Digital Library Accessing Techniques

1. Digital Library Accessing Techniques Supervisors : Prof. Michael Lyu, Prof. Irwin King Presenter : Lee Chi Wai, Anson Date : 7-12-1999

2. Outline of the presentation • Introduction • Bandwidth smoothing algorithms • Mobile agent communication model • Audio server/client delivery system • Conclusion • Question and answer

3. Introduction • Live video delivery techniques: • Challenges : decisions must be made in real-time and delay must be minimized • Solution : weaker guarantees of service or degradation in quality of service • Stored video delivery techniques: • used in digital libraries and video-on-demand systems

4. Introduction • Stored video delivery techniques(cont.): • the full video is know in advance • use pre-fetching techniques to control the picture quality • What is pre-fetching techniques? • create a pre-fetch buffer in client side • fill the pre-fetch buffer with more bytes in advance of each burst

5. Introduction • How to fill the pre-fetch buffer? • Server uses some bandwidth smoothing algorithms • Critical bandwidth allocation (CBA) • introduced by W. Feng and S. Sechrest in 1995 • Minimum changes bandwidth allocation (MCBA) • introduced by W. Feng and S. Sechrest in 1995

6. Introduction • Bandwidth smoothing algorithms (cont.) • Minimum variability bandwidth allocation • introduced by J. Salehi, Z. Zhang, J.Kurose, and D. Towsley in 1996 • Piecewise Constant Rate Transmission and Transport Algorithm • introduced by J. Mcmanus and K.Ross in 1996

7. Introduction • Problem of the client/server communication model : • static and inflexible • interaction between clients and servers requires permanent network connections • can’t support mobile computers

8. Introduction • Solution : • use Mobile Agent communication model • support asynchronous interaction of searching for information • support for mobile computers or lightweight devices

9. Bandwidth smoothing Algorithm • Aim : reduce the burstiness of the network traffic by pre-fetching video data to the client side at a series of fixed rates in advance of each burst (transmission plan) • Different bandwidth smoothing algorithms use various strategy to generate their transmission plan

10. Transmission plan • Suppose a compressed video stream consists of n frames, where frame I requires fi bytes of storage • The server must always transmit quickly enough to avoid pre-fetch buffer underflow, where

11. Transmission plan • The server must not transmit more data than • It prevent the overflow of the pre-fetch buffer (of size b)

12. Transmission plan • Hence, any valid transmission plan should follow the following constraint • where ci is the transmission rate during frame slot i of the smoothed video stream

13. Transmission plan • Creating a transmission plan involves generating m consecutive runs each with a constant bandwidth allocation rj and duration tj

14. Critical bandwidth allocation • For the need of decrease bandwidth, the CBA algorithm starts a rate decrease at the leftmost point of the frontier

15. Critical bandwidth allocation • For the need of increase bandwidth, the CBA performs a search along the frontier to locate the starting point that allows the next trajectory to extend as far as possible

16. Critical bandwidth allocation • smallest possible peak bandwidth requirement • minimum number of bandwidth increases • does not necessary have the minimum number of bandwidth decreases, since the algorithm selects the leftmost starting point

17. Minimum changes bandwidth allocation • Extends the CBA scheme to perform the linear search operation on all rate changes • smallest possible number of rate changes • minimum peak bandwidth requirement

18. Minimum variability bandwidth allocation • Reduce the variability in the rate requirements across the lifetime of the transmission plan • Initiates bandwidth changes at the leftmost point along the frontier, for both rate increases and rate decreases

19. Run boundary example • Selection of starting point of different bandwidth smoothing algorithm

20. Piecewise Constant Rate Transmission and Transport Algorithm • Aim : Generate a single run for each intervals by connecting the interaction points on the Funder curve

21. Piecewise Constant Rate Transmission and Transport Algorithm • Slope of the dotted line corresponds to the rate rj in the resulting transmission plan • vertically offsets the plan until all runs lie above Funder curve • introducing an initial playback delay in the pre-fetch buffer

22. Mobile agent • Definition of mobile agent • a program that is able to migrate from node to node on a network under its own control for the purpose of completing a task specified by a user • The agent chooses when and to where it will migrate and may interrupt its own execution and continue anywhere on the network

23. Reasons for using Mobile Agent • Asynchronous interaction by sending one active message to the service site • Reduction of network traffic

24. Reasons for using Mobile Agent • Support for mobile and lightweight devices

25. Audio Server/Client delivery system

26. Audio Server/Client delivery system • The system is implemented under Linux • Use MPEG 1 layer 3 compression technique to compress the audio file • mp3 files are stored in the same machine where the audio server is running

27. Transmission plan • mp3 bit rate = 128kbps • try to approximate the bit-rate by introducing sleep intervals between successive sending attempts • Maximum Transfer Unit(MTU)= 512 bytes • Sleep interval = MTU/(mp3 bit rate) =32000sec

28. Transmission plan • While (server buffer not empty){ • sleep interval =0.03125sec; • Record the start time; • Transmit audio packet; • Record the stop time; • Sleep for sec; • }

29. Buffer Management in Client Side • Need to buffer audio data at the beginning of playback process since some packets may arrive too late • Or some packets may arrive in burst • Use Leaky bucket Model in the client side buffer to produce a constant output bit rate (128kbps) to the decoder

30. Conclusion • We have presented four bandwidth smoothing algorithms • We have discussed the possibility of substituting client/server model with mobile agent communication model • The audio server/client delivery system has been presented

31. Future Work • Implement video server/client delivery system • Use mobile agent in the multimedia applications to delivery audio or video data

32. Question and Answer