even could not be recognized. In order to prevent such distortion, 

 therefore, it was necessary to develop and apply means for compen- 

 sating for the variation in the line by variations in the opposite 

 direction. These means are called attenuation equalizers. In the 

 telephone message circuit these attenuation equalizers are generally 

 designed to be an integral part of the telephone repeaters themselves. 



Signaling Systems 



With the exception of a few special cases all telephone circuits for 

 message telephone service are provided with a means by which signals 

 can be transmitted from one end to the other in order to call to the 

 circuit an operator (or sometimes in the case of dial systems, a machine) 

 or subscriber. In the relatively short circuits used for local telephone 

 service this signal commonly is provided either by the flow of direct 

 current which is used to light a lamp or operate a relay or by the fiow 

 of 20-cycle alternating current which is used to ring a bell or operate 

 a relay. 



Signals based upon the use of direct current are generally used in 

 cases where it is desired to have the signal continue to indicate the 

 condition of a connection throughout the conversation since the direct 

 current can continue to flow over the circuit simultaneously with the 

 voice current during conversation without interference. The most 

 extensive use of this type of signaling is for the shorter circuits. The 

 method of operation of long toll circuits generally is such that no 

 signal is required over the toll circuit during the conversation period 

 but only before and after the conversation, and for such signals alter- 

 nating current is generally used. 



In the early days the alternating currents used for signals over toll 

 circuits were generally the same as those used for signaling over local 

 circuits, namely, 16 to 20 cycles per second. As the simultaneous use 

 of toll circuits for telephone and telegraph expanded, this was modified 

 because of interference which would occur between the telegraph cur- 

 rents and the 16 or 20-cycle signaling currents. For such cases signal- 

 ing was accomplished by alternating currents of 135 cycles, sufficiently 

 high to avoid interference with the telegraph systems then in use. 



With the further development of long toll circuits having many 

 repeaters at intermediate points and also with the development of 

 carrier telephone systems, the satisfactory transmission of 135-cycle 

 current from one end of the circuit to the other became more difficult. 

 At the same time the advance in the art made possible the development 

 of satisfactory and economical signaling systems using interrupted 

 currents of 1,000-cycle frequency. With such a system the signaling 



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