A preferred data provider is selected from a plurality of data providers by receiving a request for data from a client together with client identification data, identifying a plurality of data providers capable of providing data to the client, providing the client identification data to the data providers and instructing the data providers to perform tests in order to establish a measure of the elapsed time for a signal to be sent to and received from the client, and a measure indicative of their remaining capacity for data transfer, and to make these measures available to the system. One or more preferred data providers may then be selected on the basis of the elapsed time signals and the remaining capacity signals from the data providers.