1106 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



of system. Because of the flexibility of common controls such rearrange- 

 ments are easier to make and usually cost less than in direct dial control 

 systems. Also the frequency of rearrangements is greater in the larger 

 places. Therefore this is another factor in favor of using common controls 

 for those places. 



SUPERIORITY OF COMMON CONTROL SYSTEMS WITH RESPECT TO SWITCH 

 ACCESS 



It has already been mentioned that the efficiency of trunks increases 

 as the size of the group in which they are selected increases. Recognition 

 of this fact early in the development of machine switching (about 1905) 

 led to the invention of common controls. An ordinary step-by-step selec- 

 tor has access to only ten outlets on a level. Access to more than ten 

 outlets can be obtained by providing graded multiple or by the use of 

 rotary out-trunk switches,* or by combinations of these. Whenever it is 

 necessary to employ graded multiple or rotary out-trunk switches, there 

 is still some slight loss of efficiency as compared to full access. 



In a system such as the panel system in which trunk hunting is a 

 function of the selectors, the maximum number of trunks accessible to a 

 call at any stage of selection is limited by the number of outlets accessible 

 to the switch at that stage. A panel district or office selector, for example, 

 can test a maximum of 90 trunks in a single group, 90 being the maximum 

 number of terminals to which trunks can be assigned on a single panel 

 bank, the remaining ten of the 100 terminals on a bank being reserved 

 for overflow purposes. In the step-by-step system a corresponding limi- 

 tation is avoided by a combination of graded multiple and rotary out- 

 trunk switches with the penalty of a slight loss of efficiency. Marker 

 systems avoid this limitation, also, by having the markers select trunks 

 before they select the paths to the trunks. Crossbar systems with markers 

 can readily test several hundred trunks for a given call. In some crossbar 

 systems — No. 1, for example — trunks are tested in sub-groups of forty, 

 therefore marker holding time is increased when there is more than one 

 sub-group to be tested. This increase in marker holding time is largely 

 avoided in systems like the toll crossbar systems by providing special 

 testing arrangements in which a single indication per sub-group tells 

 the marker which sub-group has one or more available trunks, whereupon 

 the marker only tests the individual trunks of a sub-group in which it is 

 assured that it can find an available trunk. 



* A rotary out-trunk switch is arranged to hunt over a single group of outgoing 

 trunks and to connect to an idle one. It is arranged fnr preselection and switches 

 not in use will advance from busy trunks. 



