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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



customer in the Madison office in the same area. However, since the 

 toll center needs the full seven digit number for completing the call and 

 since the step-by-step switches at the primary center "use up" two 

 digits (04) for its switching, the crossbar tandem equipment must 

 prefix 04 to the seven digit number. 



METHOD OF DETERMINING DIGITS TO BE TRANSMITTED 



The circuitry involved for transmitting digits as received, prefixing, 

 code conversion and for deletion involves both marker and sender 

 functions. The senders have ten registers (1 to 10) for storing incoming 

 digits and three registers (A A, AB, AC) for storing the arbitrary digits 

 that are used for prefixing and code conversion. 



On a ten-digit call into a crossbar tandem switchmg center the area 

 code XOX, the office code ABX and the station number XXXX are 

 stored in the inpulsing or receiving registers of the sender. The code 

 digits XOX-ABX are sent to the marker which translates them to 

 determine which of the digits received by the sender should be outpulsed. 

 It also determines whether arbitrary digits should be transmitted ahead 

 of the digits received and, if so, the value of the arbitrary digits to be 

 stored in the sender registers AA, AB and AC. Case 1 of Fig. 9 assumes 

 that a ten-digit number has been stored in the sender registers 1 to 10 



915 



AREA 



INCOMING 

 TOLL CALL 



LOCAL 



OFFICE 



AMARILLO 

 CROSSBAR 



TANDEM 

 OFFICE 



NUMBER 



RECEIVED 



MA2-1234 



405 

 AREA 



MA 2-1234 



Fig. 7 — Prefixing. 



