98 BELL SYSTEM TECHNICAL JOURNAL 



Brief Description of Circuit Operation 

 The operation of the system will be described by tracing the progress 

 of a call through the system. The establishment of a call from one 

 crossbar subscriber to another crossbar subscriber may be divided into 

 four stages: two in the originating end of a connection and two in 

 the terminating end. 



1. The calling subscriber is connected to a sender for the purpose of 

 registering the called number which is dialed. 



2. The subscriber sender is connected to an originating marker and 

 the marker selects the switch frames for establishing the connection to 

 an outgoing trunk. 



3. The outgoing trunk circuit is connected to a sender in the termi- 

 nating end to register the called number. 



4. The terminating sender is connected to a terminating marker and 

 the marker selects the switch frames for establishing the connection 

 to the called subscriber line. 



The first stage in the progress of a call is illustrated in Fig. 11. 

 It will be seen that the line of a calling subscriber terminates on a 

 vertical unit of a primary crossbar switch located on a line link switch 

 frame. When the subscriber receiver is lifted from the telephone 

 preparatory to dialing, a line relay is operated, as in other systems, and 

 the circuits proceed with the establishment of the connection to an 

 idle subscriber sender which will register the called number when it 

 is dialed. 



The circuit functions on this stage of the call are as follows: 

 1. The subscriber line is located by the "line link control" circuit 

 which is common to the line link frame, by a coordinate method of 

 testing. That is, the control circuit determines the primary crossbar 

 switch in which the line is located and the particular vertical unit in 

 the switch on which the line is terminated. This operation is similar 

 to the line finder operation in other dial systems, except that the 

 operation is accomplished by relay operations instead of by a mechani- 

 cally traveling brush. 



2. The line link control circuit then simultaneously selects an idle 

 line link between the primary switch in which the line appears, and a 

 secondary switch on which a group of district junctors appears which 

 has at least one idle district junctor in the group and which has 

 access to idle senders and an idle sender link. 



3. This will bring into operation the common "sender link control" 

 circuit of the sender link switch frame to which the selected group of 

 district junctors is connected. This control circuit will select an idle 

 district junctor in this group which appears on a primary switch on 



