Abstract: The present invention relates to a method and an apparatus for performing a location update of a user equipment in a mobile network, wherein it is decided on the basis of the result of a negotiation with an old location register whether to use an identification information obtained from the old location register or to retrieve the identification information from a user database. Accordingly, whenever a fall-back to a protocol version not supporting the required identification information is determined, the identification information is retrieved from the user database, to thereby guarantee that the correct identification information is used in the new location register. The identification information may comprise security or authentication parameters.

Abstract: A method of arranging data protection in a telecommunication system including a first mobile communication network, a second mobile communication network, and a mobile station supporting both of the mobile communication networks is disclosed. The method includes ciphering traffic between the mobile station and the first mobile communication network using a first cipher key; calculating a second cipher key to be used for ciphering traffic between the mobile station and the second mobile communication network in the first mobile communication network when the mobile station operates in the first mobile communication network; transmitting information necessary for calculating the second cipher key from the first mobile communication network to the mobile station when the mobile station operates in the first mobile communication network; and calculating the second cipher key at the mobile station to be used for ciphering traffic between the mobile station and the second mobile communication network.