The present invention relates to the monitoring of paths along which signals are carried in a telecommunications network, in particular to determine if a path has been altered. Each path has a transit time associated therewith for data transport along that path. The method comprises the steps of: monitoring the difference between the transit time of a first signal path and the transit time of a second signal path, such that a change in the difference between the transit times of the two paths can be detected; and, in dependence at least in part on any such detected change, generating an alarm signal. The alarm signal can be used to indicate to a customer that one of the paths has been re-routed.