Calculating Traffic flow in a communications network

A method of calculating traffic values in a communications network, the communications network comprising a plurality of nodes, the nodes being connected to one another by links, the method comprising: (a) obtaining traffic data measurements through said nodes and/or links in an initial scenario as input data; (b) deriving a traffic flow model for a modified scenario using a plurality of constraints describing the interdependency of said initial to said modified scenario; and (c) calculating values and/or upper and lower bounds of traffic values for said modified scenario from said traffic flow model using said input data. Apparatus and computer program for performing the method is also claimed.