834 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



In the United States and Canada as a whole, there are approximately 

 2,600 toll centers. The remainder of the toll connecting points — about 

 17,500 — are tributaries. 



Fig. 2 gives an idea of the variety and complexity of the network of 

 circuit groups required to interconnect the toll centers in one area. Here 

 each line represents a group of circuits, known as "intertoll trunks," 

 between two toll centers. Each group may contain anywhere from one 

 to several dozen trunks. The location of the lines on the map is unrelated 

 to the geographical routing of the trunks, and only a part of the circuit 

 groups are shown. To get a complete picture one should visualize that a 

 cluster of relatively short circuit groups radiates from each toll center 

 to its tributaries, of which there may be up to 15 or more. 



Physically, the plant consists of a network of open wire lines, cables 

 and radio systems. On these, voice frequency or carrier operation is 

 employed in each section as required to provide the necessary intertoll 

 trunks. The routes of the lines in Minnesota and Wisconsin are shown 

 by Fig. 3. In this area there are no radio routes carrying telephone cir- 

 cuits, but a radio system between Chicago and Minneapolis is in the 

 planning stage. 



Areas like Wisconsin and Minnesota must, of course, be connected to- 

 gether, and Fig. 4 shows the major Bell System toll routes that accom- 

 plish this. On a map of this kind it is not possible to include anything 

 like the detail shown in Fig. 3. One must visualize, therefore, that each 

 state contains a network of routes generally comparable to those shown 

 for Wisconsin and Minnesota. 



This then represents the interconnection problem to be met by an 

 orderly switching plan that will provide efficient, reliable and fast toll 

 telephone service between any two points. 



EARLIER TOLL SWITCHING PLANS 



Very early in the telephone industry it became evident that: (1) 

 There must be a plan for connecting circuits together. (2) Switching 

 centers with suitable equipment must be established in accordance with 

 this plan. (3) Trunks must be provided in adequate numbers to connect 

 every place to one or more switching centers and to interconnect the 

 switching centers. (4) All this must be done in a way that makes it 

 possible to provide good service at reasonable cost. 



As time went on, early plans crystallized into what became known as 

 the General Toll Switching Plan. A paper presented at the summer con- 

 vention of the A.I.E.E. in Toronto in 1930 by Dr. H. S. Osborne outlined 



