THE BELL SYSTEM 



TECHNICAL JOURNAL 



VOLUME XXXV SEPTEMBER 1956 number 5 



Copyright 1956, American Telephone and Telegraph Company 



Electronics in Telephone Switching 



Systems 



By A. E. JOEL 



(Manuscript received March 18, 1956) 



In recent years a number of fundamentals has been discovered through 

 research which place new tools at the disposal of the circuit and system de- 

 signers. Examples of this ^'new art" are concepts such as information 

 theory, dealing with the quantization and transmission of information, and 

 solid state principles from which have developed the transitor and other de- 

 vices. This paper surveys certain new art principles, techniques and devices 

 as they apply to the design of new telephone switching systems. 



Over the past forty years a great background and fund of knowledge 

 has developed in the field of telephone switching. Constant improvement 

 in available devices has resulted in increasing the scope of their appli- 

 cation. The field has almost reached a point of perfection as an art and 

 is now rapidly entering a more scientific era. 



The tools of the present day telephone system design engineer are 

 well known and some are illustrated in Figure 1. These are the relay 

 and the various forms of electromechanical switching apparatus. But 

 over the years, while the art employing these tools was developing, 

 the field of electronics has also been developing. Its applications were 

 most needed when dealing with its characteristics of sensitivity rather 



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