100 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



bering area of the called telephone the area code is not needed and seven 

 digits will suffice for completing the call. 



For example, suppose a call is originated by a customer in South 

 Bend, Indiana, destined for customer NAtional 4-1234 in Washington, 

 D.C. If it is assumed that the route to Washington is via a switching 

 center in Pittsburgh, then the crossbar tandem equipment at South 

 Bend pulses forward to Pittsburgh 202-NA4-1234, 202 being the area 

 code for the District of Columbia. Pittsburgh in turn will delete the 

 area code and send NA4-1234 to the District of Columbia terminating 

 area. 



As another example, suppose the crossbar tandem office at South 

 Bend receives a call from some foreign area destined to a nearby step- 

 by-step end office in Michigan. The crossbar tandem equipment re- 

 ceives and stores a ten-digit number comprising the area code and the- 

 seven digits for the office code and station number. Assuming that 

 direct trunks to the step-by-step end office in Michigan are available, 

 the area code and office code are deleted and the line number only is 

 pulsed forward. To meet all conditions, the equipment is arranged to 

 permit deletion of either the first three, four, five or six digits of a ten- 

 digit number. 



CODE CONVERSION 



At the present time, some step-by-step primary centers reach other 

 offices by the use of routing codes that are different from those assigned 

 under the national numbering plan. This arrangement is used to obtain 

 economies in switching equipment of the step-by-step plant and is 

 accetpable with operator originated calls. However, with the intro- 

 duction of customer direct distance dialing, it is essential that the codes 

 used by customers be in accordance with the national numbering plan. 

 The crossbar tandem control switching point must then automatically 

 provide the routing codes needed by the intermediate step-by-step 

 primary centers. This is accomplished by the code conversion feature 

 which substitutes the arbitrary digits required to reach the called office 

 through the step-by-step systems. Fig. 6 illustrates an application of 

 this feature. It shows a crossbar tandem office arranged for completing 

 calls through a step-by-step toll center to a local central office, GArden 

 8, in an adjacent area. A call reaching the crossbar tandem office for a 

 customer in this office arrives with the national number, 218-GA8-1234. 

 To complete this call, the crossbar tandem equipment deletes the area 

 code 218 and pulses forward the local office code and number. If the 



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