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Retransmission-Based Error Control for Interactive Video Applications over the Internet

Retransmission-Based Error Control for Interactive Video Applications over the Internet. Injong Rhee ACM Multimedia ’98 Presented by: Jay A. Patel (some slides adapted from authors’ original presentation). Packet Loss. Video Conferencing over Internet.

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Retransmission-Based Error Control for Interactive Video Applications over the Internet

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  1. Retransmission-Based Error Control for Interactive Video Applications over the Internet Injong Rhee ACM Multimedia ’98 Presented by: Jay A. Patel (some slides adapted from authors’ original presentation)

  2. Packet Loss Video Conferencing over Internet • Compressed video frame is packetized and transmitted • Packet loss can occur quite frequently due to congestion

  3. Motion Compensated Coding • Most video codecs use it. • Each P-frame depends on its previous frames. • I-frame is periodically transmitted. Temporal Dependency Inter Frame (P-frame) Intra Frame (I-frame)

  4. When a packet is lost…. Packet Loss Inter Frame (P-frame) Intra Frame (I-frame) Error Propagation • Conventional Solution :Transmit I-frame more often • However, compression efficiencyreduces.

  5. Retransmission in Interactive Video? TIME F0 F1 F2 F3 F1 (retransmit) retransmit lost (F1 arrives after its display) NACK F0 F1 F2 F3 • Retransmission costs = 1.5 RTT (or 3 OWT) Conventionally, late packets are discarded.

  6. Retransmission Stops Error Propagation TIME F0 F1 F2 F3 F1 (retransmit) retransmit lost F1 is recovered here NACK F0 F1 F2 F3 Don’t discard: Use late packets to recover their reference frames

  7. Handling Retransmitted ‘Late’ Packets Inverse Quantization/DCT Retransmitted Information Reference Frame Current Frame (Differences) + Display Motion Prediction Compensation

  8. Error Propagation

  9. Recovery from Error Propagation

  10. Extending Packet Deadline • Add more frame buffers at the decoder. • Adjust Temporal Dependency Distance (TDD) of a frame. • TDD: frame intervals from that frame to its temporally dependent frame.

  11. More Frame Buffers: Cascaded Buffering • Minus: More computation and buffers at the decoder. • Plus :No change in the codec and picture pattern. Reference Frame 2 Reference Frame 1 Base Reference Frame Prediction Errors + MVs ContainsFull Image Prediction Errors + MVs

  12. Extended Temporal Dependency Distance (ETDD) • Each frame depends on a frame in previous TDD • Retransmission errors delayed • Must maintain a base frame for each R-frame • Num of R-frames = value of TDD • Reduces compression • Prediction Error is computed from frames further away Each frame has TDD = 3

  13. Adjust Temporal Dependency DistancePeriodic TDDs (PTDD) • Changing Dependency • PTDD can be dynamically adjusted. • Reduced Buffers & computations. • No protection fornon-periodicframe. Intra Inter Inter Periodic

  14. Testing Methodology • H.261 encoder with PTDD = HP.261 • Compared with • H.261: base performance • INTRA-H261 • Good performance under packet loss errors • Intra-codes every block that changes significantly

  15. Video Stream • Trans-Atlantic Broadcast • 5 frames/sec, 352x288 resolution • 250 Kbits/sec • 40 sec, 190 frames, replayed for 5 minutes • Encoding method • Quantization step size 8 • 15x15 blocks w/ exhaustive search • TDD = 5 frames or 1 second

  16. Compression Efficiency

  17. Encoded Streams • Equalized Compression?

  18. Encoded Streams: Dissection

  19. Testing Methodology • Packetized using 512-byte packets • Only HP.261 is transmitted over the Internet • Trace collected from each experiment • Every hour for 8 days • ACK scheme • Receiver sends ACK for each frame received • Information on dropped packets • Retransmit restriction: 3 frames

  20. Testing Summary: Performance

  21. Conclusion • Retransmission can work for Interactive video • Requires minimal changes to the codecs • Advantage over INTRA-H.261 unclear

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