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Content for  TR 26.922  Word version:  17.0.0

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1  Scopep. 5

The present document reports the study on video telephony robustness improvements extensions in Multimedia Telephony Service for IMS (MTSI) and provides recommendation on their applicability for MTSI video telephony applications.

2  Referencesp. 5

The following documents contain provisions, which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
TR 21.905: "Vocabulary for 3GPP Specifications".
TS 22.105: "Services and service capabilities".
TS 26.114: "IP Multimedia Subsystem (IMS); Multimedia telephony; Media handling and interaction".
RFC 4588:  "RTP Retransmission Payload Format", July 2006.
RFC 6865:  "Simple Reed-Solomon Forward Error Correction (FEC) Scheme for FECFRAME", February 2013.
RFC 5109:  "RTP Payload Format for Generic Forward Error Correction", December 2007.
RFC 4585:  "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", July 2006.
K. Yamagishi, T. Hayashi, "Parametric Packet-Layer Model for Monitoring Video Quality of IPTV Services": IEEE ICC 2008, pp. 110-114, May 2008.
Q. Huynh-Thu, M. Ghanbari, "Impact of Jitter and Jerkiness on Perceived Video Quality": Proc. of the Second International Workshop on Video Processing and Quality Metrics for Consumer Electronics (VPQM), 2006.
C. Wang, X. Jiang, Y. Wang, "Video Quality Assessment Models for IPTV Services": JDCTA, April 2013.
Pierre Ferre, Dimitris Agrafiotis, Tuan Kiang Chiew, Angela Doufexi, Andrew Nix, David Bull, "Packet Loss Modelling for H.264 Video Transmission over IEEE 802.11g Wireless LANs": IEEE WIAMIS 2005.
S. Holmer, M. Shemer, M. Paniconi, "Handling Packet Loss in WebRTC": pp. 1860-1864, ICIP, 2013.

3  Definitions and abbreviationsp. 6

3.1  Definitionsp. 6

For the purposes of the present document, the terms and definitions given in TR 21.905 apply.

3.2  Abbreviationsp. 6

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply.
Audio Video
Advanced Video Coding
Audio-Video Profile with Feedback
Error Resiliency
Frames Per Second
High Efficiency Video Coding
IP Multimedia Subsystem Video Telephony
Kilo Byte
Multimedia Telephony Service for IMS
Over The Top
Picture Loss Indication
Packet Loss Rate
Quarter Video Graphics Array
Reference Picture Selection
Reference Picture Selection Indication
Round Trip Time
Video Graphics Array
Video Telephony
Video Robustness Improvements Extensions
Wireless Fidelity

4  Backgroundp. 6

The present document reports the study on video telephony robustness improvements extensions in Multimedia Telephony Service for IMS and provides recommendation on their applicability for MTSI video telephony applications. These extensions target error robustness for higher bitrate MTSI video telephony as well as inter-working with WLAN use cases where error resiliency is more important. In order to be technically competitive, e.g. to some proprietary systems, MTSI should have the capability to employ mechanisms that can offer different trade-offs between rendering delay, video rendering jitter (smoothness) and video quality that can adapt to varying channel conditions for better user experience. Retransmission, Forward Error Correction (FEC), and complementary reference picture selection indication (RPSI) AVPF feedback mechanisms offer these trade-offs. The present document first provides an overview of the additional error resiliency (ER) tools that could improve the performance of the Multimedia Telephony Service for IMS (TS 26.114). Then test conditions representative of error conditions experienced in IMS Video Telephony are presented. Following the description of the test conditions, evaluation criteria for determining the benefits of proposed tools and mechanisms is presented. Performance of the proposed ER tools is evaluated under the defined testing conditions that take into account packet loss rate/pattern, end to end delay, bitrate overhead and video smoothness (dropped frames, rendering jitter). Based on the performance results, conclusions are made in terms of recommendations for support of proposed ER tools and mechanisms for Multimedia Telephony Service for IMS.

5  Overview of video robustness improvements extensions (VTRI_EXT) toolsp. 7

5.1  Introductionp. 7

Multimedia Telephony Service for IMS (MTSI TS 26.114) defines MTSI clients' sender and receiver behaviour utilizing RFC 4585 AVPF Generic NACK and Picture Loss Indication (PLI) feedback messages for ER. Current error correction scheme provides basic error correction through codec level error resiliency (ER) mechanisms. Transport and application level error resiliency schemes such as Retransmission (NACK), Forward Error Correction (FEC) along with advanced codec level ER schemes such as Reference Picture Selection (RPS) provide alternative error correction mechanisms that offer different performance trade-offs. The performance of error correction schemes varies with end-to-end delay, channel bandwidth and packet loss rate.

5.2  Retransmissionp. 7

Retransmission (NACK) scheme RFC 4588 provides efficient error correction in terms of bandwidth under short round-trip-time (RTT) cases with low packet loss rates. The efficiency of retransmission scheme becomes more pronounced at higher bitrates since selective retransmission of lost packets instead of entire pictures are needed. Under low RTT scenarios it can provide low video rendering jitter dependent on the de-jittering mechanism at the cost of additional delay. If additional delay cannot be accommodated, then retransmission can still provide recovery from error with video freezes during recovery similar to the existing error resiliency scheme in TS 26.114.

5.3  Forward error correctionp. 7

Forward Error Correction (FEC) schemes RFC 6865 and RFC 5109 provide a mechanism that balances video quality and end-to-end delay. FEC schemes can adapt to varying channel error conditions. FEC is suitable for high RTT channels with high packet loss rates where retransmission leads to high video rendering delay and codec based recovery mechanisms like RPSI, PLI lead to frequent video freezes and/or corruptions. FEC schemes are complemented by retransmission (NACK) or RPSI, PLI feedback mechanisms to address FEC failure cases.

5.4  Reference picture selectionp. 7

Reference picture selection indication (RPSI) feedback message in AVPF RFC 4585 that is currently not supported in TS 26.114 offers establishment of common reference point for recovery between the sender and the receiver. In essence it provides codec level ER mechanism similar to the transport layer ER mechanism supported by the generic NACK message in TS 26.114.

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