TELEPHONE SYSTEM OF THE UNITED STATES 63 



telephone system in the United States to give satisfactory service 

 between any two points in the country. In order to accompHsh this 

 it is necessary to make arrangements for a minimum number of 

 switches between any two points. Also the toll circuits which will 

 be used as parts of the built-up connections must be designed for a 

 very high standard of transmission so that the overall efficiency of 

 the built-up connection will be satisfactory. 



Arrangements have recently been worked out in the United States 

 for meeting requirements of switched traffic more satisfactorily than 

 has heretofore been possible. These arrangements may be briefly 

 described as follows: 



At different points in the country there have been selected a group 

 of eight very important switching points shown in Fig. 52. These 

 eight regional centers will all be interconnected by high grade groups 

 of circuits directly, that is, without intermediate switch. Through- 

 out the country there are selected about 147 important switching 

 points known as primary outlets also indicated in Fig. 52, each of 

 which is directly connected to at least one of the regional centers. 

 Each of the remaining 2,576 toll offices in the country will be con- 

 nected to at least one of these important switching points. Further- 

 more, within limited areas, such for example as a State, all important 

 switching points will be directly interconnected. Within such an 

 area, therefore, any two toll offices can be connected together with 

 not more than two intermediate switches. Also, every toll office can 

 be connected to a regional center with not more than one switch and 

 through that center can reach any other toll office in any part of the 

 country with a minimum number of switches. 



To insure adequate transmission on the switched connections, each 

 of the important switching points will be provided with means for 

 automatically inserting gain in the connection when two toll circuits 

 are switched together so that the overall connection may be operated 

 at the highest possible efficiency. This will, in general, be done by 

 automatic adjustment of the gain of terminal repeaters permanently 

 installed in circuits which must, in general, because of transmission 

 limitations be operated at a lower efficiency when used for terminal 

 business than when used as parts of a built-up connection. 



Maintenance of Toll Service 



With the present network of long distance lines in the United States, 

 it is common to have 20 or more repeaters installed on each of the 

 longer circuits and this number will increase greatly with the further 

 extension of toll cable. The maintenance of service over these long 



