1. Apparatus for producing an electronic model of a communications network, the apparatus comprising: a data collection system for obtaining data relating to probes inserted at a plurality of first network nodes and using the probes to obtain network data for those nodes, a traffic analysis arrangement for analysing, from an external location, traffic to and from one or more second nodes in which probes are not inserted; using the traffic analysis to calculate at least one predefined parameter; and using the at least one parameter to define a probability distribution representing a behaviour profile between the second nodes and the external location; and modelling apparatus for creating an electronic model of said network by combining said data collected from the first nodes and the behaviour profile.