Closed-Loop Algorithms - Scalable Continuous Media Streaming Systems - page 304

WAN Trunk LAN Trunk

Media Server

Disk

Disk

Prefix Proxy

Figure 17.9 Using a prefix proxy to cache and serve the initial portion of the media stream

much higher for the WAN link than the residential network. This motivates the use of regional
caches to reduce the amount of data transferred over the WAN link.
One immediate application of caching is to integrate it with patching. Observe that in
patching the patching streams are primarily used to send the initial portion of themedia stream-
called the prefix of the media stream. Thus, if the prefix data are stored in the cache as shown
in Figure 17.9, the patching streams can then be served entirely from the cache through the
prefix proxy. In this way only full streams that are shared by many users will be transferred
over the costly WAN link while the numerous patching streams are served by the prefix proxy.
Because of the proximity of the prefix proxy to the clients, the latency in starting the patching
streams can also be shortened.
More generally, we can extend caching beyond just themedia prefix. Prefix caching, however,
is particularly effective as the prefix duration is short and common across all clients requesting
the same media stream, thus reducing the cache size requirement. If the prefix is sufficiently
small, it is even possible to bypass the disk altogether and simply store the prefix in physical
memory, thus eliminating another potential I/O bottleneck. Interested readers are referred to
the literature for many sophisticated caching strategies [18-21].

17.5 Piggybacking
The previous techniques are all based on changing the transmission schedule or the reception
schedule to merge clients onto a shared multicast data stream. In the fifth technique described
in the following, called piggybacking , the principle is to change the playback schedule so that
multiple clients will eventually converge to the same playback point, at which they can be
merged and served using a single multicast data stream [22-25].
To change the playback schedule it means the client plays back the media stream either
faster or slower than normal playback speed. Note that here playback speed does not equate
to data rate. For example, in video the media playback speed is typically defined in frames
per second (fps). Thus to increase playback speed the client can display the video frames in
a frame rate higher than normal, which also consume media data at a higher data rate, or the
media server can send a video streamwith frames discarded periodically, thus achieving higher