A Rate/Quality Controlled MPEG Video Transmission System in a TCP-Friendly Internet Scenario

1
A Rate/Quality Controlled MPEG Video Transmission
System in a TCP-Friendly Internet Scenario

• Francesco Licandro, Giovanni Schembra
• Dipartimento di Ingegneria Infomatica e delle
  Telecomunicazioni
• University of Catania
• E-mail addresses (flicandro, schembra)_at_diit.unict
  .it

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Presentation outline

• Motivations and paper target
• Background
• TFRC TCP-friendly rate control
• TM-5 MPEG rate control algorithm
• The MPEG video transmission system
• Performance analysis
• Conclusions and future work

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Paper target

• Definition of a Video-over-IP adaptive-rate
  quality-controlled transmission system

• Scenario BEST EFFORT Internet

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Problems of Video-over-IP

• From the user point of view
• Available bandwidth discovery
• Bandwidth variation
• From the network point of view
• TCP friendliness
• Network utilization maximization

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The idea

• Use of
• the TCP-friendly TFRC algorithm
• to discover the available bandwidth
• to be friendly with TCP sources sharing the same
  bottleneck link
• the MPEG TM-5 rate control algorithm
• to change the transmission rate according to the
  network congestion situation

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A brief background

• TCP-friendly TFRC algorithm
• MPEG and TM-5

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TFRC (TCP-Friendly rate control)

• Target to control the rate of a real-time source
  in order to be friendly with TCP flows sharing
  the same bottleneck links
• Equation-based protocol it estimates the
  throughput according to the following equation

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TFRC declared objectives

• TFRC was defined to support real-time traffic,
• and to provide sources with a
• rate-control algorithm to achieve
• fairness towards TCP traffic

but
only the fairness aspects have been considered up
to now
no attention has been paid to real-time
sources using TFRC
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TFRC protocol scheme
TX
RX
PR
TFRC sender
TFRC receiver

• TFRC receiver
• PR calculates the loss rate
• PR sends feedback to the TFRC sender every time
  it receives a packet
• this allows RX to calculate the RTT and to know
  the loss rate

• TFRC sender
• divided into two parts TX and RX
• this division not present in the original TFRC
  definition
• the TFRC sender in mind of the TFRC authors was a
  GREEDY SOURCE, with always something to transmit

• RX entity
• applies the equation and proposes the maximum
  rate to the source
• TX entity
• receives from the source the packets to be
  transmitted, and the rate to apply in their
  transmission

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MPEG source and TM-5

• MPEG video source is an adaptive-rate source
• the emission rate can be tuned through the
  quantizer scale parameter (qsp), q
• the most widely used algorithm to change the
  emission rate by varying q is the TM-5
• The TM-5 rate control algorithm works in three
  steps
• Target bit allocation
• Rate control
• Adaptive quantization

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TM-5 Target bit allocation

• This step estimates the number of bits available
  to encode the frames of the beginning GoP

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TM-5 Rate control

• It is performed macroblock by macroblock
• It is based on the state of three virtual
  buffers, one for each kind of frame
• Each virtual buffer is a counter having memory of
  the past
• it is positive if credits have been accumulated
• it is negative if debts have been accumulated
• From the state of the virtual buffers, the
  reference value of q is set for each macroblock

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TM-5 Adaptive quantization

• This step modulates the reference value of the
  quantizer scale according to the spatial activity
  in the macroblock to be encoded

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The MPEG video transmission system

• Targets
• the output bit rate has to follow the bandwidth
  available in the network
• user requirements have to be met, in terms of
  encoding quality (mean PSNR and PSNR
  oscillations)

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Network Bandwidth Smoother

• It receives the network bandwidth estimated by
  the TFRC
• It has the aim of eliminating the high
  frequencies ofthis process
• To this end, it uses a low-pass filter with an
  Exponential Wighted Moving Average (EWMA)

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Rate/Quality MPEG Video source
Video source
Rate controller
MPEG encoder
Packetizer
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RQ-source Rate Controller

• Target to calculate the quantizer scale
  parameter for each macroblock, according to a
  given law
• Input data the budget to be allocated to each
  beginning GoP
• Three different Rate controller laws
• rate controller using the classical
  TM-5
• rate controller derived from
  , by imposing an hysteresis mechanism to
  guarantee better quality stability
• rate controller derived from
  , by imposing the memory-less property at the
  beginning of each GoP

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rate control mechanism
H3

• Rate controller applying an hysteresis mechanism
  to the classical TM-5
• Hysteresis mechanism
• divide the PSNR range in L levels, F1, F2, , FL
• the frame quality must remain at the same PSNR
  level for at least H frames (H hysteresis
  parameter)
• the frame quality can step by at most one PSNR
  level up or down if the PSNR associated to the
  quantizer scale suggested by RCTM5 does not
  belong to one of the allowed PSNR levels, the qsp
  providing the PSNR closest to the suggested one
  (at most one PSNR level up or down) will be chosen

The PROBLEM during the hysteresis period a large
number of credits or debts may be accumulated
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rate control mechanism

• Memory-less rate controller
• Defined as an extension of the
• Target to avoid the fact that, due to
  hysteresis, such a great number of credits or
  debts are accumulated that, when it is possible
  to change level (after H frames), the source will
  not follow the behavior required by the TFRC
• The memory is deleted at the beginning of each GoP

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Case study
360 km 2 ms
RQ source
Router
Router

• Network topology
• easy, but representing the worst case for the
  quality stability of the video source (the same
  source has to face up to congestion situation)
• Movie
• 90 minutes of The silence of Lambs, encoded
  with the GoP structure IBBPBB
• Link capacity
• 2 Mbits/s
• Simulative approach
• Network simulator ns-2
• Video-trace generator for ns-2 (www.diit.unict.it/
  arti/video.htm)

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Encoding quality levels
PSNR level F1 39.2, 49.2 dB
PSNR level F2 36.2, 39.2 dB
PSNR level F3 35.0, 36.2 dB
PSNR level F4 33.7, 35.0 dB
PSNR level F5 31.5, 33.7 dB
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Rate processes
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Loss process
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PSNR process
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Quality-level process
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Conclusions

• The paper defines an MPEG video transmission
  system for the best-effort Internet
• TFRC rate control algorithm is used to discover
  the available bandwidth
• Three TM5-like source rate controllers have been
  defined to follow the bandwidth calculated by TFRC

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Peculiarities of our approach
Encoding video for transmission on the Internet
must follow a right tradeoff between
TCP-friendliness and encoding quality
Variation of encoding parameters does not provide
an infinitesimal fine tuning of the rate
IN ADDITION
TCP dynamics are deleterious for real-time video
sources
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Future works

• TCP-friendliness evaluation
• Introduction of channel encoding techniques (e.g.
  Forward Error Correction - FEC) to protect video
  from losses caused by the TFRC behavior (TFRC
  increases bandwidth until losses do not occur)

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Support material

• Material relating to this topic, and an extended
  version of this paper can be found at

www.diit.unict.it/arti