444 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



BACKGROUND AND OBJECTIVES 



In many recent designs of dial telephone central offices, especially those 

 in use in large urban areas, the subscriber's dial does not control directly 

 the setting of switches leading toward the desired destination as was 

 the case in early dial systems. Instead the information is received first 

 by a register circuit which is selected from a group of such register cir- 

 cuits and is connected to the calling subscriber's line on the origination 

 of a call. The register cooperates with other complex circuits to ascertain 

 the location of idle trunks to the called subscriber's office and possible 

 routes through the switching network to these trunks, and to control the 

 selection and use of one such path to this called office. In the called office 

 another register circuit, frequently of a type different from that into 

 which the subscriber originally dialed, is selected from a group of such 

 circuits and the directory number of the called subscriber is transmitted 

 to it from the register-sender circuit in the calling office. In the ter- 

 minating office the procedure of locating and testing the called line 

 and switching paths to it, and of establishing a connection over one of 

 these paths is accomplished through the use of additional control cir- 

 cuits. These various circuits which are used in setting up a conversa- 

 tional path are called common control circuits. 



Each type of common control circuit is provided in sufficient number 

 to halidle the expected traffic. The number required is, of course, related 

 to speed of operation since the shorter the holding time of a circuit, i.e., 

 the length of time a circuit takes to complete its functions for one call, 

 the more calls such a circuit can complete in a given time. The holding 

 time of a control circuit is, in turn, dependent upon the operating speed 

 of the equipment controlled. Furthermore, control circuits of the same 

 type, if more than one of a given type is required, will have added to 

 their normal functioning time during busy traffic periods a delay time 

 interval since they must not interfere with each other's actions in the 

 controlled equipment. Common control circuits, such as dial pulse regis- 

 ters, which receive information directly from subscribers must be engi- 

 neered on the basis of an average holding time which allows for the 

 variable reaction times, hesitations, partial usages and other personal 

 idiosyncrasies of subscribers. Present designs of automatic central offices 

 require a number of each type of control circuit and auxiliary circuits 

 for selecting and connecting the control circuits as required in the opera- 

 tion of the system. These control circuits and connectors embrace a 

 considerable fraction of the space and cost of such an office. 



Dr. T. C. Fry, at the time he was Director of Switching Research 



