CROSSBAK TANDEM AS A TOLL SWITCHING SYSTEM 



101 



call is switched to an alternate route via the step-by-step primary 

 center, it will be necessary for the crossbar tandem equipment to delete 

 the area code 218 and substitute the arbitrary digits 062 to direct the 

 call through the switches at the primary center, since the toll center 

 requires the full seven digit number for completing the call. 



PREFIXING DIGITS 



It may be necessary to route a call from one area to another and back 

 to the original area for completion. Such a situation arises on a call 

 from Amarillo to Lubbock, Texas, both in area 915 when the crossbar 

 tandem switching equipment finds all of the direct paths from Amarillo 

 to Lubbock busy as illustrated on Fig. 7. The call could be routed to 

 Lubbock via Oklahoma City which is in area 405. A seven-digit number 

 for example, MAin 2-1234, is received in the crossbar tandem office at 

 Amarillo. Assuming that the call is to be switched out of the 915 area 

 through the 405 area and back to the 915 area for completion, it is 

 necessary for the crossbar tandem office in Amarillo to prefix 915 to the 

 MAin 2-1234 number so that the switching equipment in Oklahoma 

 City will know that the call is for the 915 area and not for the 405 area. 



Prefixing digits may also be needed at crossbar tandem offices to 

 route calls through step-by-step primary centers. The crossbar tandem 

 office in Fig. 8 receives the seven digit number MA2-1234 for a call to a 



701 



AREA 



218 



AREA 



NUMBER 



RECEIVED 



218-GA8-1234 



CROSSBAR 

 TANDEM 



NUMBER 



OUTPULSED 



062-GA8-1234 



STEP-BY-STEP 

 PRIMARY CENTER 



ALTERNATE 

 ROUTE 



DIRECT 

 ROUTE 



GA8-I234 



i^ 



GA8-t234 



SX S 



TOLL 

 CENTER 



GA8-1234 



LOCAL 

 CUSTOMER 



Fig. 6 — Code conversion. 



