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In WLANs, secure access can be provided operating at link layer, at network layer (IP) or at application level, and several solutions have been implemented in current public/private access networks; however most of them are proprietary and the interworking between different operators is still an open issue. At the same time, the 3GPP is working in integrating the WLAN access with the rest of 3GPP/UMTS network infrastructure. In this paper we propose a generic layer-two-independent open solution for secure authentication in a heterogeneous wireless access scenario based on 3G SIM credentials and on the SIP protocol. The solution follows the 3G specification for the authentication in the IMS, with the only addition of a new parameter in a specific header of the SIP protocol. Therefore the security infrastructure of the 3G networks can be fully reused also in scenarios in which the visited network does not provide 3G services. The proposed solution has been implemented in a testbed in which UMTS, 802.11, and Bluetooth has been used as access technologies.