Curtis St. John
7.2 Notes
Clients request compressed video/audio files that reside on servers. These servers can be normal web
servers or they can be special streaming servers tailored for the audio/video streaming application. The
majority of streaming audio/video traffic is transported by TCP, but can also work with UDP. Once the
requested file starts to arrive, the client begins to render the file.
Some client systems and server applications allow for interactivity, such as pause/resume functions, and
allowing the user to jump midway into a the streaming file instead of viewing the whole thing. The Realtime streaming protocol (RTSP) allows for this interactivity.
Users often request audio/video files through a browser, but then play the file using a Media Player. The
media player performs functions such as:
Decompression: Files are usually compressed to save space on disk storage and also for network
bandwidth. The media player must decompress the file before it can play it.
Jitter Removal: Packet jitter is the variability of source-to-destination delays of packets within
the same packet stream. Since the audio and video must be played out with the same timing
with which it was recorded, a receiver will buffer received packets for a short period of time to
remove this jitter.
Stored audio/video can reside either on a web server that delivers the audio/video to the client over
HTTP, or on a dedicated audio/video streaming server using HTTP or some other protocol. The
streaming of multimedia via HTTP has become popular because firewalls will often allow HTTP traffic to
pass through while proprietary protocols are blocked.
In many implementations of streaming multimedia over HTTP, client-side-functionality is split into two
parts. The browsers job is to request a metafile that provides information about the multimedia file that
is to be streamed over HTTP. This metafile is then passed from the browser to the media player, whose
job is to contact the HTTP server, which then sends the multimedia file to the media player over HTTP.
A proprietary streaming server requires two servers. One server, the web server, serves web pages,
including metafiles. The second server, the Streaming Server, serves the audio/video files. Both servers
can run on the same end system or on two different end systems.
RTSP allows a media player to control the transmission of a media stream. RTSP is an out-of-band
protocol.