Information Technology Reference
In-Depth Information
(
a
1
−
t
m
)
(
a
i
−
t
m
)
…..….
Static Ch:
1
2
... i
n
1
User 1:
w
1
User 2:
1
2
w
2
User i:
1
2
...
i
w
i
User n:
1
2
...
i
n
w
n
time
a
1
a
2
a
i
a
n
t
m
+1
t
m
……
Figure 19.7
Timing diagram for admitting a group of dynamically-admitted users
As an illustration, consider a dynamic multicast channel serving
n
dynamically-admitted
clients as shown in Figure 19.7. Let
a
i
be the time client
i
arrives in the system and the nearest
multicast cycle starts at
t
m
and
t
m
+
1
respectively, where
t
m
<
a
1
<
<
a
n
<
(
t
m
+
1
−
δ
).
Each client upon arrival will begin caching data from a static multicast channel while waiting
for an available dynamic channel to begin playback. Note that the later a client arrives in the
batch, the longer it must receive data from the dynamic multicast channel to make up for the
missed data transmitted by the static multicast channel. Eventually all clients in the batch will
reach their cached data position and the dynamic multicast channel is released. Therefore, the
channel holding time of the dynamic multicast channel is equal to (
a
n
−
a
2
...
2
t
m
), i.e., dominated
by the last client joining the batch.
Compared to TVoD systems, a SS-VoD client must have the capacity to receive twomulticast
channels concurrently and have a local buffer large enough to hold up to
T
R
seconds of video
data. Given a video bit-rate of 3Mbps (e.g., high-quality MPEG4 video), a total of 6Mbps
downstream bandwidth will be needed for the initial portion of the video session. For a two-
hour movie served using 25 static multicast channels, the buffer requirement will become
108MB. This can easily be accommodated today using a small hard disk in the client, and in
the near future simply by using memory as technology improves.
19.2 Interactive Controls
To provide a complete VoD service, interactive playback controls such as pause-resume, slow
motion, seeking, etc. will also need to be supported. Among these, pause-resume is likely to be
the control most frequently performed in typical movie-on-demand applications. Intuitively,
performing an interactive control in SS-VoD essentially breaks the client away from the current
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