Common Control Telephone 

 Switching Systems 



By OSCAR MYERS 



(Manuscript received August 1, 1952) 



In the development of dial telephone switching systems two fundamentally 

 different arrangements have been devised for controlling the operations of the 

 switches. In one arrangement the switch at each successive stage is directly 

 responsive to the digit that is being dialed. Systems using this method of 

 operation are called direct dial control systems, an example being the step- 

 by-siep system a^ commonly used in the Bell System. In the other arrange- 

 ment the dialed information is stored for a short time by centralized control 

 equipment before being used in controlling the switching operations. Systems 

 using the second arrangement are known as common control systems, ex- 

 amples of which are rotary, panel and crossbar. These two arrangements 

 have different economic fields of use, the direct dial control being better suited 

 for the smaller telephone exchanges and the common controls for the larger 

 exchanges, especially those in metropolitan areas. A history of the evolution 

 of these types of switching systems is presented, followed by a discussion of 

 their comparative merits for various fields of use. 



HISTORY 



Invention of machines for switching telephone connections started 

 shortly after the invention of the telephone. A forerunner of the step-by- 

 step system, the Connolly and McTighe "girlless" telephone sj^stem,* 

 was patented in 1879 and the first patent on the Strowger step-by-step 

 systemf was issued in 1891. The first commercial installation of auto- 

 matic switching equipment was made at La Porte, Indiana, in 1892. This 

 installation used step-by-step mechanisms. 



In the early 1900's many telephone engineers regarded full automatic 

 switching as uneconomical but technically feasible if restricted to single 

 office exchanges with individual flat rate lines. They were, however, un- 



* U. S. Patent 222,458— 1879— Connolly and McTighe. 

 t U. S. Patent 447,918— 1891— Almon B. Strowger. 



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