BELL SYSTEM TECHNICAL JOURNAL 



1921-1928 ACTUAL 



1929-1933 ESTIMATED 



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1921 '22 '23 '24 '25 '26 '27 '28 '29 '30 '31 '32 1933 



END OF YEAR 



2.7 3,6 5.6 8.7 12.5 15.5 18.7 22 27 33 38 42 47 

 PER CENT DIAL STATIONcs OF TOTAL 



Fig. 15. 

 Fig. 15 — Relation between manual, dial and total stations — -Bell owned stations. 



Step-hy-Step Dial System 



The step-by-step system is used in the Bell System in single office 

 areas and in the smaller and medium sized multi-office areas where 

 the number of central offices is limited and consequently the trunking 

 problems are not complicated. Step-by-step equipment is in service 

 in 194 offices to which are connected 1.6 million stations. 



The fundamental unit of the step-by-step dial system is the selector 

 illustrated in Fig. 16. This switch has a capacity for 100 terminals 

 placed ten on a level and ten levels high, thus making possible the 

 selection of any one of a hundred lines. By placing several selectors 

 in series a network of central offices may be built up, each office 

 serving 10,000 telephones. 



The selectors are mounted on iron frameworks and the terminals 

 are cabled to cross-connecting frames so that any grouping can be 

 made as may be demanded by the number of calls which the par- 

 ticular selector handles. A typical arrangement of selectors is shown 

 in Fig. 17. 



