NGN release 1 makes provision for a category of services which are referred to as real-time conversational multimedia services, as shown in Supplement 1 to the ITU-T Y.2000-series Recommendations. These include PSTN/ISDN simulation services and other services. When these real-time conversational multimedia services are provided by using an IMS-based service environment, the implementations will use SIP protocol between the application/service support functions and the service control functions within the service stratum of the NGN architecture. IMS-based NGNs which meet these service requirements will be capable of supporting real-time conversational multimedia services with enhanced service features, as well as allowing for enhanced implementation scenarios. This Recommendation defines the service requirements, service features, service architecture and implementation scenarios of IMS-based real-time conversational multimedia services.