Patent ID: 8447298

Claim:
A mobile station comprising: an AS (Access Stratum) function corresponding to an AS terminated between the mobile station and a radio base station; and an NAS (Non Access Stratum) function corresponding to an NAS terminated between the mobile station and a mobile switching center, wherein the AS function is configured to activate a first timer, when a quality deterioration of a radio link established with the radio base station is detected; the AS function is configured to detect a failure in the radio link and to activate a second timer, when the quality deterioration of the radio link is not recovered before the first timer expires; the AS function is configured to transition to an idle state, when an inter-RAT cell is discovered during a time that the second timer is activated, and to notify, to the NAS function, the failure; and the NAS function is configured to transmit, in response to the notification, a location registration request signal to a mobile switching center in a radio communication system to which the inter-RAT cell belongs.