1. Reducing Bandwidth Requirement for Delivering Video Over Wide Area Networks With Proxy Server Wei-hsiu Ma and David H. C. Du IEEE Transactions on Multimedia, December 2002

2. Outline Concept - Smoothing CBR transmission (Server) (VBR video) Without proxy With proxy Frame staging selection algorithm Just-In-Time (JIT) Minimum Difference (MD) Chunk algorithm Simulation results Proxy stream consideration

3. Smoothing Server Client Buffer Server Client Unsmoothed Smoothed

4. Transmission constraint

5. CBR transmission without proxy Assume a CBR r is given, a feasible transmission schedule can be achieved by taking advantage of client buffer and startup delay. Open the buffer underflow curve. Move the straight line with slope r along the Time axis until the whole line is above the buffer consumption schedule L(t).

6. Transmission schedules R(T) : increase r R ” (t) : increase d

7. Movie profile Too expensive!! Why lower bit rate implies long delay and larger buffer?

8. CBR transmission with proxy Store some of video frames in the proxy. If buffer underflow takes place at Frame 8: Some Frame i ≤ 8 can be staged in the proxy. Frame i does not need to be transmitted over WAN. Stream synchronization Central stream (from central server) Proxy stream (from the proxy)

9. Two synchronization models

10. Frame staging scenario CBR Transmission Schedule Original Consumption Schedule L(t)

11. Just-In-Time algorithm Scan the transmission schedule and buffer underflow curve. Whenever the transmission schedule is lower than the consumption curve, the frame is selected to be stored in the proxy. The total number of staged frames selected according to the JIT algorithm may be small.

12. Minimum Difference algorithm If underflow happens at Frame i, the MD algorithm selects a frame before Frame i with the smallest possible size to be staged in the proxy to avoid underflow. MD attempts to provide smaller buffer requirement.

13. JIT and MD JIT MD

14. Chunk algorithm (1/3) Chunk: A whole segment of continuous frames which is selected to be staged in the proxy or to be delivered by the server. A chunk can be as small as a frame or as large as a whole video. The JIT and MD algorithms select a frame each time. Storage management at the proxy is difficult. Increase the synchronization burden at the client. Chunk algorithm gathers the staged frames continuously as many as possible by utilizing the client buffer size.

15. Chunk algorithm (2/3) 2 1

16. Chunk algorithm (3/3) The underflow/overflow curve consists Flat periods - all of the frames are staged in the proxy. Ascending (nonflat) periods Long periods imply larger chunks. There may be 4 conditions Overflow in a flat period – switch period Underflow in an ascending period – switch period Overflow in an ascending period Staging a frame is not enough Staging more frames to raise overflow curve Underflow in a flat period May occur after raising overflow curve Report failure (should increase buffer size)

17. Comparison (Client / Proxy) Motion JPEG MD is inefficient in some cases!!

18. Number of proxy chunks versus client buffer size

19. Number of proxy chunks

20. Proxy stream consideration The shape of the buffer underflow/overflow curve of the proxy stream is opposite to the central stream ’ s curve. VBR transmission is adopted for the proxy stream. Use “ Optimal Smoothing ”

21. Optimal smoothing B(t) D(t)

22. Central stream and proxy stream transmission

23. Client buffer partition B = B C + B P The server sends data in average rate r 1, and the proxy sends data in average rate r 2. Is B C : B P = r 1 : r 2 optimal? Simulation result shows that B C : B P should be 1 : 1.

24. Combination cases versus buffer size B C : B P 1:72:63:54:4