6 Functional requirements

26.2353GPPDefault codecsPacket switched conversational multimedia applicationsTS

SIP protocol itself does not mandate any codecs. Standardisation of codecs does not prevent the use of other codecs that can be signalled using the SDP protocol. 3G PS multimedia terminals shall be able to use the same audio and video codecs applied in 3G-324M [8]. This will ensure the interoperability with 3G circuit switched multimedia telephony.

6.1 Audio

3G PS multimedia terminals offering audio communication (including PoC services) shall support AMR narrowband speech codec [9], [10], [11] to [12].

The AMR wideband speech codec shall be supported when the 3G PS multimedia terminal supports wideband speech working at 16 kHz sampling frequency [16], [17], [39], [40].

The usage of telephone-event media format is recommended for DTMF.

Annex D provides guidelines for using audio in the context of PoC services.

6.2 Video

3G PS multimedia terminals offering video communication shall support ITU-T recommendation H.263 [6] [19] baseline (Profile 0) Level 45.

H.263 [6] [19] version 2 Interactive and Streaming Wireless Profile (Profile 3) Level 45 should be supported.

ISO/IEC 14496-2 [13] (MPEG-4 Visual) Simple Profile at Level 3 should be supported with the following constraints:

– Number of Visual Objects supported shall be limited to 1.

– The maximum frame rate shall be 30 frames per second.

– The maximum f_code shall be 2.

– The intra_dc_vlc_threshold shall be 0.

– The maximum horizontal luminance pixel resolution shall be 352 pels/line.

– The maximum vertical luminance pixel resolution shall be 288 pels/VOP.

– If AC prediction is used, the following restriction applies: QP value shall not be changed within a VOP (or within a video packet if video packets are used in a VOP). If AC prediction is not used, there are no restrictions to changing QP value.

H.264 (AVC) [41] Baseline Profile at Level 1.1 [42] should be supported with constraint_set1_flag=1 and without requirements on output timing conformance (Annex C of [41]). Each sequence parameter set of H.264 (AVC) shall contain the vui_parameters syntax structure including the num_reorder_frames syntax element set equal to 0.

The H.264 (AVC) decoder in a PSS client shall start decoding immediately when it receives data (even if the stream does not start with an IDR access unit) or alternatively no later than it receives the next IDR access unit or the next recovery point SEI message, whichever is earlier in decoding order. The decoding process for a stream not starting with an IDR access unit shall be the same as for a valid H.264 (AVC) bitstream. However, the client shall be aware that such a stream may contain references to pictures not available in the decoded picture buffer. The display behaviour of the client is out of scope of this specification.

NOTE 1: Terminals may use full-frame freeze and full-frame freeze release SEI messages of H.264 (AVC) to control the display process.

NOTE 2: An H.264 (AVC) encoder should code redundant slices only if it knows that the far-end decoder makes use of this feature (which is signaled with the redundant-pic-cap MIME/SDP parameter as specified in [43]). H.264 (AVC) encoders should also pay attention to the potential implications on end-to-end delay.

NOTE 3: If a codec is supported at a certain level, then all (hierarchically) lower levels shall be supported as well. Examples of lower levels include Level 10 for H.263 Profile 0 and 3, Level 0 for MPEG-4 Visual Simple Profile and Level 1 for H.264 (AVC) Baseline Profile. However, as for instance Level 20 is not hierarchically lower than Level 45 of H.263 Profile 0 and 3, support for Level 45 does not imply support for Level 20.

NOTE 4: All levels are minimum requirements. Higher levels may be supported and used for negotiation.

NOTE 5: If a codec is supported at a certain level, it implies that on the receiving side, the decoder is required to support the decoding of bitstreams up to the maximum capability of this level. On the sending side, the support of a particular level does not imply that the encoder may produce a bitstream up to the maximum capability of the level.

6.3 Real time text

3G PS multimedia terminals offering real time text conversation should support ITU-T Recommendation T.140 [25] Text Conversation presentation coding.

6.4 Interactive and background data

SIP signalling offers initialisation of packet switched interactive or background class reliable data services as well. However specification of such data services are outside the scope of the present document.

6.5 Speech Enabled Service

3G PS multimedia terminals offering speech enabled services should support the DSR Extended Advanced Front-end codec [37]

Speech enabled services may also be supported with AMR or AMR-WB audio codecs, however it is noted that there is a substantial performance advantage from DSR [45].