IP Multimedia Subsystem (IMS) allows the use of unlicensed, non-dedicated and nondeterministic access networks for delivering IP multimedia services. Providing end-to-end Quality-of-Service (QoS) for resource demanding real-time services (e.g. real-time multimedia) over such networks is a challenging task due to varying resource availability of the network and the end-devicei during a single sessionii. A common solution to this problem is adapting service quality and type according to availability of system resources, which requires end-to-end monitoring and signaling of resource availability during a single session. This paper presents an extension of the IMS architecture for real-time resource availability monitoring and signaling. The novelty of the proposed extension is twofold. It takes into account uncontrolled access networks with no resource reservation on the one hand, and end devices on the other. Two system architecture components are introduced for monitoring and signaling of the real-time resource availability in both networks (e.g. bandwidth, buffer space), and end-devices (e.g. battery, CPU, memory, storage), namely a Resource Manager (RM) and a Resource Availability Server (RAS). Our solution does not require any modifications of the IMS architecture and can be implemented as a plug-in
