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. Videotelephony provides a specific use of the generic capabilities of the IMS. The present document provides requirements to investigate the capabilities of the IMS to provide Videotelephony, and may be used for a "gap analysis" of the features and capabilities provided by the IMS. The specific services and interactions described in the present document are based on other service specifications. The IMS is an inherently multimedia service control platform, and should be able to provide all of these services. The interactions between the user and the terminal and the terminal and the network are not provided. The services and capabilities provided by a TISPAN NGN are described in TS 181 005 [2]. 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.

The present document defines the requirements for a videotelephony service which may be supported by a TISPAN NGN. These requirements form the basis for the definition of network capabilities. The present document provides interoperability service requirements for interconnection between existing networks and a TISPAN NGN, and between TISPAN NGN. The present document only provides requirements for IP multimedia based networks. 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 of scope of the present document and are described in other documents. The applicability of PSTN/ISDN simulation services to the videotelephony service requirements are defined in the present document. The requirements in the present document are described from the user point of view. The requirements do not take into account capabilities of existing protocols defined for the IMS. The evolution or modifications to these protocols are beyond the scope of the present document.