to meet requirements for toll dialing on 

 a national basis, with other signaling arrangements limited to the shorter 

 trunks. The requirements, design considerations, main features, and method 

 of operation for the in-band system are outlined in this paper. 



INTRODUCTIOX 



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 recjuirements for 

 the toll telephone plant and for automatic toll switching systems de- 

 scribed in recent papers in this Journal/-^ 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 arrange- 

 ments for intertoll telephone trunks which are the links between tele- 

 phone s^^'itching systems, therefore, become most important. 



Dial operation in the past has been based upon dc signaling which is 



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