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The 3GPP has specified the IP multimedia subsystem (IMS) for the provisioning of multimedia services in UMTS Release 5 and later. Interconnection at the service layer between 3GPP and LAN networks requires interworking between IMS and WLAN functionalities. Studying several interconnection scenarios and the main functionalities of IMS, this article analyzes how the interconnection of 3GPP and WLAN networks may be performed in order to support different levels of service interconnection. Special attention is paid to interconnection at the session negotiation level, using SIP (the base protocol of the IMS) to provide session negotiation with QoS and AAAC support.