For the economic planning of an alternate routing network the number of circuits in a high-usage group should be determined so that the annual charges for the whole network arrangement are at a minimum. This is done under the constraint that given requirements for the grade of service are fulfilled. In the optimum arrangement, the cost per erlang of carrying a marginal amount of traffic over the high-usage route or over the alternative route is the same.
ITU-T E.522 SPANISH-1993 Referenced Document
ITU-T E.506 Forecasting International Traffic - Telephone Network and ISDN - Quality of Service@ Network Management and Traffic Engineering (Study Group II) 18 pp