Yiu-Wing Leung, Senior Member, IEEE, and
Tony K. C. Chan
IEEE Transactions on multimedia
March 2003

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Introduction
VOD system architecture
Broadcast delivery schemes
Interactive operations
Design considerations and examples
Conclusions

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Client-Server Design
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Maintain a dedicated video stream for each customer
Use batching policy to server more concurrent customers
Customers must wait before starting a VOD session (called access delay )

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Broadcasting Design
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Periodic broadcasting
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Broadcast multiple streams of the same video at staggered times periodically
Staggered broadcasting
Similar to periodic, but perform an interactive operation

Video archives
•Connect to an optical fiber and provide logical channels
•Contain a lot of videos
•Broadcast over multiple optical channels according to a broadcast delivery scheme
Proxy
•Logical unit for reception and transmission
•Receives the video from optical channel, and transmits it with video playback rate

Scalability
•To add storage and optical fibers if not sufficient
•VOD warehouse in distributed site and nearest customers

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Each video is organized into pages
A video consists of n=9 pages and these are broadcast over C=3 channels
Two types of broadcast delivery schemes
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Basic broadcast delivery
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Interleaved broadcast delivery

Video archives broadcast diagram

Proxy receives the shaded pages

Proxy delivers the retrieved pages to the customer

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Buffer size
Proxy retrieves video at the channel bit rate (50Mbps), and delivers video at the video playback rate
(1.5Mbps), so must have temporary storage
Maximal buffer size
(R c
- R v
) * (Tc / p) = R v
* Tc
Retrieval rate : R c
Delivery rate : R v
Duration of a slot : Tc / p

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Tuning time
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When proxy has retrieved all the pages from one channel, it tunes its receiver to another channel.
The maximum permissible tuning time is Tc seconds.
Slot duration
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Depend on Tc, R c
, R v
Proxy retrieves a page from channel in one slot : (R c bits
Tc / p) bits
Proxy delivers this page to the customer in p+1 slots : R v
(p+1)Tc/p
=> Tc / p = Tc*R v
/ (R c
– R v
)

•Divide each page into m minipages, and interleave them in a cycle.
•Page i divided into m minipages, referred to as minipages i
1
, i
2
, …, i m

A page (or m minipages) must last for one cycle and one minislot

Proxy delivers the retrieved pages to the customer

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Buffer size
(1)
(2) x1 = (R c
– R v y1 = (x1 – R v
) * (Tc / mp) = R v
* (2Tc / mp) ) = R
* Tc( 1+1/mp-1/p) / m v
* Tc / m 2 p
(3)
(4)
(5) x2 = (y1 + (R c y2 = (x2 – R v
– R v
) * (Tc / mp) ) = R
* (2Tc / mp) ) = 2R v v
* Tc( 1+2/mp-1/p) / m
* Tc / m 2 p
X3 = (y2 + (R c
– R v
) * (Tc / mp) ) = R
Maximum buffer size is R v
Tc / m v
Tc / m

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Tune time
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Proxy retrieved all the minipages of a page from one channel.
Tuning must be done within p minislots.
Maximum permissible tuning time is (Tc / mp) * p = Tc / m
Minislot duration
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Depends on Tc, R c
, R
Proxy retrieves channel : R c
Tc / p bits
Proxy delievers minislots : R m m v
, m minipages of a page from an optical minipages to the customer in mp+1 v
*(mp+1)Tc / mp
=> Tc / mp = Tc R v
/ (m*R c
– R v
)

 Interleaved broadcast scheme support better interactive operations

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Pause : Tc is smaller, the approximate is more similar to the ideal one
 Fast forward :
(1) Play a small portion of video at normal rate
(2) Minipage level is better than page level

 Fast rewind :

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Design issue
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Optical bandwidth : an optical fiber provide 5 Gbps, so if one channel needs 50 Mbps, and can provide 100 channel to use.
I/O speed and channel bit rate : we can match the I/O speed of a disk with the bit rate of an optical channel, so system requires an small capacity disk.
Video playback rate and duration : Different video can occupy different number of channels, therefore can accommodate video with different playback rate (e.g., MPEG-1 and MPEG 2) and different duration (e.g., 90min and 120min).

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Design parameters
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Cycle duration Tc
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If Tc is larger, a channel can broadcast more pages in a cycle
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If Tc is larger, the mean access delay is longer. If service can specify an acceptable mean access delay T * , then Tc can be chosen to 2T *
Number of minipages per page m
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A page divide into m minipages can reduce each proxy buffer size
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The actual tuning time must be equal to or smaller than the maximum permissible tuning time Tc / m
Each minipage may have to contain at least a certain number of frames
(e.g., contain at least one GOP of nine frame for MPEG)

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Video is compressed by MPEG with nine frames per GOP.
Because T * =30s, so Tc=2T * =60S
Each video require
[(1.5*106*90*60) / (50*106*60)] = 3 channels. There are 50 video program, so require two optical fibers
Because tuning time cannot not be larger than Tc /
1000 m, so 10*10 -3 ≤ 10/m => m ≤
But since each minipage contain at least two GOP of frames
Tc ≥ m( 2*9 / 30 ) => m ≤ 100
Buffer size : R v
Tc / m = 109.9 Kbytes

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Change acceptable mean access delay T * =5s, so Tc=2*5=10
Each video program requires 18 optical channels, so requires ten optical fibers, where each optical fiber accommodates five video programs.
Consequently, it provides a better quality (i.e., shorter access delay and better interactive operation, but use more optical fibers.

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Adopt both the client-server paradigm and the broadcast delivery paradigm.
The system can easily be scaled up to serve more concurrent customer and provide more video .
Provide interactive operations which are approximations of the ideal ones.
The access delay is small
Each video stream only requires a small buffer size for temporary storage.