Introduction - Scalable Continuous Media Streaming Systems - page 17

. . .

Playback Time

Figure 1.15 A media stream with time-varying playback bit-rates

These five dimensions serve to illustrate the complexities and possibilities in engineering a
media streaming system and thus are not meant to be exhaustive nor the most important.
For illustrative purposes, we will consider the streaming of a media streamwith time-varying
playback bit-rates as shown in Figure 1.15. We assume that the media stream is divided into
constant-duration segments of T seconds, i.e., the playback duration for each segment is the
same. The size of each segment, however, is variable. Let r i be the rate at which segment i (0,
1, . . . ) is consumed in playback. Our goal is to deliver this media stream from a media server
to a media client over the network.

1.6.1 Trade-off in Capacity
For simplicity, we will assume that the media stream is the only traffic in the network, which
has a finite and fixed bandwidth available. We first consider a simple solution by trading off
network capacity. Specifically, knowing the bit-rates of all the media segments, we can simply
allocate network bandwidth according to the maximumbit-rate of all segments, i.e., we allocate
a network bandwidth of C , as illustrated in Figure 1.16.

The obvious shortcoming of this simple solution is that except for the segment(s) with peak
bit-rate, some of the allocated network bandwidth will be unused and thus wasted. With a fixed
amount of total network bandwidth available, this solution results in guaranteed delivery but
reduced usable streaming capacity.

= max r i |∀ i

{ }

1.6.2 Trade-off in Time
Observing the inefficiency of the previous solution we proceed to consider another dimension
of trade-off - time. Specifically, the media stream Figure 1.15 has its peak bit-rate in the first
segment. Thus, another strategy to stream the media is to send the initial segment at a bit-rate
lower than the playback bit-rate. Now this implies that the first media segment will take more
than T seconds to arrive at the receiver and so the playback must be delayed accordingly,