CROSSBAR TANDEM AS A TOLL SWITCHING SYSTEM 99 



to Pittsburgh and on its last attempt it would test the final group to 

 Indianapolis. If the call were routed to Pittsburgh or Indianapolis, the 

 switching equipment at these points would attempt by first choice and 

 alternate routes to reach Youngstown. The final choice backbone route 

 would be via Indianapolis, Chicago, St. Louis, Pittsburgh, Cleveland to 

 Youngstown. Should all the trunks in any of the final groups tested be 

 busy no further attempt to complete the call is made. It is unlikely 

 that so many alternate routes would be provided in actual practice 

 since crossbar tandem can test only a maximum of 240 trunks on each 

 call and, in the case illustrated, the final trunk group to Indianapolis 

 may be quite large. 



The method employed by the crossbar tandem marker in selecting 

 the direct route and subsequent alternate routes is shown in simplified 

 form on Fig. 5. As a result of the translating operation, the marker 

 selects the first choice route relay, corresponding to the called destina- 

 tion. Each route relay has a number of contacts which are connected to 

 supply all the information recjuired for proper routing of the call. Several 

 of these contacts are used to indicate the equipment location of the 

 trunks and the number of trunks to be tested. The marker tests all of 

 the trunks in the direct route and if they are busy, the search for an 

 idle trunk continues in the first alternate route which is brought into 

 play from the "route advance" cross-connection shown on the sketch. 

 As many as three alternate routes in addition to the first choice route 

 can be tested in this manner. 



STORING AND SENDING FORWARD DIGITS AS REQUIRED 



The crossbar tandem equipment at control switching points must 

 store all the digits received and send forward as many as are required to 

 complete the call. 



The called number recorded at a switching point is in the form of 

 ABX-XXXX if the call is to be completed in the same numbering 

 plan area. If the called destination is in another area, the area code 

 XOX or XIX precedes the 7 digit number. The area codes XOX or XIX 

 and the local office code ABX are the digits used for routing purposes 

 and are sufficient to complete the call regardless of the number of switch- 

 ing points involved. Each control switching point is arranged to ad- 

 vance the call towards its destination when these codes are received. 

 If the next switching point is not in the numbering area of the called 

 telephone, the complete ten-digit number is needed to advance the 

 call toward its destination. If the next switching point is in the num- 



