Efficient Support for Interactive Browsing Operations in Clustered CBR Video Servers IEEE Transactions on Multimedia, Vol. 4, No.1, March 2002 Min-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 server Delivery node

Server architecture Storage node Storing video data Dealing its own disk scheduling algorithm 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 sequence 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 t 2, so that at time t 3 block 3 can be delivered. Consider an fast-forward operation of ration f starts at t k. The maximum delay

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 reallocation and delay of some requests.

Maximum (total) buffer requirement (prefetching 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.