- Video on Demand
(VoD) is the
"deluxe"
version of the
Pay-per-view
service of cable or satelite systems.
- The (old) Pay-per-view service:
- In "Pay-per-view", the cable or satelite company offers
a certain event showing at a fixed time.
- Viewers who wish to view the showing purchase the viewing
rights in advance, and wait until the show time
before viewing the event.
- Obviously, for live events
(like a soccer match - we Dutch like soccers, not football :-))
viewers have to wait to view the event.
- But many offerings of the "Pay-per-view" channels are stored videos - like popular (and not so popular) movies.
- In "Pay-per-view", the cable or satelite company offers
a certain event showing at a fixed time.
- Video on Demand service:
- Is applicable to recorded video and audio events
- A viewer who wants to watch a certain event (e.g., a new movie)
at any moment simply pays the fee and
after a (reasonably) short delay, the viewer will be able
to see the showing
- Thus, consumer can view a given program at his/her on convenience.
- Is applicable to recorded video and audio events
- Before we present algorithm that perform
Video on Demand ,
(VoD)
we need to go into some details about the operating environment
and the performance measures that are important in Video on Demand
- Typically, a private
(i.e., owned by a company, e.g., Comcast)
network is typically used to guarantee
good performance.
(Public networks are open to anyone, e.g., Internet)
The VoD technique discussed can certainly be applied in the Internet (with more or less success depending on the amount of bandwidth)
-
Architecture of a Video on Demand System:
- Network bandwidth is
divided
into multiple frequency channels
(simular to how radio stations sharing the air waves)
- A video server broadcasts video streams:
one video stream per channel.
- User has a
Set Top Box
at home that he/she uses
to access the different video streams by tuning into
a specific channel or channels.
- Set top boxes can tune to
multiple channels
at the same time
- Set top boxes contain a hard disk to store video (temporally) that it has downloaded from the video server (user can playback the stored video at a later time ).
- Set top boxes can tune to
multiple channels
at the same time
- The user can
make requests
(through the set top box) for movies.
- Network bandwidth is
divided
into multiple frequency channels
(simular to how radio stations sharing the air waves)
- The requested movie will then be delivered to the user
after a certain time.
The delay between the request and the time that the user sees the movie is called the response time
- Sender and receivers are connected by a high capacity
communication medium (a communication channel)
- The full capacity of the communication channel is very large
and it is usually divided into many subchannels
of lower bandwidth capacity
(Somewhat akin to a big pipe divided into a number of narrower pipes):
(This is done with "airwaves": the full range of radio frequency has been divided up into "bands" - each radio station occupies a small frequency range for its broadcast use)
- When it is divided into subchannels, typically, each subchannel has the same data rate (capacity)
- The effect of the division of the whole channel into subchannels
has the following effect:
- Picture above shows the transmission of
10 M bits
of data using the whole channel and one subchannel
- The same amount of data transmitted using a
(low bandwidth) subchannel
will take longer time.
- On the other hand, when using (multiple) subchannels, you can transmit multiple video streams simultaneously
- Picture above shows the transmission of
10 M bits
of data using the whole channel and one subchannel
- Note that the total amount of data rate is unchanged
- If the channel is
not divided into
subchannels,
then the source must send the different
video objects
sequentially:
- If the channel is
divided into
subchannels,
then the source can send the different
video objects
simultaneously,
one video on a subchannel:
- The channel will almost always be divided into subchannels
- Because the original (undivided) channel is
rarely refered to,
I often use
"channel" to refer to
"subchannel" -
the meaning should be clear from the context.
(To relate to reality: a channel is in fact a cable channel)
- We require that receivers have disk memory space
to store video packets
- The pioneering device is the "Tivo": click here