which the transmission performance of toll circuits of the new types 

 can best be maintained at the desired standards. This involves a 

 determination of the kind of tests and the limits of adjustment neces- 

 sary for the different types of apparatus included in these circuits, the 

 frequency of tests and the desirable range of performance results for 

 the maintenance of a high quality of service over these circuits, with 

 the least practicable expense for their maintenance. Maintenance 

 routines of this sort are developed from time to time with each new 

 type of circuit and amended to accord with modifications in the details 

 of the circuits or to take advantage of the results of field experience. 



Special Services 



With a nation-wide network of poles, wires and circuits available 

 for telephone message purposes, and with its accumulated knowledge 

 concerning technical communication problems the Bell System, as the 

 demand has arisen, has naturally been in a position to analyze the 

 technical requirements of the special communication services and to 

 provide suitable facilities for them. The earliest demand for intercity 

 circuits for special services were for private telephone circuits between 

 telephones in different cities and for private line telegraph circuits. 

 Since that time developments in the communication art, such as radio 

 broadcasting and the transmission of pictures over wires, have created 

 additional demands. 



The toll wire plant of the Bell System can be used either inter- 

 changeably or simultaneously for telephone message service and many 

 of the special services. In addition, the telephone message service and 

 practically all the special services make common use of many other 

 parts of the toll plant, such as poles, conduits, buildings and power 

 plants. 



Some of the special services which make use of telephone circuits or 

 of circuits similar to telephone circuits involve special requirements 

 for satisfactory transmission. This is best illustrated by the trans- 

 mission of programs for radio broadcast stations, a service which is 

 given on a nation-wide basis over the toll plant of the Bell System. 

 The principal reason for the wide difference in technical requirements 

 of program transmission circuits and of telephone message circuits is 

 that, unlike the message circuits, program transmission circuits are 

 required to transmit music as well as speech. The satisfactory recep- 

 tion of transmitted music requires the transmission of a broader band of 

 frequencies than is necessary for speech alone. The national program 

 transmission networks of the country at the present time, consistent 

 with the requirements of radio broadcast art, transmit a band of 



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