The IMS (IP Multimedia Subsystem) architecture is the key control for next generation networks (NGN). IMS gives network operators the opportunity to extend their services, including voice and multimedia communications and deliver them in new environments with new goals. Its security is paramount, especially authentication. In IMS, authentication is divided into two phases a PS (Packet-Switch) domain-level with the 3GPP-AKA protocol, and a second at IMS level using the IMS-AKA protocol. In our first contribution, we propose a new IMS authentication mechanism that improves the IMS-AKA in terms of security and more efficient in the use of bandwidth and processing time. Based on the results obtained, we can confirm that our protocol can save at least 21.5% of SIP/Cx traffic compared to the IMS-AKA and resists to attack reaching the confidentiality and integrity of data in an IMS registration (validated by AVISPA). In our second contribution, we propose a new Service provisioning model: Virtual Walled-Garden. This new model allows the user accessing all the applications, even the external ones transparently, simulating a walled-garden environment. This model will create a trust link between IMS domain and external services, and will reduce the burden of both end users and SPs through a Single Sign-On (SSO) feature, using identity federation. We also introduce the notion of security level to classify the SPs in a Multi-level model.