538 BELL SYSTEM TECHNICAL JOURNAL 



telephone plant. The teletypewriter switching plan is designed to 

 provide on the most economical basis the circuits necessary for satis- 

 factory connection between any two stations in the country without 

 any special line-up or adjustment of the circuits or apparatus. 



Each switching point has a direct connection to each of the sub- 

 scriber stations within its area (except for a few stations which are 

 connected to the switchboards by a single channel carrier circuit 

 operating over regular toll telephone circuits when a connection to 

 these stations is desired). In addition it has direct toll circuits to 

 one or more of the other switching points. Eight cities of considerable 

 importance from the standpoint of switching in the national network 

 are designated as "regional centers." These cities, New York, 

 Atlanta, Chicago, St. Louis, Dallas, Denver, San Francisco, and Los 

 Angeles, are interconnected largely by high grade direct circuits and 

 ultimately will be interconnected completely by such facilities. Each 

 of the regional centers has direct circuits to a number of smaller centers 

 designated as "routing outlets" within a given area, which are also 

 interconnected by direct circuits. 



The other switching centers, called "teletypewriter centers," which 

 are not required by their position in the networks to handle through 

 business, have direct circuits to one or more routing outlets and may 

 have direct connections to similar nearby centers if traffic justifies it. 



The appHcation of the teletypewriter switching plan is illustrated in 

 Fig. 4. Considering only the toll circuits of the basic routes (solid 

 lines connecting switching centers in the figure), it may be noted that 

 within any area where the routing outlets are interconnected by direct 

 circuits, the maximum number of teletypewriter toll lines required for 

 connection between two stations in the area is 3. A very large per- 

 centage of the connections can, of course, be made with only one or 

 two toll links. It may also be seen that, assuming all regional centers 

 to be interconnected by direct circuits, a maximum of 5 toll links will 

 serve to connect any two stations in the country, using only the basic 

 routes. 



In addition to these basic toll routes, supplementary routes are 

 provided wherever the traffic warrants, as indicated by the dashed 

 lines in the figure. These supplementary routes may be direct circuits 

 between two teletypewriter centers, between a teletypewriter center 

 and a routing outlet or regional center other than that through which 

 it is normally served, or between a routing outlet in one area and a 

 routing outlet or regional center in another regional area. It is obvious 

 from the figure that the efTect of these supplementary routes is to 

 reduce the number of toll links and consequently the number of 

 switches involved in certain connections. 



