26.2353GPPDefault codecsPacket switched conversational multimedia applicationsTS
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 3GPP 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.