VOD: Video On Demand

Edward Chow

The materials presented here are adapted from Dave Sincoskie's pioneer paper, "System Architecture for a Large Scale Video On Demand Service," Computer Network and ISDN Systems, 22, (1991) 155-162. The topic is related to real-time, multimedia storage/communications systems.

Video On Demand Example

Video On Demand Service

Provides service similar to the popular video-tape rental service.
The term, Interactive video, covers broader services.

Technological Assumptions

Video Material

Disk Technology Trend

Disk size:
1980  200MB
1988  2GB
1998  35 GB based on annual increase of 33%
1998  AV Disk 9GB common

Transfer rate:
The transfer rate is controlled by the linear bit density and rotational speed.
Ultra wide SCSI drives transfer data at 40MBps (8ms access time, 512k cache, 9.1GB, $800).
Enhanced IDE and Ultra DMA drives transfer data at 33MBps (9.5ms access time, 512k cache, 6.4GB, $259)
RAID data striping can multiply the transfer speed of single drive.
The real-time Audio/Video applications require fast response time and sustain delivering speed. Disk cache at hard drive may have to be turned off or speeded up to meet the requirements.
Assume no change in rotational speed, both linear density and radial density increase twice resulting four times increase in disk density,
by 1998 the disk may have 83 Mbps transfer rate.

Central Office Switch

Video on Demand System Architecture


View with More Detailed Multimedia Storage System

Usage Scenario

  1. A single customer watching a movie that no other customer is viewing. A connection is setup between customer and tape player.
  2. A popular movie, when the first customer requests it, it will be cached into the copier memory.

Copier Memory

Is it correct that since the disk head covers 12.5 min movies, it will take 12.5min to read the same frame again?

Stop-Start Buffer

A good research project will be to modify the above design to allow full function rewind and fast forward.
Bonus Exercise: Discuss the impact of including fast rewind and fast forward on the memory requirement of VoD and propose the design of a stop-start buffer and its operation for handling such new features.

System Trade-offs