CROSSBAR DIAL TELEPHONE SWITCHING SYSTEM 91 



operation of the circuits. A simplified block diagram of the principal 

 equipment units of the system is shown in Fig. 7. It will be noted 

 that in general there are three types of equipment units: 



1. The transmission battery supply and supervisory circuits consisting 



of the "district junctors" and the "incoming trunks." 



2. The crossbar switch frames. 



3. The common "control" circuits, the "senders" and the "markers." 



The "district junctor" and the "incoming trunk" circuits are 

 composed principally of small relays. The district junctors furnish 

 the talking battery for the calling subscribers and supervise the 

 originating end of connections. The incoming trunks control the 

 ringing of the called subscriber bells, furnish talking battery for the 

 called subscribers, and supervise the terminating end of connections. 



The switch frames, which consist almost entirely of crossbar switches, 

 provide the means for switching between the subscriber lines, the 

 district junctors and the incoming trunks. Switch frames also are 

 used for switching the district junctors and the incoming trunks to 

 the senders. 



The "senders" consist principally of small relays and their functions 

 are similar to those of the operators at a manual switchboard. The 

 "subscriber senders" register the called numbers from the subscriber 

 dials and transmit the necessary information to the "markers," to 

 the "terminating senders" and to the manual operator positions in 

 manual offices for completing connections to the called lines. The 

 subscriber senders also control the operation of the selectors in distant 

 panel offices. The "terminating senders" in the terminating end of 

 the crossbar office receive the numerical digits of the called numbers 

 from the subscriber senders of any dial office and transmit the required 

 information to the "terminating markers" for setting up the connec- 

 tions to the called lines. 



The "markers" are the most important control circuits in the 

 system. They are composed of both small and multi-contact relays. 

 There are two types, one for originating traffic and one for terminating 

 traffic. The operating time of the markers is short, considerably less 

 than one second, and consequently only three or four markers of each 

 type are required in the average office. 



The "originating markers" determine the proper trunk routes to 

 the called office. They have access to all outgoing trunk circuits and 

 all the crossbar switch frames that are used for establishing the 

 connections to the called office trunks. They test the trunk group 

 to find an idle trunk to the called office, and also test and find an idle 



