School of Computing Science Simon Fraser University

1. School of Computing ScienceSimon Fraser University Bounding Switching Delay in Mobile TV Broadcast Networks Cheng-Hsin Hsu Joint Work with Mohamed Hefeeda January 19, 2008

2. Motivations: TV Evolution—Mobile • Batterypowered • Mobile, wireless • Small screens, lower bit rates

3. Most mobile devices (phones, PDAs, ...) are almost full-fledged computers Users like to access multimedia content anywhere, anytime Longer Prime Time viewing  More business opportunities for content providers Market research forecasts (by 2011) 500 million subscribers, 20 billion Euros in revenue Already deployed (or trial) networks in 40+ countries [http://www.dvb-h.org] Mobile TV: Market Demand & Potential 3

4. Over (current, 3G) cellular networks Third Generation Partnership Project (3GPP)  Multimedia Broadcast/Multicast Service (MBMS) Pros: leverage already deployed networks Cons: Limited bandwidth (<1.5 Mb/s)  very few TV channels, low quality, and higher energy consumption for mobile devices (they work mostly in continuous mode) Mobile TV: Multiple Technologies 4

5. Mobile TV: Multiple Technologies • Over Dedicated Broadcast Networks • T-DMB: Terrestrial Digital Media Broadcasting • Started in South Korea • Builds on the success of Digital Audio Broadcast (DAB) • Limited bandwidth (< 1.8 Mbps) • DVB-H: Digital Video Broadcast—Handheld • Extends DVB-T to support mobile devices • High bandwidth (< 25 Mbps), energy saving, error protection, efficient handoff, … • Open standard • MediaFLO: Media Forward Link Only • Similar to DVB-H, but proprietary (Qualcomm)

6. This is called Time Slicing Supported (dictated) in DVB-H and MediaFLO Need to construct Feasible Time Slicing Schemes No receiver buffer under/over flow instances No overlap between bursts Energy Saving for Mobile TV Receivers Bit Rate Burst Overhead To R Off r Time 6

7. Users usually flip through many channels Long/variable delays are annoying In fact, users have complained long channel switching delay on DVB-H phones YouTube MWC 2008: Channel change comparison DVB-H vs. MediaFLO Our experience with Nokia N92/N96 (> 5 secs) Goal: bound maximum switching delay without sacrificing energy saving for mobile receivers Controlling Channel Switching Delay 7

8. Switching delay has multiple components Time slicing delay (our focus) Frame refresh delay (till an I-frame arrives) Add more/redundant I-frames [Vadakital 07] Move I-frames closer to start of burst [Rezaei 07, 08] Processing and Decoding delays Controlling Channel Switching Delay Burst R Channel Switch Off r1 Time Slicing Delay Time 8

9. Reduce inter-burst periods  wastes energy Reduce delay from 1.5 to 0.25 sec  Controlling Delay: Current Approach #1 energy saving drops from 90% to 55% 9

10. DVB-H standard [EN 102377, May 2007] Suggests bundling multiple channels in one group  virtually zero switching delay within a group But, Delay across groups can be large Devices receive all data of the bundle  wastes energy How do we group channels in the first place (manual)? Controlling Delay: Current Approach #2 10

11. Use simulcast Broadcast each TV channel over two burst trains One optimized for delay (bootstrap) The other optimized for energy saving (primary) Devices tune to bootstrap bursts for fast playout, then tune to primary bursts for high energy saving Systematically construct optimaltime slicing schemes Three variations SIMU : traditional video systems (nonscalablecodecs) SIMU-S: scalable codecs SIMU-S+: scalable codecs, bandwidth limited networks Controlling Delay: Our Approach 11

12. Low quality not noticed during flipping Scalable codecs facilitate stream management SIMU-S+ less energy saving than SIMU-S , but better bw utilization Controlling Delay: Our Approach • SIMU-S • SIMU • SIMU-S+ 12

13. Bounding Switching Delay target switching delay dm Our Algorithms time slicing scheme full quality rate r reduced quality rate rl {<start_time, burst_size>} • Run at the base stations to multiplex TV channels into a traffic stream

14. Time Slicing Scheme – SIMU/SIMU-S • Primary bursts: • Bootstrap bursts:

15. Correctness and Performance – SIMU/SIMU-S • Prove the scheme is feasible • Show the scheme maximizes energy saving • First, show our scheme outperforms any scheme that does not employ simulcast idea • Then, show our scheme is optimal among all simulcast schemes • Analytically derive energy saving • for devices receiving bootstrap bursts • for devices receiving primary bursts

16. Comparison on Energy Saving • SIMU-S Primary: More than 95% energy saving

17. Comparison on Network Utilization • SIMU/SIMU-S incur (controllable) BW overhead • SIMU+ is BW efficient, but results in lower energy saving than SIMU/SIMU-S

18. Our algorithms are implemented in the IP Encapsulator Real Implementation 18

19. Testbed for DVB-H networks 19

20. Experimental Setup • Implemented SIMU-S scheme in C++ • Broadcast 8 TV channels for 10 min • Set the target delay to be 500 msec • Collect detailed logs that contain • time and size of each burst

21. Experimental Setup (cont.) • Based on logs, wrote a utility to emulate a million of users • Randomly switching channels • let average watch time for each channel be 100 sec • Compute switching delay and weighted energy saving

22. Theoretical and empirical data match SIMU much better than Current Analytical and Empirical Energy Saving Curves 22

23. SIMU-S achieves the target switching delay bound Channel Switching Delay 23

24. SIMU-S increases energy saving from 74% to 93% in real testbed Energy Saving 24

25. Studied the problem of controlling switching delay Proposed and analyzed three optimal (in terms of energy saving) time slicing schemes Implemented and evaluated SIMU-S in a real testbed It met the delay bound while achieving 93% energy saving Demo Conclusions 25

26. Sample Video Shot from our Testbed • Burst analysis for SIMU: 2 primary& 2 bootstrap trains 26