COMMON CONTROL SWITCHING SYSTEMS 1107 



The maximum access of ten terminals on a level in ordinary step-by- 

 step is not inherent in the system and might be overcome by a difTerent 

 switch design. A review of how a direct dial control system operates will 

 help to clarify this point. At each switching stage, two actions take place. 

 First, the switch follows the dial pulses until it reaches a group of outlets 

 corresponding to the dialed digit. Then in the interval following this 

 digit and before the pulses of the ne.xt digit arrive the switch hunts over 

 the outlets for an idle path to reach the next stage. The number of paths 

 from a switch level is therefore limited by the number of terminals the 

 switch can hunt over in the interdigital interval. Assuming, for example, 

 an interdigital interval of six-tenths of a second and a hunting speed of 

 100 terminals per second, GO outlets could be provided. However, if such 

 a high speed of hunting could be attained, and the 60 outlets were pro- 

 vided, 60 terminals would be required per group even for small ones Avhich 

 are in the majority. Hence such a switch would be wasteful of terminals. 

 Direct dial control systems have generally employed switches with ten 

 outlets per level although special arrangements such as twin levels have 

 been employed to increase the number of outlets. A twin level switch 

 provides terminals for two trunks at each rotary step and thus twenty 

 trunks per level can be reached. 



TRUNK ECONOMIES FROM TANDEM OPERATION WITH COMMON CONTROL 

 SYSTEMS 



An important factor in trunk economies is the ability to use tandems. 

 The numbering difficulties that direct dial control systems have with 

 tandems have already been discussed. Tandems permit major tiTink 

 economies on two scores. First, tandem routings take advantage of the 

 efficiency which results from concentrating the smaller items of traffic 

 and handling them over common trunk groups. Fig. 7 shows how this 

 economy is attained. Ten offices completely interconnected by one-way 

 trunks require 90 interoffice trunk groups. Ten offices interconnected 

 only by way of tandem require only 20 groups. The groups by Avay of 

 tandem are larger in size than the individual direct groups they replace 

 and because of increased efficiency with group size fewer trunks are 

 required. 



There is a second possibility for an increase of efficiency, an example 

 of which occurs when part of the offices are in business districts and part 

 in residential districts. The peaks of trunked traffic from these different 

 types of offices frequently occur at different hours, hence the trunks 

 through tandem can be provided more economically for a given grade of 



