Introduction - Scalable Continuous Media Streaming Systems - page 11

1.5 Challenges in Building Continuous Media Streaming Systems
The previous sections outlined the general architecture of continuous media systems and the
concept of media streaming. In this section we will discuss the main challenges in the design
and implementation of continuous media streaming systems.

1.5.1 Continuity
As briefly mentioned in Section 1.4, once we adopted the streaming model for media data
delivery, we will need to ensure that media data fragments are delivered to the client in time to
maintain playback continuity. A key decision then is to decide when to begin playback after
the streaming process is started. As depicted in Figure 1.9, once playback begins, the playback
time for all media data fragments will be fixed (ignoring interactive playback control). Thus,
if the client defers the playback start time, the whole playback schedule will be deferred as well,
thereby allowing more time for data delivery. We cannot, however, defer playback indefinitely
as that will increase the start-up delay experienced by the user - a crucial performance metric.
As we will see in the rest of the topic, these two conflicting objectives will occur frequently in
the design of continuous media streaming systems.

1.5.2 Known and Unknown Variations
A continuous media streaming system comprises many system components. At the highest
level we have the media server and a number of media clients (Figure 1.10). For on-demand
streaming servers the media data are typically stored in local storage devices such as hard disks,
and then retrieved into memory for processing, and finally transmitted through the network
interfaces into the network to the clients. The media client will receive the data arriving through
a network interface, temporarily store them in memory buffers while waiting for decoding and
playback to the user.

Server

. . . . . . . . . . . . . . . . . . . . . . . .

Client

Send req

Delay

Once playback begins, the playback
schedule for all media data are set.

Figure 1.9 Relation between start-up delay and the playback schedule