The present document contains a specification for default multimedia codecs to be used within 3GPP specified IP Multimedia Subsystem (IM Subsystem). IM Subsystem as a subsystem includes specifically the conversational IP multimedia services, whose service architecture, call control and media capability control procedures have been defined in 3GPP specifications TS 24.229, and are based on the 3GPP adopted version of IETF Session Initiated Protocol (SIP). The term codec is usually associated with a single media type. In case of packet switched transport domain, which IM Subsystem will depend on, the individual media types are independently encoded and packetised to appropriate separate Real Time Protocol (RTP) packets. These packets are then transported end-to-end inside UDP datagrams over real-time IP connections that have been negotiated and opened between the terminals during the SIP call as specified in TS 24.229. From the codec definition viewpoint, the UEs operating within IM Subsystem need to provide encoding/decoding of the derived codecs, and perform corresponding packetisation/depacketisation functions. Logical bound between the media streams is handled in the SIP session layer, and inter-media synchronisation in the receiver is handled with the use of RTP time stamps. Finally, since 3GPP networks are inherently error prone, error detection and/or correction must also be provided by the individual codecs within IM Subsystem, since they have a comprehensive view of the bit stream they produce and therefore can apply the most efficient form of error detection and/or correction.

The present document introduces the set of default codecs for packet switched conversational multimedia applications within 3GPP IP Multimedia Subsystem. Visual and sound communication are specifically addressed. The intended applications are assumed to require low-delay, real-time functionality. The present document is applicable, but not limited, to services such as PS video telephony and Push to talk over Cellular (PoC) as well as Combined CS and IMS services (CSI). For Multimedia Telepphony Service for IMS (MTSI), all media handling, including codecs, is defined in TS 26.114. This specification does not apply for MTSI. The applicability of this specification to GERAN is FFS.