306 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



is in a vertical direction and it is only when a busy condition or a special 

 function such as PBX hunting is encountered that the call shifts to the 

 right or left. On a particular call, the only control functions performed 

 are those marked with an x. 



In the situation illustrated, the size of the office is assumed to be such 

 that two groups of intraoffice trunks are provided. The marker makes 

 a more or less random choice of the first group to be tested, and, if this 

 group proves to be all busy, will automatically test the second group. 



This brief description of the Number 5 Crossbar System is not intended 

 to be exhaustive. It discusses only the important features of the system 

 and those which will help make the description of the throwdown ma- 

 chine more intelligible. Certain more involved items will be discussed 

 in greater detail in later sections concerned with the functioning of the 

 throwdown machine. 



TIME 



I POSSIBLE DELAYS 



CONTROL FUNCTIONS i AND DIFFICULTIES 



RECEIVER OFF HOOK :^ 



DELAY FOR MARKER (INCLUDES 



LINE LINK FRAME MARKER CONNECTOR ^ qelAY TILL CONNECTOR ACHIEVES 



CONNECTS TO MARKER I PREFERRED POSITION IN GATEJ 



MARKER HUNTS FOR IDLE TRUNK LINK vv ^ ., , pp^.-Tpp- „, .^^ 

 FRAME WITH IDLE REGISTER 1^~ * ^^^ REGISTERS BUSY 



MARKER SEIZES CHOSEN TRUNK LINK FRAME-X TRUNK LINK FRAME DELAY 



MARKER LOCATES IDLE REGISTER X 



MARKER SEIZES LINE LINK FRAME ^X LINE LINK FRAME DELAY 



MARKER IDENTIFIES CALLING LINE -L MARKER MAY IDENTIFY ANOTHER 



I CALLING LINE 



MARKER CONNECTS TO LINKS AND i^ CHANNEL MISMATCH 



JUNCTORS AND MATCHES CHANNEL ^ * CHANNEL MISMATCH 



MARKER OPERATES CROSSPOINTS X 



MARKER RELEASES LINE LINK FRAME X 



AND TRUNK LINK FRAME ^ 



MARKER RESTORES TO NORMAL X 



DIAL TONE TO SUBSCRIBER ^X 



DIALING 1 



I 



END OF DIALING-- ^X 



* MARKER MUST TAKE ALTERNATIVE ACTION 

 IF IT ENCOUNTERS THIS CONDITION 



Fig. 3 — Establishing a dial tone call. 



