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Client
c
a
Client
c
b
Playback Point
Playback rate:
r+
∆
r
Client
a
catching up
c
b
Playback Point
Playback rate:
r+
∆
r
Client
c
a
catches up with
c
b
c
a
and
c
b
Playback Point
Playback rate:
r
Figure 17.10
Illustration of the piggybacking technique
playback speed (relative to the actual event recorded by the video) without consuming media
data at higher data rate. On the other hand, to decrease the playback speed, the client can simply
reduce the playback frame rate, which also results in lower media data consumption rate.
Figure 17.10 illustrates the piggybacking process. There are two clients
c
a
and
c
b
in the
system initially in different playback points of the media stream. To merge them into a single
multicast stream we can increase the playback rate of client
c
a
from the normal speed
r
to
r
0. Thus the difference in playback point between the two clients will
decrease and eventually client
c
a
will catch up with client
c
b
at the same playback point. At
this time the system can then use a single multicast stream to serve both clients, which now
continue playback at the
same
playback speed.
In addition to speeding up the playback speed of the latecomer, we can also slow down
the playback speed of the early-starter (e.g., client
c
b
in Figure 17.10), or apply both speed-
up and slowdown simultaneously. Clearly, the larger the rate increase/decrease (i.e.,
+
where
≥
in
Figure 17.10), the faster the clients can be merged. However, too much playback rate variation
will be noticeable or even objectionable to the end users. Previous studies have suggested
that playback rate variation up to 5% can be applied without objectionable effect on the end
users, and this can be further extended using advanced signal processing techniques such as
time-scale modification for audio [26-29].
17.6 Summary
In this chapter we have reviewed four orthogonal closed-loop algorithms for multicast stream-
ing, namely batching, patching, caching, and piggybacking. These four approaches are com-
plementary and hence can be combined to form even more sophisticated architectures. For
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