Multimedia Telephony Service for IMS (MTSI), here also referred to as Multimedia Telephony, is a standardized IMS telephony service that builds on the IMS capabilities to establish multimedia communications between terminals within and in-between operator networks. The terminals connect to the IMS using either a fixed access network or a 3GPP access network. The objective of defining a service is to specify the minimum set of capabilities required in the IP Multimedia Subsystem to secure multi-vendor and multi-operator inter-operability for Multimedia Telephony and related Supplementary Services. The objective also includes defining procedures for inter-working between different clients and networks. The user experience of multimedia telephony is expected to be equivalent to or better than corresponding circuit-switched telephony services. Multimedia telephony also exploits the richer capabilities of IMS. In particular, multiple media components can be used and dynamically added or dropped during a session. The present document specifies a client for the Multimedia Telephony Service for IMS (MTSI) supporting conversational speech (including DTMF), video and text transported over RTP with the scope to deliver a user experience equivalent to or better than that of Circuit Switched (CS) conversational services using the same amount of network resources. It defines media handling (e.g. signalling, transport, jitter buffer management, packet-loss handling, adaptation), as well as interactivity (e.g. adding or dropping media during a call). The focus is to ensure a reliable and interoperable service with a predictable media quality, while allowing for flexibility in the service offerings. The present document describes two client types:

• An MTSI client in terminal which uses a 3GPP access (LTE, HSPA, or EGPRS) to connect to the IMS. These clients are described in Clauses 5 - 17 and Annexes A - M.
• An MTSI client in terminal which uses a fixed access (corded interface, fixed-wireless interface, e.g. Wi-Fi, Bluetooth or DECT/NG DECT) to connect to the IMS. These clients are described in Clause 18.

MTSI clients using 3GPP access and MTSI clients using fixed access have many common procedures for the media handling. This specification aligns the media handling by using cross references whenever possible. This does not mean that 3GPP terminals must support fixed access, nor does it mean that fixed terminals must support 3GPP access. The scope includes maintaining backward compatibility in order to ensure seamless inter-working with existing services available in the CS domain, such as CS speech and video telephony, as well as with terminals of earlier 3GPP releases. In addition, inter-working with other IMS and non-IMS IP networks as well as traditional PSTN is covered. The client may also support the IMS Messaging service and Group 3 facsimile transmission. The scope therefore also includes media handling for non-conversational media using MSRP and UDPTL-based Facsimile over IP (FoIP). The specification is written in a forward-compatible way in order to allow additions of media components and functionality in releases after Release 7.