Variations of telephone traffic

The use of Erlang's probability equation for determining the number of circuits required in exchange systems requires a knowledge of the average busy-hour traffic, and this must be determined experimentally, as the average over a large number of short busy hour periods. To ensure that traffic is stationary over these periods a comparison can be made of the observed with the theoretical standard deviation of the observations. Theoretical standard deviations are derived, for constant and varying holding times, certain reasonable approximations being made, and the application of these results is discussed.