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BELL SYSTEM TECHNICAL JOURNAL 



to the routings involved in order to trunk economically either large 

 or small volumes of trafific. The panel system was developed to 

 meet these requirements and is now installed in a number of such 

 metropolitan centers, notably, New York, Chicago, Philadelphia, 

 Boston, Detroit, Cleveland, St. Louis, Pittsburgh, Baltimore, San 

 Francisco, Buffalo, Kansas City, Seattle, Providence and Omaha. 

 Panel equipment is in service in 128 offices to which are connected 

 1.6 million stations. 



Fig. 20 — ■Installation of panel dial equipment. The unused floor space is provided 



for future growth. 



In the panel system the fundamental switching unit is a large switch 

 consisting of five banks of 100 terminals each. The selectors, by 

 which contact is made with any one of the 500 terminals, move ver- 

 tically on both sides of the terminal banks. A typical panel frame 

 having capacity for 60 selectors is illustrated in Fig. 18. 



In the panel system the selectors do not follow in synchronism with 

 the impulses of the dial as in the step-by-step system. Rather, a 

 group of apparatus known as the " sender " records the impulses and 

 in turn directs the operation of the several selectors in the train until 

 the called terminal is reached. By this means the trunking arrange- 

 ments and the numbering scheme can be designed independently of 

 each other. This, combined with the large capacity of the panel 



