CROSSBAR TANDEM AS A TOLL SWITCHING SYSTEM 107 



provides all the information necessary for routing the call to the central 

 office involved. 



CUSTOMER DIRECT DISTANCE DIALING 



Crossbar tandem will provide arrangements permitting customers in 

 step-by-step offices to dial their own calls anywhere in the country. 

 Centralized automatic message accounting previously mentioned will 

 be used for charging purposes. While the basic plan for direct distance 

 dialing provides for the dialing of either seven or ten digits, it will be 

 necessary for the customer in step-by-step areas to prefix a three-digit 

 directing code, such as 112, to the called number. This directing code 

 is required to direct the call through the step-by-step switches to the 

 crossbar tandem office so that the seven or ten digit number can be 

 registered in the crossbar tandem office. 



When a customer in a step-by-step office originates a call to a distant 

 customer whose national number is 915-CH3-1234, he first dials the 

 directing code 112 and then the ten-digit number. The dialing of 112 

 causes the selectors in the step-by-step office to select an outgoing trunk 

 to the crossbar tandem office. The incoming trunk in the crossbar tandem 

 office has quick access to a three-digit register. The register must be 

 connected during the interval between the last digit of the directing 

 code and the first digit of the national number to insure registration of 

 this number. This arrangement is used to permit the customer to dial 

 all digits without delay and avoids the use of a second dial tone. If this 

 arrangement were not used, the customer would be required to wait 

 after dialing the 112 until the trunk in the tandem crossbar office could 

 gain access to a sender through the sender link circuit which would 

 then signal the customer to resume dialing by returning dial tone. 



After recording the 915 area code digits in the case assumed, the 

 CH3-1234 portion of the number is registered directly in the tandem 

 sender which has been connected to the trunk while the customer was 

 dialing 915. When the sender is attached to the trunk, it signals the 

 three-digit register to transfer the 915 area code digits to it via a con- 

 nector circuit. Thus when dialing is complete, the entire number 915- 

 CH3-1234 is registered in the sender. 



Crossbar tandem is being arranged to serve customers of panel and 

 No. 1 crossbar offices for direct distance dialing. At the present time, 

 ten digit direct distance dialing is not available to these customers 

 because the digit storing equipments in these offices are limited to 

 eight digits. Developments now under way, will provide arrangements 

 for expanding the digit capacity in the local offices so that ultirnately 



