826 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



5. The entire plan must be such as to provide satisfactorily for growth, 

 for flexibility to meet changing conditions and for minimum overall 

 costs of operation. 



FUNDAMENTAL PLANS FOR TOLL PLANT 



Mr. Pilliod's paper, pages 832 to 850, discusses the fundamental lay- 

 out of plant for nationwide operator toll dialing. This is subject to 

 changes from time to time with further specific studies, as is the case 

 with all far-reaching fundamental plans of this type. The additional 

 requirements imposed by nationwide customer dialing are still under 

 study as will be discussed a little later. 



The national toll switching plan is modified so that there may be a 

 maximum of eight toll circuits switched together to connect any two 

 telephones compared with the previous limit of five. In order to handle 

 the entire traffic of the country, approximately 100 control switching 

 points are necessary at which highly intelligent common control switch- 

 ing systems of the No. 4 crossbar type will be placed. 



A very important feature of the layout is a trunking plan providing 

 for a high degree of use of alternate routes. To design all of the toll cir- 

 cuit groups of the country for a no-delay service would be very expen- 

 sive. However, taking advantage of the extreme rapidity of automatic 

 switching and the ability to build into the machine capacity for using 

 a large number of .alternate routes, a trunking system has been devised 

 in which only about one-sixth of the toll circuit groups of the country 

 need be engineered on a very liberal basis. These are called final groups 

 and are the groups to which the machine ultimately appeals if all of the 

 more direct circuit groups are busy. These more direct circuit groups 

 can then be engineered on a basis providing for high usage of the cir- 

 cuits, recognizing that when one group is busy the machine appeals to 

 another and so on until as a last resort the final group is used. 



In determining means for handling all of the toll messages with a 

 relatively small number of control switching points, tremendous ad- 

 vantage was derived from modern transmission developments, par- 

 ticularly carrier systems which give a great economy from the concen- 

 tration on a long distance route of large numbers of telephone circuits 

 - numbers often running into the thousands. As a result, a considerable 

 degree of circuitous routing and back hauling of circuits is economical if 

 by these means the circuits can be concentrated on heavy routes. This 

 in turn lends itself to a plan using a minimiun of control switching points. 



