Abstract: Typically data migration between HLR nodes is implemented by suspending operation of the HLR function in a mobile communications network while the data is transferred. This operation inevitably results in considerable network disruption. The present invention provides a method of on-line active data migration in which a subscriber diversion is implemented before subscriber data is sequentially transferred from one HLR node to another. The diversion redirects particular subscriber transactions from the second to the first HLR nodes if the subscriber has not been migrated from the first HLR node. The method allows the transfer to be implemented while the network HLR function remains on-line.