MTSI clients in terminals offering video communication shall support:

• H.264 (AVC) [24] Constrained Baseline Profile (CBP) Level 1.2;
• H.265 (HEVC) [119] Main Profile, Main Tier, Level 3.1. The only exception to this requirement is for the MTSI client in constrained terminal offering video communication, in which case the MTSI client in constrained terminal should support H.265 (HEVC) Main Profile, Main Tier, Level 3.1.

In addition they should support:

• H.264 (AVC) [24] Constrained High Profile (CHP) Level 4.0;
• H.265 (HEVC) [119] Main Profile, Main Tier, Level 4.0.

For backwards compatibility to previous releases, if H.264 (AVC) [24] Constrained High Profile Level 3.1 is supported, then H.264 (AVC) [24] Constrained Baseline Profile (CBP) Level 3.1 should also be offered. H.264 (AVC) shall be used without requirements on output timing conformance (Annex C of [24]). 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. H.265 (HEVC) Main Profile shall be used with general_­progressive_­source_­flag equal to 1, general_­interlaced_­source_­flag equal to 0, general_­non_­packed_­constraint_­flag equal to 1, general_­frame_­only_­constraint_­flag equal to 1, and sps_­max_­num_­reorder_­pics[ i ] equal to 0 for all i in the range of 0 to sps_­max_­sub_­layers_­minus1, inclusive, without requirements on output timing conformance (Annex C of [119]). For both H.264 (AVC) and H.265 (HEVC), the decoder needs to know the Sequence Parameter Set (SPS) and the Picture Parameter Set (PPS) to be able to decode the received video packets. A compliant H.265 (HEVC) bitstream must include a Video Parameter Set (VPS), although the VPS may be ignored by the decoder in the context of the present specification. When H.264 (AVC) or H.265 (HEVC) is used it is recommended to transmit the parameter sets within the SDP description of a stream, using the relevant MIME/SDP parameters as defined in RFC 6184 for H.264 (AVC) and in RFC 7798 for H.265 (HEVC), respectively. Each media source (SSRC) shall transmit the currently used parameter sets at least once in the beginning of the RTP stream before being referenced by the encoded video data to ensure that the parameter sets are available when needed by the receiver. If the video encoding is changed during an ongoing session such that the previously used parameter set(s) are no longer sufficient then the new parameter sets shall be transmitted at least once in the RTP stream prior to being referenced by the encoded video data to ensure that the parameter sets are available when needed by the receiver. When a specific version of a parameter set is sent in the RTP stream for the first time, it should be repeated at least 3 times in separate RTP packets with a single copy per RTP packet and with an interval not exceeding 0.5 seconds to reduce the impact of packet loss. A single copy of the currently active parameter sets shall also be part of the data sent in the RTP stream as a response to FIR. Moreover, it is recommended to avoid using a sequence or picture parameter set identifier value during the same session to signal two or more parameter sets of the same type having different values, such that if a parameter set identifier for a certain type is used more than once in either SDP description or RTP stream, or both, the identifier always indicates the same set of parameter values of that type. The video decoder in a multimedia MTSI client in terminal shall either start decoding immediately when it receives data, even if the stream does not start with an IDR/IRAP access unit (IDR access unit for H.264, IRAP access unit for H.265) or alternatively no later than it receives the next IDR/IRAP 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/IRAP access unit shall be the same as for a valid video bit stream. However, the MTSI client in terminal shall be aware that such a stream may contain references to pictures not available in the decoded picture buffer. The display behaviour of the MTSI client in terminal is out of scope of the present document. An MTSI client in terminal offering H.264 (AVC) CBP support at a level higher than Level 1.2 shall support negotiation to use a lower Level as described in RFC 6184 and RFC 3264. An MTSI client in terminal offering H.264 (AVC) CHP support at a level higher than Level 3.1 shall support negotiation to use a lower Level as described in RFC 6184 and RFC 3264. An MTSI client in terminal offering video support shall include in the SDP offer H.264 CBP at Level 1.2 or higher. An MTSI client in terminal offering video support for H.265 (HEVC) [119] Main Profile, Main Tier, Level 3.1, should normally set it to be preferred. An MTSI client in terminal offering H.265 (HEVC) shall support negotiation to use a lower Level than the one in the offer, as described in RFC 7798 and RFC 3264. If a codec is supported at a certain level, then all (hierarchically) lower levels shall be supported as well.

MTSI clients in terminals offering real time text conversation shall support:

• ITU-T Recommendation T.140 [26] and [27].

T.140 specifies coding and presentation features of real-time text usage. Text characters are coded according to the UTF-8 transform of ISO 10646-1 (Unicode). A minimal subset of the Unicode character set, corresponding to the Latin-1 part shall be supported, while the languages in the regions where the MTSI client in terminal is intended to be used should be supported. Presentation control functions from ISO 6429 are allowed in the T.140 media stream. A mechanism for extending control functions is included in ITU-T Recommendation T.140 [26] and [27]. Any received non-implemented control code must not influence presentation. A MTSI client in terminal shall store the conversation in a presentation buffer during a call for possible scrolling, saving, display re-arranging, erasure, etc. At least 800 characters shall be kept in the presentation buffer during a call. Note that erasure (backspace) of characters is included in the T.140 editing control functions. It shall be possible to erase all characters in the presentation buffer. The display of the characters in the buffer shall also be impacted by the erasure.

MTSI clients supporting still images shall support HEVC encoded encoded images conforming to the HEVC bitstream requirements of clause 5.2.2. Still images encoded using the HEVC shall have general_­progressive_­source_­flag equal to 1, general_­interlaced_­source_­flag equal to 0, general_­non_­packed_­constraint_­flag equal to 1, general_­frame_­only_­constraint_­flag equal to 1. For HEVC encoded images/image sequence, the display properties are carried as SEI and VUI within the bitstream, and the RTP timestamps determine the presentation time of the images.