Section: 6.1.4 Location update
Content: In case a user identifies itself using a TMSIo/LAIo pair that was assigned by the visited VLRn the IMSI can normally be retrieved from the database. If this is not the case, the visited VLRn should request the user to identify itself by means of its permanent user identity. This mechanism is described in 6.2. In case a user identifies itself using a TMSIo/LAIo pair that was not assigned by the visited VLRn and the visited VLRn and the previously visited VLRo exchange authentication data, the visited VLRn should request the previously visited VLRo to send the permanent user identity. This mechanism is described in 6.3.4, it is integrated in the mechanism for distribution of authentication data between VLRs. If the previously visited VLRo cannot be contacted or cannot retrieve the user identity, the visited VLRn should request the user to identify itself by means of its permanent user identity. This mechanism is described in 6.2.