MACHINE SWITCHING TELEPHONE SYSTEM 



79 



The incoming trunk to the machine switching office terminates 

 on an "incoming selector," which is of the type already described. 

 The machine switching office has a capacity for 10,000 numbers, but 

 the incoming selector has capacity of only 500 trunks, so that the 

 same arrangement is employed as on the district selectors; that is, 

 the incoming selector chooses one of a number of other selectors, 

 called "final selectors," which have access to the subscribers' lines. 

 Since each group of final selectors has access to 500 subscribers, 20 



TO DISTRICT FR. 

 OR OFFICE FR. IN 

 DISTANT OFFICE 



CALLED 

 SUBSCRIBER 



Fig. 20 — Diagram of Incoming and Final Frames 



groups of finals will be necessary to care for the full 10,000 numbers. 

 On the incoming selector frames, therefore, appear 20 groups of trunks, 

 each group leading to a different frame of final selectors. 



The method of selection is the same as described for the district 

 and office selectors; that is, first the incoming selector, under control 

 of the sender in the originating office, trips the proper brush, chooses 

 the proper group, and finally chooses an idle trunk leading to a final 

 selector. The final selector then goes through the process of brush, 

 group, and subscriber's terminal selection. The terminal selection 

 is under the control of the sender which counts line by line in the 

 group of ten, until the desired one is reached. If the called line is 

 idle, it is rung, and the calling subscriber is advised of that fact by 

 hearing the audible ringing signal. If the called line is busy it is not 

 connected, but an intermittent buzz, recognized as the busy signal, is 

 sent back to the calling subscriber. If the called number is that of a 

 P. B. X. having several trunks, the final selector automatically hunts 

 for an idle one. If the final selector, after testing all the P. B. X. 

 trunks finds them all busy, it sends back the busy signal. 



