Selective Retransmission of MPEG Video Streams over IP Networks

1
Selective Retransmission of MPEG Video Streams
over IP Networks

• Árpád Huszák, Sándor Imre
• Budapest University of Technology and Economics
  Department of Telecommunications
• Mobile Communications and Computing Laboratory
• Phoenix project

2
Motivation

• Multimedia streaming is becoming increasingly
  popular in IP networks
• New wireless technologies
• 3G, 4G mobile networks
• WiMAX
• WLAN
• Two major issues arise in spite of the popularity
  of multimedia applications in mobile networks
• limited bandwidth restricts high bit rate video
  transmission
• wireless transmission by its nature may introduce
  higher rate of errors during transmission

3
Introduction

• Multimedia applications in mobile networks
• Transmission over unreliable wireless channels
• The frequent errors should cause deterioration of
  the quality of multimedia streams
• Delay and packet loss caused by handovers
• New method to increase the quality of audio/video
  streams in mobile environment
• Differentiation of packets within a media stream
• Selective retransmission of important packets

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Selective retransmission

• Differentiation of packets within an MPEG media
  stream
• MPEG frames
• (I) Intra frame coded, key-frame (max. priority)
• (P) Predicted
• (B) Bidirectional
• Error in a key-frame (I) propagates to all other
  frames till the next key-frame
• It is advisable to handle these frames on a
  different way
• Retransmission of the damaged part of the
  key-frame will significantly raise the quality of
  the video stream

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Selective retransmission

• Transport protocol for selective retransmission
• Unreliable (no automatic retransmission)
• Packet numbers
• Acknowledgements
• Information on lost packets
• Information about the network (RTT)
• Congestion control (TCP-like, TFRC)
• DCCP
• RFC4340, March 2006
• think of DCCP as TCP minus bytestream semantics
  and reliability
• or as UDP plus congestion control, handshakes,
  and acknowledgements

DCCP Datagram Congestion Control Protocol
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Selective retransmission scheme

• Packet identifying
• Temporal storage of dedicated packets in buffers
• Delete only when acknowledgement arrives
• Each DCCP packet has an individual sequence
  number that makes possible to detect packet losses

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Examination

• The packet drop probability
• The retransmission probability is calculated as
  follows
• In case of interactive media the RTT must be
  analyzed to decide whether the total delay is
  acceptable. In our solution the delay is

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Examination
The retransmission probability in the function of
size(I)/size(GoP) ratio according to the actual
channel bit-error probability
Retransmission probability in function of
bit-error probability
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Examination

• Comparison of UDP, UDPLite and DCCP
  Retransmission Algorithm
• There are N frames in a GoP, therefore single
  I-frame bit correction leads to the improvement
  of N bits increasing the MPEG quality
  significantly

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Testbed

• DCCP Selective Retransmission testbed
• Linux 2.6.14
• Client-Server side applications
• Adjustable packet loss ratio

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Results
With Sel. Retr.
Original
Without Sel. Retr.

• Visible difference in MPEG video quality
• Our algorithm radically reduces the bit
  corruption ratio and achieves significant
  increase of quality

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Results

• MPEG - peak-to-peak signal-to-noise ratio (PSNR)
  dB
• 3-4 dB gain was achieved with our Selective
  Retransmission method

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Future plans

• The additional delay caused by our method should
  be examined
• Time and bandwidth limitations should be examined
• The algorithm can be extended to allow multiple
  retransmissions realizing TCP-like reliable
  transmission
• The evaluations were done on MPEG streams, but
  the selective retransmission algorithm is capable
  for other data type transmissions where the data
  can be differentiated

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Conclusion

• In this presentation a new selective
  retransmission algorithm was presented and
  studied
• The results obtained show that the algorithm
• radically reduces the bit corruption ratio
• achieves significant increase of quality
  especially in wireless networks with high
  bit-error ratio

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• Thank you for your attention!