Crossbar Tandem as a Long Distance 

 Switching System 



By A. O. ADAM 



(Manuscript received March 4, 1955) 



Major toll switching features are being added to the crossbar tandem 

 switching system for use at many of the important long distance switching 

 centers of the nationwide network. These include automatic selection of one 

 of several alternate routes to a 'particular destination, storing and sending 

 forward digits as required, highly flexible code conversion for transmitting 

 digits different from those received, and a translating arrangement to select 

 the most direct route to a destination. The system is designed to serve both 

 operator and customer dialed long distance traffic. 



INTRODUCTION 



The crossbar tandem switching system,^ originally designed for switch- 

 ing between local dial offices, will now play an important role in nation- 

 wide dialing. New features are now available or are being developed that 

 will permit this system to switch all types of traffic. As a result, crossbar 

 [ tandem offices will have widespread use at many of the important switch- 

 ing centers of the nationwide switching network. 



This paper briefly reviews the crossbar tandem switching system and 

 its application for local switching, followed by discussion of the general 

 aspects of the nationwide switching plan and of the major new features 

 required to adapt crossbar tandem to this plan. 



CROSSBAR TANDEM OFFICES USED FOR LOCAL SWITCHING 



Crossbar tandem offices are now used in many of the large metropolitan 

 areas throughout the country for interconnecting all types of local dial 

 offices. In these applications they perform three major functions. Basi- 

 cally, they permit economies in trunking by combining small amounts of 



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