1038 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 

 NUMBERING PLAN 



For nationwide dialing it is necessary that each customer have 

 a distinctive universal number. This numbering system is accom- 

 plished by dividing the country and Canada into about ninety num- 

 bering areas. Each of these areas is assigned a distinctive three digit 

 code which, in order not to conflict with local office three digit codes, 

 has either the digit "1" or "0" as the second digit. Within these num- 

 bering areas each local office will have a distinctive non-confficting, 

 name and number code. Since each customer in an office has a distinctive 

 number, a corresponding distinctive nationwide universal number is 

 thus provided. To reach a customer outside the local numbering area 

 will require the dialing of three digits for the area code, three digits for 

 the office code and four or five digits for the line number. Thus by dialing 

 ten or eleven digits a connection can be made to any customer an3rwhere 

 within the country and Canada. Fig. 1 shows the present numbering 

 area code assignments for the United States and Canada. 



TOLL LINE SWITCHING NETWORK 



A second requirement for nationwide dialing is the provision of about 

 70 strategically placed automatic switching toll offices called control 

 s^vitch points (CSP) throughout the United States and Canada. The 

 switching system used at each of these 70 or so CSP offices have sev- 

 eral new features as follows: 



1. Six digit translation. 



2. Ease in changing and adding routings. 



3. Automatic alternate routing. 



4. Code conversion. 



5. Storing and sending forward digits as needed. 



BASIC SWITCHING ARRANGEMENT 



In the CSP offices the transmission paths are established through 

 crossbar switches mounted on the incoming and outgoing link frames 

 as shown in Fig. 2. The setting up of the connection through these 

 switches and the linkages is controlled by equipment common to the 

 office which is held in use only long enough to set up each connection. 



The major items of common control equipment are the senders, de- 

 coders, markers and card translators. 



The sender's function is to receive and register the digits of the 

 called destination, to transmit the area and, if required, the office codes 



