1016 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 195G 



by judiciously introducing redundancy into the equipment, the chance 

 of simultaneous failures of any two identical parts should be extremely 

 improbable.^- With automatic trouble locating, the maintenance forces 

 will not be reciuired to have a thorough understanding of the device 

 characteristics and the circuitry used. Centralized repair of defective 

 units as in modern telephone transmission systems^^ and perhaps even 

 expendability of defective units are a distinct possibility. 



As a result of some of these maintenance considerations it is quite 

 likely that equipment in the future, besides being smaller and more com- 

 pact, will appear more generally in enclosed low cabinets rather than 

 exposed frames. The administrative control may be from consoles 

 rather than vertical panels. More attention will be paid to appearance. 

 The appurtenances, such as ladders required for high frames in electro- 

 mechanical systems, may be eliminated. 



Another change in concept which may come with electronics in tele- 

 phone switching is the form of the power supply. Present day telephone 

 systems use a centralized single voltage dc distribution system with 

 reserve battery. The wide variety of devices and associated voltages, 

 and the need for close regulation in some portions of electronic systems 

 make a reliable ac distribution system with individual power rectifiers at 

 the point of use appear quite attractive. To insure reliability of service 

 the ac distribution must be continuous and not dependent directly upon 

 the commercial sources. 



There is no question that reliability is imperati\'e if electronic switch- 

 ing systems are to survive among electromechanical systems which 

 have achieved a high degree of reliability over a long period of years. 

 The device reliability of the first electronic system may not be com- 

 parable since some of the components of the electronic switching sys- 

 tems will not in their initial applications be as reliable as the least reli- 

 able component in our present day systems. Reliability will be earned 

 and this will probably require considerable effort. Even if initially some 

 devices employed in electronic systems do not measure up to the present 

 high standard which has been set, continuity of high (luality service is 

 a must. It is, therefore, necessary to design a system which will mask 

 the shortcomings of any individual electronic component. ^^ As their 

 reliability is proven an optimum balance will be sought between system 

 redundancy and component quality. Telephone engineers familiar only 

 with the high degree of reliability of present day apparatus will have to 

 accommodate themselves to the characteristics of new electronic de- 

 vices. 



