MPEG-4 and scalable streams The MPEG team has
MPEG-4 and scalable streams The MPEG team has proposed a different way to cope with variable network bandwidth. The server transmits a basic low-resolution stream. Additional helper streams can carry more detail. If the bandwidth is available then these extra streams allow a better quality picture to be assembled by the player. MPEG-4 also supports scalable encoding. This means that a basic player may decode only part of the stream to create the video, albeit at a lower quality than a more complex player, which can decode and display all the stream information. Loading content Whether you are using a managed service or doing your own serving, the first step is to deliver your content to the streaming servers. The encoding probably takes place near the video editing facility or, for a live webcast, at the venue. The servers have to be located close to an Internet backbone, unless you are streaming only over a local area. So in all probability the encoder and server are separated geographically. The simplest way to deliver the content is via a file transfer, using FTP. Some encoding systems have the ability to transfer a file automatically, immediately after the encoding has finished. Live streaming The file can, of course, be sent on a CD-ROM. If the content is a live broadcast, then neither of these methods is suitable; it has to be streamed. This is covered in the chapter on live webcasts. The media encoder typically connects to the server using TCP for a bidirectional control link and a unidirectional media stream, using UDP. It is very important that the circuit used for this connection has more than sufficient bandwidth and a high QoS; that means low packet loss and timing jitter. Any data loss or corruption will be visible by all receivers of the webcast. This generally means using an uncontended circuit like T-1/E-1, rather than a domestic circuit like ADSL or a cable modem. Stream serving 217 control (TCP) origin server to players media encoder media data (UDP) Figure 11.6 Encoder connections. 1
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