04 BELL SYSTEM TECHNICAL JOURNAL 



the machine switching office receive the information as to the desired 

 number from "A" operators in the distant manual office, exactly as 

 is done in the case of manual operation. 



The introduction of machine switching equipment does not require 

 radical changes in private branch exchanges. The private branch 

 exchange is provided with dials, and calls to the central office are 

 dialed by the private branch exchange attendant or by the extension 

 user in the same way that the ordinary subscriber dials. No change 

 in the private branch exchange is required for handling incoming 

 calls. An idle trunk in the private branch exchange group is selected 

 by the mechanism in the machine switching office, in much the same 

 way as an individual subscriber's line is selected. 



Numbering System 



One of the unique advantages of the plan developed for designating 

 telephone numbers, to which reference has already been made, is 

 that it does not necessitate the abandonment of the existing manual 

 listings. It requires no change except that the first three letters of 

 the office name are set out more prominently. Simple as this change 

 in the form of listing appears, until it was developed by the Bell 

 System no satisfactory method of designating telephone numbers for 

 machine switching offices in large cities was known. 



Many plans had been proposed, to all of which there were serious 

 objections. Some of them required changing the whole system of 

 manual designation, others the use of combinations difficult for the 

 subscriber to use. In small cities a numbering plan employing 

 only digits is sometimes practicable, but in such a large area as we 

 are considering, such a plan would involve seven digits. The sub- 

 scriber's number would take the form of say 786-3549. Such numbers 

 would be difficult for operators to use and for the subscribers to carry 

 in mind and would require that every subscriber's number in the 

 entire area be changed before the first machine switching office could 

 be cut into service. With the new system, the subscriber's number 

 and office in general remains as before. It is necessary to change only 

 a few conflicting office names in order to make them fit into the system. 



Description of the Equipment 



A detailed description of each unit employed in this system would 

 be impracticable, in this connection, but a brief description of the 

 more important ones will be of interest. 



Sender. The use of the sender makes practicable the introduction 

 of machine switching in large metropolitan areas where, of necessity, 



