Error Control Techniques for Interactive Lowbit Rate Video Transmission over the Internet.

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Error Control Techniques for Interactive Low-bit
Rate Video Transmission over the Internet.

• Injong Rhee
• Department of Computer Science
• North Carolina State University

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Video Conferencing over Packet-Switching Networks.

• Compressed video frame is packetized and
  transmitted.
• Packet loss can occur quite frequently due to
  congestion
• and user mobility.

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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)
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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 efficiency reduces.

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Myth retransmission is not useful for
interactive video
TIME
F0 F1 F2 F3 F1 (retransmit)
retransmit
lost
(F1 arrives after its display)
NACK
F0 F1 F2 F3
Conventionally, late packets are discarded.
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Retransmission stops error propagation.
TIME
F0 F1 F2 F3 F1 (retransmit)
retransmit
lost
F1 is recovered here
NACK
F0 F1 F2 F3
Dont discard Use late packets to recover their
reference frames
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Handling retransmitted late packets in decoder
Inverse Quantization/DCT
Reference Frame
Current Frame Info
Retransmitted Information.
Current Diff.erence Frame
Old Reference Frame

Display
Motion Prediction Compensation
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Error Propagation
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Recovery from Error Propagation
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Error Recovery Technique

• Recovery from Error Spread using Continuous
  Update (RESCU)
• Focus on error spread/propagation
• Continuous Update
• Retransmission (this paper)
• Forward error correction
• Combined with Layer coding (this paper)

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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.

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Add more reference frame buffers.Cascaded
Buffering

• Minus More computation and buffers at the
  decoder.
• Plus No change in the codec and picture
  pattern.

Base Reference Frame
Reference Frame 1
Reference Frame 2
Contains Full Image
Prediction Errors MVs
Prediction Errors MVs
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Adjust Temporal Dependency Distance Periodic TDDs
(PTDD)

• Changing Dependency.
• f
• PTDD can be dynamically adjusted.
• Reduced Buffers computations.
• No protection for non-periodic frame.

Intra Inter Inter
Periodic
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Continuous UpdatesRetransmission Forward Error
Correction

• Retransmission
• Retransmit lost packets within PTDD.
• Retransmission delay requires larger PTDD.
• Forward Error Correction
• send parity packets over a PTDD period.

Periodic frame
Periodic frame
Parity Packet for periodic frame
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Protecting non-periodic frames RESCU Quality
Assurance Layering

• .
• HP stream is sent with forward error correction.
• RESCU contributes to reducing bit rates.

HP
HP
HP
HP
LP
LP
LP
LP
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protecting non-periodic frames Change temporal
dependency

• Errors in non-periodic frames do not propagate.
• As PTDD increases,
• Compression efficiency decreases.

Periodic Inter Inter
Periodic
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Compression Efficiency(MPEG-4 Class A, AKIYO)
RESCU
RESCU QAL
H.261
Intra-H261
AVG PSNR (dB)
AVG Bytes/Frame
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Error Resilience (H.261 v.s. RESCU)
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RESCU QAL
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Error Resiliencevarious loss groups
RESCU QAL
Intra-H.261
PSNR
RESCU
H.261
Loss Rate
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Conclusions

• RESCU accommodates delays in recovery
  (retransmission or FEC).
• error resilience for interactive video.
• high compression efficiency.
• need I-frames less frequently.
• Good for bandwidth-constrained, lossy networks
  (e.g., mobile wireless networks).
• Much more work to be done.

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Error ResilienceRetransmission over various
network delays
50 ms 100 ms 150 ms 200 ms 250 ms 300 ms
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