Rural automatization in the Swedish telephone system

WHEN AN automatization of the Swedish telephone system came under serious consideration at the beginning of the 1920´s, there were available two quite distinct types of selectors. One of these selectors was the L. M. Ericsson machine-driven 500-line selector and the other was the Swedish crossbar selector. Telephone plants under consideration at that time for automatization chiefly comprised large telephone plants in densely built-up areas. It was a difficult matter in those days to make a choice between the two types of selectors. One arrangement which would enable the crossbar selector to compete economically with the 500-line selector for large telephone plants was the employment of marker control. Thus marker systems were proposed as early as the beginning of the 1920´s by advocates of the crossbar selector. The Board of Telecommunications, however, decided on several grounds not to adopt such systems at that time. One reason was the possible service risks of common control systems which were not sufficiently eliminated until later. Another reason was that in the 1920´s it was not possible to determine the magnitude of the decrease in first cost, plausible in itself, that might be obtained with marker systems. Determination of that point required a mathematical treatment that was not developed until later. The choice was therefore given to the L. M. Ericsson machine-driven 500-line selector for the automatization of the large exchanges in question.