83G THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



extensive use of carrier than would have been practicable with a slower 

 rate of growth. 



Consideration of these factors which offer an opportunity to improve 

 service has led to the gradual reorientation of the fundamental plans 

 for the intertoll trunk plant which is now under way. 



The New General Toll Switching Plan 



Mechanization of switching and the use of improved transmission 

 instrumentalities permits the design of the switching plant to be con- 

 trolled primarily by the balance between the costs of transmission facil- 

 ities and of switching facilities. 



The new general toll switching plan contemplates as many as eight 

 intertoll trunks in tandem on the most complex connections to be estab- 

 lished. These eight trunks can be interconnected at switching points as 

 described later. The plan further contemplates that wherever possible, 

 the traffic will by-pass intermediate switching points. The number of 

 switches that can be avoided depends on the volume of traffic between 

 the two points concerned and on the traffic load at the time the con- 

 nection is established. 



The proposed plan provides a systematic grouping of switching points. 

 Under this arrangement, each ordinary Toll Center (TC) serves a cluster 

 of nearby tributary points and has trunks to a "home" Primary Outlet 

 (PO) which serves a cluster of toll centers. In some cases it appears 

 practicable to utilize a simplified switching system at a PO, and in order 

 to distinguish this type of center it has been designated a Tandem 

 Outlet (TO). In turn, each PO or TO has trunks to a "home" Sectional 

 Center (SC) which serves a section of the country varying in size from 

 part of a state to all of several states depending on the density of the 

 population. Similarly, The United States and Canada are divided into 

 nine regions, each having a Regional Center (RC) serving as a central 

 switching point for all sectional centers in the region. One of these RC's 

 (St. Louis) is termed the National Center (NC). All of the higher orders 

 of switching centers also act in the capacity of each of the lower centers. 

 For example, any specific SC also acts as a PO and as a TC. 



This arrangement is illustrated in Fig. 5, which covers approximately 

 the same area as Fig. 1, portraying the toll connecting routes. Hibbing, 

 Minnesota, is shown as a representative toll center with the tributaries 

 it serves. It is in the service area of the Duluth Tandem Outlet, the 

 approximate boundaries of which are indicated. Duluth lies in the Min- 

 neapolis "section," which includes a large portion of Minnesota, and is 



