78 BELL SYSTEM TECHNICAL JOURNAL 



talking connection is completed. During the conversational period 

 the connection is held under control of the calling telephone, and when 

 the telephone receivers are replaced, the central office equipment and 

 the telephones are released for use on other calls. The equipment, 

 of course, transmits the busy tone signal to the calling subscriber if 

 the called line is found busy, and automatically routes a call for a 

 discontinued or an unassigned line to an operator who informs the 

 subscriber of the status of such a line. 



Operators and associated switchboards are provided in the dial 

 system to handle certain classes of calls and to render assistance to 

 subscribers when required. Calls to these operators are established 

 in response to the dialing of operators' codes in a manner similar to 

 the establishment of calls to other subscribers. 



Operators are usually provided to complete calls terminating in a 

 dial office which are originated by subscribers served by manual 

 offices. 



Prior to the introduction of the crossbar system, the Bell System 

 employed two general types of dial central offices. These are the 

 well known step-by-step and panel systems. 



The step-by-step system has been used generally in the smaller 

 cities which are frequently served by a single central office or by a 

 relatively small number of offices and where the trunking problems 

 are consequently less complicated. The switches of the step-by-step 

 system are controlled directly by the impulses from the subscriber 

 dials and, necessarily in conformity with the dial, the system operates 

 on a decimal basis. The selectors of this system are first moved 

 under control of the dial to any one of ten vertical positions, corre- 

 sponding to the numeral of the digit dialed, and in the case of trunk 

 hunting switches is then automatically rotated over a row of ten trunk 

 terminals to find an idle trunk during the interdigital time of the dial. 



The step-by-step switch thus has access to ten different groups of 

 trunk terminals with ten terminals each. The location of the trunk 

 groups on the switches is governed by the digits dialed and conse- 

 quently the relocation of a group necessitates directory changes. 

 These limitations in trunk access and flexibility are not material 

 handicaps in the smaller cities throughout the country where the 

 system is giving excellent service. 



The panel system meets the complex service requirements of the 

 larger cities with their large volume of traffic and multiplicity of central 

 offices. In these cities the number of trunk groups is large and the 

 number of trunks in the groups varies widely. Further, the number of 

 groups and their sizes are frequently changed by the introduction of 



