CROSSBAR DIAL TELEPHONE SWITCHING SYSTEM 109 



The lines of a PBX may be assigned to non-consecutive numbers 

 within the usual 10,000 series, and with the exception of the numbers 

 dialed, they may be assigned to line numbers in a special group of 

 2500 outside of the 10,000 series. These features reduce the necessity 

 for number changes due to the growth of private branch exchanges, 

 and conserve subscriber line numbers in the office. The lines of a 

 PBX group can be distributed over several line link frames and over 

 two number group connectors to equalize the terminating traffic load 

 n the case of busy private branch exchanges. 



2. The marker recognizes numbers dialed which are unassigned, 

 disconnected or changed numbers, and automatically routes such 

 calls to an operator who will inform the calling subscribers as to the 

 status of the numbers called. 



3. In case the called number is on a party line, the marker deter- 

 mines from one of the test terminals which station of the line is to be 

 rung, and signals the incoming trunk to provide the proper ringing. 



4. The marker tests the continuity of the circuit paths to be used 

 to the called line before establishing the connection, to insure that the 

 connection is properly set up and that it will be held under control 

 of the subscriber telephone after the marker disconnects. The marker 

 also tests for short-circuits, crosses and grounds, and in case of a 

 failure due to any inoperative condition it will connect itself to the 

 common trouble indicator and leave a record of the trouble and its 

 location and give an alarm to the maintenance force. 



Figure 15 shows the complete talking connection through the various 

 trunks and switch frames as finally established after all of the common 

 control circuits have been released. 



On a call to a subscriber served by a panel dial office, the connection 

 is routed through the district link and the office link frames in the 

 same manner as on a call terminating in a crossbar office, but in this 

 case the idle trunk chosen on the office link frame terminates in an 

 incoming panel switch in the distant panel dial office. The subscriber 

 sender of the crossbar office causes the incoming and final selectors in 

 the terminating panel office to select the called subscriber line without 

 the aid of any terminating senders in either office. When the sub- 

 scriber sender has completed these functions it will be released and 

 the connection will be established from the calling line over the inter- 

 office trunk circuit and through the terminating panel incoming and 

 final selectors to the called line. On this type of call the subscriber 

 sender operates in the same manner as though the called line were in a 

 crossbar office, and the selectors in the panel office operate in the same 

 manner as though the call had originated in another panel office. 



