108 BELL SYSTEM TECHNICAL JOURNAL 



frame. These terminals correspond to the director number of the 

 subscriber Hne. A number group connector generally has access to 

 the test terminals of several hundred line numbers depending on the 

 terminating traffic to the lines. 



2. The marker will obtain a connection through the number group 

 connector to the busy test terminal of the particular called line and 

 determine whether the line is busy or idle. 



3. It will determine from the two other test terminals, the identifica- 

 tion of the line link frame where the called line appears, and the hori- 

 zontal group of line links which has access to the called line. In addi- 

 tion, it determines the type of ringing to be applied to the called line 

 from the circuit conditions on the test terminal. 



4. Assuming that the called line is idle, the marker will connect, 

 through the line choice connector circuit, to the line link frame and to 

 the ten line links which have access to the called line. 



5. It will then connect, through the connector circuit, to the in- 

 coming link frame associated with the incoming trunk to which the 

 calling subscriber line is now connected. The incoming link frame 

 identification was obtained from the sender link control circuit 

 through the sender as previously mentioned. 



6. The marker will then select an idle channel through the incoming 

 link and line link frames as previously described. This channel will 

 consist of an "incoming link," a "line junctor," and a "line link" all 

 to be connected in series. The marker then operates the proper 

 selecting and holding magnets of the crossbar switches in each frame 

 which establishes the connection from the incoming trunk to the called 

 subscriber line. 



7. The marker will then cause the incoming trunk to start the 

 proper ringing over the called subscriber line and to transmit the 

 ringing tone signal over the trunk to the calling subscriber. 



8. At this point the terminating marker and the terminating sender 

 will have completed their functions and, together with the terminating 

 sender link frame, will be released. The complete connection will then 

 be established from the calling line to the called line and the con- 

 versational circuit completed when the called subscriber answers. 



If the terminating marker finds the called line busy it will cause the 

 incoming trunk circuit to transmit a busy tone to the called subscriber. 

 The terminating marker has the following other important functions: 

 1. If the call is for a PBX (Private Branch Exchange) the condition 

 on one of the test terminals of the called line in the number group 

 connector will inform the marker that the line is one of a group of 

 lines. The marker will test all of the lines in the group, testing up to 

 as many as twenty simultaneously, and will select an idle one. 



