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)@diit.unict.it

2. 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

3. to better exploit network bandwidth No bandwidth guarantees Paper target • Definition of a Video-over-IP adaptive-rate quality-controlled transmission system Scenario: BEST EFFORT Internet

4. 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

5. 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

6. A brief background • TCP-friendly TFRC algorithm • MPEG and TM-5

7. Maximum allowed rate where: 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:

8. 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

9. SOURCE TFRC protocol scheme • 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 TX RX PR TFRC sender TFRC receiver • 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

10. MPEG source and TM-5 • MPEG video source is an adaptive-rate source • the emission rate can be tuned through the quantizer scaleparameter (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

11. RQ-source Network bandwidth smoother TFRC 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)

12. For each TFRC rate modification event Transformation in a continuous-time step function 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):

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

14. 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

15. The source behavior may depend mainly on the past, and not on the present rate control mechanism H=3 • 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 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 The PROBLEM: during the hysteresis period a large number of credits or debts may be accumulated

16. 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

17. RQ destination Case study 360 km 2 ms RQ source • 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) Router Router

18. 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

19. Rate processes

20. Loss process

21. PSNR process

22. Quality-level process

23. 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

24. Video source cannot exactly follow the rate calculated by the TFRC We allowed local TFRC rate violation to account encoding quality Its better that video sources do not exactly follow TCP dynamics 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

25. 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)

26. Support material • Material relating to this topic, and an extended version of this paper can be found at: www.diit.unict.it/arti