TISPAN NGN bases the provision of real-time communication services on the 3GPP specified IMS. This is a flexible tool that can provide a wide range of service interactions between terminals and networks. Multimedia Telephony with PSTN/ISDN Simulation Services provides a specific use of the generic capabilities of the IMS. The goal of the present document is to provide requirements to investigate the capabilities of the IMS. A key role of the Multimedia Telephony with PSTN/ISDN Simulation Services is to provide PSTN/ISDN service simulation and thereby provide opportunities for Network Operators to smooth the transition from a PSTN/ISDN to the new NGN services available using the IMS. Therefore, the specific services and interactions described in the present document are to a large extent based on supplementary services already deployed within PSTN/ISDN networks. The IMS should be able to provide all of these services in a similar manner, although the identical interactions between the user and terminal and the terminal and the network is not required. The present document can therefore be used as the requirements for a "gap analysis" of the features and capabilities provided by the IMS that are required to deploy similar services to PSTN/ISDN supplementary services. The IMS is an inherently multimedia service control platform. Therefore Multimedia Telephony with PSTN/ISDN Simulation Services, whilst drawing on the existing service features in the PSTN/ISDN, is not limited to voice. The capability, for example, to divert requested communications under certain circumstances is seen as a useful tool for users, regardless of the media used (voice, video, etc.). The Services and Capabilities provided by a TISPAN NGN are described in TS 181 005 [1]. The requirements for services, described in the present document, also take account of the interactions required between interconnected networks - both between NGN and the interconnection of NGN with legacy networks. The aim of the present document is to assist network operators and service providers to deploy NGN multimedia services step by step to their users.

The present document defines the requirements for multimedia telephony services which may be supported by a TISPAN NGN. These requirements form the basis for the definition of network capabilities. An important aspect of the Multimedia Telephony with PSTN/ISDN Simulation service is to provide PSTN/ISDN like services. Nevertheless the requirements are independent of the media that is used during the communication. The present document covers part of the transition from PSTN/ISDN to an NGN. Interconnection between the existing networks and a TISPAN NGN is also covered. The present document only provides requirements for services using IMS. Services provided by a TISPAN NGN to support legacy terminals and interfaces (PSTN/ISDN emulation) are defined in existing PSTN/ISDN documents. Requirements for PSTN/ISDN Emulation are out the scope of the present document and are described in other documents. New NGN services are out the scope of the present document and are described in other documents. The applicability of PSTN/ISDN simulation services to services other than Multimedia Telephony (e.g. instant messaging) is not defined. Some PSTN/ISDN simulation services (for example communication waiting) are clearly not applicable to some real-time communication services. The requirements in the present document are described from the user point of view to assist in the transition of phone services to IP multimedia services with a similar behaviour. The requirements do not take into account capabilities of existing protocols. The evolution or modifications to these protocols are beyond the scope of the present document.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

From the human user point of view, Multimedia Telephony with PSTN/ISDN simulation services, provides a suite of service that are, in most aspects, the same as those already existing in the PSTN/ISDN. The services described are not limited to voice media as in PSTN/ISDN, but generally applicable regardless of the media used (voice, video, etc.). In addition to the capabilities to establish a bi-directional point-to-point communication between two parties, a selected number of PSTN/ISDN Simulation services, corresponding to perceived popular PSTN/ISDN supplementary services are included in the present document. PSTN/ISDN simulation services are described from a user's perspective and are similar or almost identical to the corresponding supplementary service in PSTN/ISDN. However, from a functionality distribution perspective, the present document places no particular requirements on the functionality distribution between network and terminals. Further, it is perceived that the need to preserve the split of functionality provided for supplementary services in an ISDN or PSTN is not a requirement in an NGN.