In telecommunication networks, traffic volume is a measure of the total work done by a resource or facility, normally over 24 hours, and is measured in units of erlang-hours. It is defined as the product of the average traffic intensity (in erlang) and the period of study (in hours). A traffic volume of one erlang-hour can be caused by two circuits being occupied continuously for half an hour or by a circuit being half occupied (0.5 erlang) for a period of two hours. Telecommunication operators are vitally interested in traffic volume, as it directly dictates their revenue.