CROSSBAR TANDEM AS A TOLL SWITCHING SYSTEM 



93 



CROSSBAR TANDEM SWITCHING ARRANGEMENT 



The connections in a crossbar tandem office are established through 

 crossbar switches mounted on incoming trunk link and outgoing office 

 link frames shown on Fig. 1. The connections set up through these 

 switches are controlled by equipment common to the crossbar tandem 

 office which is held only long enough to set up each individual connec- 

 tion. Senders and markers are the major common control circuits. 



The sender's function is to register the digits of the called number, 

 transmit the called office code to the marker and then, as subsequently 

 directed by the marker, control the outpulsing to the next office. 



The marker's function is to receive the code digits from the sender 

 for translation, return information to the sender concerning the de- 

 tails of the call, select an idle outgoing trunk to the called destination 

 and close the transmission path through the crossbar switches from the 

 incoming to the outgoing trunk. 



GENERAL ASPECTS OF NATIONWIDE DIALING 



Operator distance dialing, now used extensively throughout the 

 country, as well as customer direct distance dialing are based on the 

 division of the United States and Canada into numbering plan areas, 

 interconnected by a national network through some 225 Control Switch- 

 ing Points (CSP's) equipped with automatic toll switching systems. 

 ^ An essential element of the nationwide dialing program is a universal 

 numbering plan^ wherein each customer will have a distinctive number 

 which does not conflict with the number of any other customer. The 

 method employed is to divide the United States and Canada geographi- 



INCOMING 



TRUNK FROM 



ORIGINATING 



OFFICE 



TANDEM 



TRUNK 



TRUNK LINK FRAME 



9 ? 



TRUNK LINK 

 CONNECTOR 



SENDER LINK 



SENDER LINK 

 CONTROL CIRCUIT 



SENDER 



OFFICE LINK FRAME 



<? 9 



OFFICE LINK 

 CONNECTOR 



J 4_ 



MARKER 



CONNECTOR 



OUTGOING 

 TRUNK 



MARKER 



Fig. 1 — Crossbar tandem switching arrangement. 



