94 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



cally into more than 100 numbering plan areas and to give each of these 

 a distinctive three digit code with either a 1 or as the middle digit. 

 Each numbering plan area will contain 500 or fewer local central offices 

 each of which will be assigned a distinctive three-digit office code. 

 Thus each of the telephones in the United States and Canada will have, 

 for distance dialing purposes, a distinct identity consisting of a three 

 digit area code, an office code of two letters and a numeral, and a sta- 

 tion number of four digits. Under this plan, a customer will dial 7 digits 

 to reach another customer in the same numbering area and 10 digits to 

 reach a customer in a different numbering area. 



A further reciuirement for nationwide dialing of long distance calls is 

 a fundamental plan"* for automatic toll switching. The plan provides a 

 systematic method of interconnecting all the local central offices and 

 toll switching centers in the United States and Canada. As shown on 

 Fig. 2, several local central offices or "end offices" are served by a single 

 toll center or toll point that has trunks to a "home" primary center 

 which serves a group of toll centers. Each primary center, has trunks to 

 a "home" sectional center which serves a larger area of the country. 

 Similuj-ly, the entire toll dialing territory is divided into eleven very 

 large areas called regions, each having a regional center to serve all the 

 sectional centers in the region. One of the regional centers, probably 

 St. Louis, Missouri, will be designated the national center. The homing 

 arrangements are such that it is not necessary for end offices, toll centers, 

 toll points and primary centers to home on the next higher ranking 

 office since the complete final route chain is not necessary. For example, 

 end offices may be served directly from any of the higher ranking switch- 

 ing centers also shown in Fig. 2. 



Collectively, the national center, the regional centers, the sectional 

 centers and the primary centers will constitute the control switching 

 points for nationwide dialing. The basic switching centers and homing 

 arrangements are illustrated in Fig. 3. 



TANDEM CROSSBAR FEATURES FOR NATIONWIDE DIALING 



The broad objective in developing new features for crossbar tandem 

 is to provide a toll switching system that can be used in cities where 

 the large capacity and the full versatilit}^ of the No. 4 toll crossbar 

 switching system-'' may not be economical. 



The application of crossbar tandem two-wire switching systems at 

 primary and sectional centers has been made possible by the extended 

 use of high speed carrier systems. The echoes at the 2-wire crossbar 

 tandem switching offices can be effectively reduced by providing a high 



