A.K. Erlang (1878-1929) studied the problems of telephone traffic at the beginning of the 20th century when he worked for the Copenhagen Telephone Company. By the use of the theory of probabilities he formulated the maths to calculate the loss and waiting time of estimated call traffic. To pay tribute to his work the unit "Erlang" denotes the telephone traffic:     Erlang = average holding time of a line (e.g. 5 min) * arrival rate (calls/min) In the Excel sheet provided on this page there is an example of a traffic profile for a determined number of lines. Please feel free to ask me for explanation: n_reimelt@yahoo.de Useful References: - Introduction to Erlang - Call routing in telephone networks - Efficient Algorithm for Erlang's Formula (Erlang B) - Queueing theory books on line