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

Francesco Licandro, Giovanni SchembraDipartimento di Ingegneria Infomatica e delle Telecomunicazioni

University of Catania

E-mail addresses: (flicandro, schembra)@diit.unict.it

Presentation outline

Motivations and paper targetBackground TFRC: TCP-friendly rate control TM-5: MPEG rate control algorithm

The MPEG video transmission systemPerformance analysisConclusions and future work

Paper target

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

to better exploit network bandwidth

Scenario: BEST EFFORT Internet

No bandwidth guarantees

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

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

A brief background

TCP-friendly TFRC algorithm

MPEG and TM-5

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 linksEquation-based protocol: it estimates the throughput according to the following equation:

23218

33

32

ppp

tp

RTT

sR

RTO

Maximumallowed rate

where:

losses reveal toused timeout:

timetrip-round :

rate losspacket :

sizepacket :

RTOt

RTT

p

s

TFRC declared objectivesTFRC was defined to support real-time traffic,

and to provide sources with a rate-control algorithm to achievefairness towards TCP traffic

but …

only the fairness aspects have been considered up to now

no attention has been paid to real-time sourcesusing TFRC

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

TFRC protocol scheme

TX RX PR

pnoseq ,_

pktDATA _

pktDATA _

TFRCsender

TFRCreceiver

pktDATA _ )(mTOUT )(tTIN

SOURCE

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

MPEG source and TM-5MPEG 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

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)

pktDATA _ )(mTOUT )( nTFRC tT

Networkbandwidthsmoother

)(tTIN

TFRC

RQ-source

Network Bandwidth Smoother

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

)(ˆ)1()()(ˆ11 nSnTFRCnS tTtTtT

nnnSIN ttttTtT , )(ˆ)( 11

Transformation in a continuous-timestep function

For each TFRC rate modification event

Rate/Quality MPEG Video source

pktsDATA _

Videosource

MPEG encoder

Rate controller

PacketizerSamplersampling frequency= GoP

)(tTIN

)()( hT GoPIN

bitsDATA _

q

)(mTOUT

Videosource

MPEG encoder Packetizer

Rate controller

RQ-source Rate ControllerTarget: to calculate the quantizer scale parameter for each macroblock, according to a given lawInput data: the budget to be allocated to each beginning GoPThree 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

[TM5]RC

)(H[TM5]RC [TM5]RC

),( MLH[TM5]RC )(H

[TM5]RC

rate control mechanism

Rate controller applying an hysteresis mechanism to the classical TM-5Hysteresis mechanism:

divide the PSNR range in L levels, 1, 2, …, L

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 RC[TM5] 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

)(H[TM5]RC

1234 H=3

The PROBLEM:during the hysteresis period a large number of credits or debts maybe accumulated

The source behavior may depend mainly on the past, and not onthe present

Memory-less rate controllerDefined 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

rate control mechanism

),( MLH[TM5]RC

)(H[TM5]RC

TRR PREVNEW TRNEW

Case study

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)

RQ source RQ destination

Router Router

360 km2 ms

Encoding quality levels

PSNR level 1

[39.2, 49.2] dBPSNR level 2

[36.2, 39.2] dB

PSNR level 3

[35.0, 36.2] dBPSNR level 4

[33.7, 35.0] dBPSNR level 5

[31.5, 33.7] dB

comparison )( - )( )()( hThT GoPOUT

GoPINRate processes

]5[TMRC )(]5[

HTMRC

),(]5[MLH

TMRC

Loss process

]5[TMRC )(]5[

HTMRC

),(]5[MLH

TMRC

PSNR process

]5[TMRC )(]5[

HTMRC

),(]5[MLH

TMRC

Quality-level process]5[TMRC )(

]5[HTMRC

),(]5[MLH

TMRC

Conclusions

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

We allowed local TFRCrate violation to account

encoding quality

Peculiarities of our approachVariation of encoding parameters does not

provide an infinitesimal fine tuning of the rate

IN ADDITION

TCP dynamics are deleterious for real-time video sources

Video source cannot exactly follow the rate calculated by the TFRC

Its better that video sources donot exactly follow TCP dynamics

Encoding video for transmission on the

Internet must follow a right tradeoff between

TCP-friendliness and

encoding quality

Future works

TCP-friendliness evaluationIntroduction 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)

Support material

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