Information Technology Reference
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Allocated network bandwidth
. . .
Media bit-rate
Playback Time
Figure 1.19 Reducing the peak bit-rate by skipping the transmission of some media data (a solution
with trade-off in media quality)
media bit-rate variability can be reduced very effectively. We will revisit this topic in Chapter 7
and investigate the streaming of variable bit-rate media in mixed-traffic networks.
1.6.4 Trade-off in Quality
So far we have assumed that the media server must send all media data to the media client. If we
remove this constraint, i.e., allowing some media data to be skipped, then we can also lower the
network bandwidth required in delivering the media stream. This is illustrated in Figure 1.19
where some of the media data in the segment with peak bit-rate are not transmitted.
With incomplete media data, however, the media client obviously will not be able to recon-
struct the original compressed media stream for playback. The amount of quality degradation
incurred depends heavily on the media encoding algorithm employed, the data skipping algo-
rithm used, the amount of data skipped, as well as the type of the skipped data (e.g., headers,
video data, audio data, etc.).
In addition to simply skipping data for transmission, another approach is to dynamically
reshape the media segments to a lower bit-rate before transmission. For example, knowing that
the sixth media segment in Figure 1.19 requires a bit-rate higher than the network bandwidth
available, the media server could re-encode the media segment to a lower bit-rate that is
within bandwidth limit. This re-encoding can be done, for example, by decoding the media
segment and then re-encoding it at a lower bit-rate (by discarding more information); or it can
be done using a media transcoder than can reduce the media bit-rate without going through
the complete decoding/encoding processes, resulting in less quality degradation and possibly
lower processing complexity as well. We further explore media transcoding in Section 2.5 and
adaptive streaming in Chapter 8.
1.6.5 Trade-off in Complexity
The final trade-off to be discussed is complexity. In multimedia streaming media data are
almost always compressed before transmitted over the network. Thus the choice of compression
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