Efficient Support for Interactive Browsing Operations in Clustered CBR Video Servers

IEEE Transactions on Multimedia, Vol. 4, No.1, March 2002Min-You Wu and Wei Shu

Outline

Introduction Video server architectures The prefetching approach The grouping approach Simulation results

Implementing fast-forward

Increasing network bandwidth Displaying frames at a higher rate

Without increasing network bandwidth Reserve a separate encoded video file

for each ratio Block-skipping Frame-skipping

A clustered video serverDelivery node

Server architecture Storage node

Storing video data Dealing its own disk scheduling algorith

m Delivery node

Receiving requests from clients Buffering and resequencing video blocks

from storage nodes Sending video blocks to clients

Distribution of video blocks

A video file is partitioned into many video blocks (CBR)

Video blocks are evenly distributed over N storage nodes in a round-robin fashion. Block i is stored in the disk (i mod N)

Scheduling of video blocks for normal play

Time cycle: time to play a block

Time slot: storage node can service multiple requests in a time cycle

The prefetching approach Block-skipping

When performing an fast-forward operation of ratio f, a block is retrieved after skipping f-1 blocks.

To avoid a hot spot, the number of storage nodes (N) and fast-forward ratio (f) must be relatively prime.

Otherwise, a video file can be distributed to a subset of storage nodes.

For example, if N=4 and f=2, then only storage node 0 and 2 will be accessed.

Layout and access pattern of fast-forward for block-skipping

N = 5, F = 3

Prefetching approach for block-skipping

Play

retrieve

Delay The delivery node retrieves blocks in the seq

uence of 0, 6, 12, 3, 9, 15, …and it delivers blocks in the sequence of 0, 3, 6, 9, 12, 15, …

The block 0 needs to be delayed to time t2, so that at time t3 block 3 can be delivered.

Consider an fast-forward operation of ration f starts at tk. The maximum delay

kNkfNk

modmax0

Example of maximum delay for different ratios f

Frame-skipping Skipping frames within a video block.

Base substream – contains the frames for fast-forward

Enhancement substream – contains the frames only for normal play.

Prefetching approach for frame-skipping

The grouping approach The pace is defined as the speed of play. The grouping approach divides requests

into different groups based on their paces.

Groups of request with different paces

Operations

Change the membership of a request Adding Removing

Change the number of time slots allocated to a group Expanding Shrinking

Grouping approach for block-skiiping

Frame-skipping

Access f sub-blocks in a time slot

Comparison Prefetching approach

Fully utilize the system bandwidth Initial delay Require some buffer space

Grouping approach Does not require extra buffer space The shrinking operation may lead to reall

ocation and delay of some requests.

Maximum (total) buffer requirement (prefetching approach)

Maximum (total) buffer requirement (prefetching approach)

Average delay and relocation (grouping approach)

Average delay and relocation (grouping approach)

Discuss

Interactive 20% ~ 50% Pace = 1: 50% ~ 80% (5 ~ 6 time

slot) Pace = 5: 10% ~ 25% (2 time slot) Pace = 11: 10% ~ 25% (2 time slot)

Expanding and shrinking operations are rarely invoked.