Application of the Theory of Probability to Telephone Trunking Problems

I F telephone plants were provided in such quantities that when a subscriber makes a call there would be immediately available such switching arrangements and such trunks or paths to the desired point as may be necessary to establish the connection instantly, it would require that paths and switching facilities be provided to meet the maximum demand occurring at any time, with the result that there would be a large amount of plant not in use most of the time.