The automatic line concentrator and subscriber telephone applications
H. G. Evers, J. R. Wylie · Transactions of the American Institute of Electrical Engineers Part I Communication and Electronics · 1957
WHERE a large number of telephone subscriber lines are to be served, it is possible to show statistically that only a small percentage of them will require service at a given interval. Telephone operating companies have used this fact to economic advantage in providing central office switching equipment, by concentrating the traffic from large groups of lines into a much smaller number of primary switching trunks. Generally speaking, 100 lines may normally be served with only from 10 to 15 primary trunks without degrading service beyond the point where one call in one hundred will encounter an appreciable delay, so that the economy of this approach is obvious.