320 likes | 412 Vues
This presentation covers different bandwidth smoothing algorithms for efficient video delivery in digital libraries. It discusses challenges in live and stored video delivery techniques and explores solutions using pre-fetching and Mobile Agent communication models.
E N D
Digital Library Accessing Techniques Supervisors : Prof. Michael Lyu, Prof. Irwin King Presenter : Lee Chi Wai, Anson Date : 7-12-1999
Outline of the presentation • Introduction • Bandwidth smoothing algorithms • Mobile agent communication model • Audio server/client delivery system • Conclusion • Question and answer
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
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
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
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
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
Introduction • Solution : • use Mobile Agent communication model • support asynchronous interaction of searching for information • support for mobile computers or lightweight devices
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
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
Transmission plan • The server must not transmit more data than • It prevent the overflow of the pre-fetch buffer (of size b)
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
Transmission plan • Creating a transmission plan involves generating m consecutive runs each with a constant bandwidth allocation rj and duration tj
Critical bandwidth allocation • For the need of decrease bandwidth, the CBA algorithm starts a rate decrease at the leftmost point of the frontier
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
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
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
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
Run boundary example • Selection of starting point of different bandwidth smoothing algorithm
Piecewise Constant Rate Transmission and Transport Algorithm • Aim : Generate a single run for each intervals by connecting the interaction points on the Funder curve
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
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
Reasons for using Mobile Agent • Asynchronous interaction by sending one active message to the service site • Reduction of network traffic
Reasons for using Mobile Agent • Support for mobile and lightweight devices
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
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) =32000sec
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; • }
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
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
Future Work • Implement video server/client delivery system • Use mobile agent in the multimedia applications to delivery audio or video data