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 addresses: (flicandro,

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

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 links Equation-based protocol: it estimates the throughput according to the following equation: Maximum allowed rate where :

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

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 TXRXPR TFRC sender TFRC receiver 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-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

TM-5: Target bit allocation This step estimates the number of bits available to encode the frames of the beginning GoP Budget for the new GoP

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

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

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) Network bandwidth smoother TFRC RQ-source

Network Bandwidth Smoother It receives the network bandwidth estimated by the TFRC It has the aim of eliminating the high frequencies of this process To this end, it uses a low-pass filter with an Exponential Wighted Moving Average (EWMA): Transformation in a continuous-time step function For each TFRC rate modification event

Rate/Quality MPEG Video source Video source MPEG encoder Rate controller Packetizer Sampler sampling frequency= GoP Video source MPEG encoderPacketizer Rate controller

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

rate control mechanism Rate controller applying an hysteresis mechanism to the classical TM-5 Hysteresis 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=3 The PROBLEM: during the hysteresis period a large number of credits or debts may be accumulated The source behavior may depend mainly on the past, and not on the present

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 rate control mechanism

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 ( RQ source RQ destination Router 360 km 2 ms

Encoding quality levels PSNR level  1 [39.2, 49.2] dB PSNR level  2 [36.2, 39.2] dB PSNR level  3 [35.0, 36.2] dB PSNR level  4 [33.7, 35.0] dB PSNR level  5 [31.5, 33.7] dB

Rate processes

Loss process

PSNR process

Quality-level process

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

We allowed local TFRC rate violation to account encoding quality Peculiarities of our approach 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 Video source cannot exactly follow the rate calculated by the TFRC Its better that video sources do not 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 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)

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