CROSSBAR DIAL TELEPHONE SWITCHING SYSTEM 101 



3. The sender link control circuit tests the circuit paths chosen 

 from the line circuit to the sender before disconnecting from the 

 connection, in order to insure the proper establishment of the con- 

 nection. In case of a failure the control circuits will make repeated 

 trials to establish the connection over different paths and give an alarm 

 to the maintenance force. 



4. Emergency control circuits are provided for use in case the 

 regular control circuits are removed from service for maintenance 

 reasons. 



The next stage in the progress of the call is illustrated in Fig. 12. 

 In this stage of the call the principal control unit is the originating 

 marker. Its major function is to control the switches in the establish- 

 ment of the connection to an idle outgoing trunk circuit to the called 

 office, which may terminate in a distant office or in the same office as 

 the calling subscriber. 



When the subscriber sender connects to the originating marker 

 through the connector circuit, the sender transfers the called office 

 code indication and the district link frame identification to the marker 

 circuit. The called office code indication causes the operation of a 

 "route" relay in the marker corresponding to the particular office 

 called. 



There are a number of "route" relays in each marker and one is 

 assigned to each called office routing. The route relay is connected 

 as required by the office code to which it is assigned, so that it will 

 direct the marker to the trunks of the called office and to the office 

 link switch frame on which these trunks appear and indicate the 

 number of trunks in the group. The route relay also is connected to 

 determine the type of the called office, such as Crossbar, Panel, or 

 Manual, and to set up the corresponding circuit conditions in the 

 subscriber sender to enable the sender to handle the connection 

 properly after the marker has been released. The connections of 

 the route relay contacts to the control relays in the marker are made 

 flexible so as to permit the assignment of any route relay to any 

 office code and to permit changes to be made from time to time in the 

 route information, changes in trunk group sizes and location, changes 

 in the type of terminating office, etc. The route relays and associated 

 flexible connection facilities represent a considerable portion of the 

 marker equipment, especially in large metropolitan offices where 

 several hundred central offices are involved. 



When the route relay is operated, the marker proceeds with the 

 establishment of the connection as follows: 



1. It connects to the office link frame on which the trunks to the 



