The data stored in location registers are automatically updated in normal operation; the main information stored in a location register defines the location of each mobile station and the subscriber data required to handle traffic for each mobile subscriber. The loss or corruption of these data will seriously degrade the service offered to mobile subscribers; it is therefore necessary to define procedures to limit the effects of failure of a location register@ and to restore the location register data automatically. This Technical Specification defines the necessary procedures. The basic principle is that restoration should be based on radio contact to avoid faulty data being spread in the system. Subscriber data for supplementary services must also be correctly restored@ although the impact on service of corruption of supplementary service data is less severe. Procedures for supporting these functions are defined in Technical Specification GSM 09.02. The MAP operation 'IMSI Attach' is used only in MAP version 1; in MAP version 2 the same function is performed by the MAP operation 'Update Location Area'. References in this specification to IMSI attach apply only to MAP version 1 network entities. If the restoration of subscriber data in the VLR is triggered by Location Updating or IMSI Attach@ the VLR retrieves subscriber data from the HLR by sending an 'Update Location' request@ which triggers one or more 'Insert Subscriber Data' operations from the HLR. The 'Update Location' request may also be used to send the LMSI to the HLR. If the restoration of subscriber data in the VLR is triggered by a 'Provide Roaming Number' request@ the behaviour of the VLR depends on whether it is implemented according to MAP version 1 or MAP version 2. For MAP version 2@ the VLR retrieves subscriber data from the HLR by sending a 'Restore Data' request@ which triggers one or more 'Insert Subscriber Data' operations from the HLR. The 'Restore Data' request is also used to send the LMSI to the HLR. For MAP version 1@ the VLR retrieves subscriber data from the HLR by sending a 'Send Parameters' request with parameter type 'Subscriber Data'@ which cannot be used to send the LMSI to the HLR. The VLR number and MSC number in the subscriber data in the HLR are updated by the 'Update Location' procedure.