532 BELL SYSTEM TECHNICAL JOURNAL 



of the intercontinental circuits. In order that this may be accom- 

 plished in an economical and orderly way a general toll switching plan 

 has been adopted for the entire continental area. This plan establishes 

 a fundamental basis for the routing of connections involving more 

 than one toll circuit through the establishment of about 150 important 

 switching centers to which all of the 2500 toll centers of the country 

 will be directly connected. These 150 switching centers are inter- 

 connected by groups of high grade toll circuits either directly or for 

 distant parts of the country through the intermediary of "regional 

 centers" of which there are eight in the continental area. 



By means of this plan it is possible to allocate each group of toll 

 circuits to one of several broad classifications depending upon its 

 position in the general toll switching plan, and to apply standard 

 transmission requirements to each such broad classification. These 

 requirements include the requirement that the efifective net loss of all 

 direct circuits shall not exceed 9 db, and that circuits designed for 

 use in switched connections shall have minimum working net losses 

 not exceeding 3 db for end links, 4 db for circuits between regional 

 centers and 3.5 to 4 db for the remaining intermediate links inter- 

 connecting the important switching centers. When several circuits 

 are connected together to form a long switched connection, the 

 overall crosstalk effects are not appreciably increased over the effects 

 of an individual circuit. Singing effects will usually not be limiting 

 since long switched connections are seldom established without at 

 least one intervening four-wire circuit. On the other hand echo 

 effects increase fairly rapidly even with circuits equipped with echo 

 suppressors and therefore, in selecting facilities which will meet the 

 requirements of minimum working net loss previously specified, the 

 echo effects are generally controlling on long connections. Since these 

 circuits are also used for direct circuit connections, higher net losses, 

 which w^ill be satisfactory from the crosstalk and singing standpoints 

 in the terminating condition, are obtained by adding pads at one or 

 both terminals of the circuit for this condition. 



A more complete statement of the transmission requirements applied 

 to toll cable circuits in the United States is given in Table II. For 

 convenience there are also given in this table the current transmission 

 requirements for international circuits adopted by the C. C. I. In 

 order to make the comparison as nearly as possible on a comparable 

 basis the international circuit requirements of the C. C. I. are compared 

 with the requirements for American toll circuits interconnecting two 

 regional centers, and the requirements for the national terminal are 

 compared with the American requirements for connections from the 



