Overview of streaming media

What is streaming media?

Streaming media is a term used to describe media files that are served in discrete paced individual packets rather than in bulk, playing while they are being transmitted over the network to the media player on the client computer. In contrast, conventional Web files, which are downloaded through a file transfer, must be downloaded entirely before the user can view them. Commonly requested types of streaming media are video and audio. Streaming media also includes interactive media, cartoon-like animations, panoramic data, and more.

Live versus on-demand streaming media

Streaming media is delivered in the following ways:

Live media streams
Live media streams occur in real time, like the news program that you watch on your television set. Some organizations record a live media stream and then broadcast the media stream to their employees or customers at a specified time. All users who have requested the media stream see the same media stream at the same time. Users are not able to rewind or fast-forward the media stream.
On-demand, or previously recorded, media streams
Users can request these on-demand media streams at a time most convenient to them. Users can rewind the media and fast-forward on-demand media streams. On-demand streaming content is commonly referred to as VOD (video on demand).

NetCache supports both of these types of streaming media.

Streaming media presentation versus a unique stream

Discussions in this chapter refer to streaming media presentations and unique streams.

Streaming media presentation

Streaming media presentation is a general term used to describe the delivery of live or on-demand streaming media. Multiple unique streams can make up a streaming media presentation.

Unique stream

Each stream has the following characteristics:

Bandwidth speed
Media type, for example, audio or video
Thinning parameters
The streaming server drops a consistent number of frames to try to make the stream that is being delivered to the client one that the client can handle.

A stream is unique if one or more of the characteristics in the previous list are different from another stream that is part of the same streaming media presentation.

For example, even though two clients request the same streaming media presentation, if one client notifies the server that it requires a 28.8-kbps video stream and another client notifies the server that it requires a 56-kbps video stream, the two clients are requesting different streams. Similarly, a 56-kbps video stream is different from a 56-kbps audio stream.

Note
When a content creator creates a streaming media presentation, the creator can encode multiple bit rates. One advantage of providing multiple bit rates for a streaming media presentation is that clients can negotiate with the streaming server for a stream at a bit rate that the client's network connection can handle. Another advantage of a multi-bit rate file is that only a single URL needs to be published. Conversely, if multiple single-bit rate files are published, a separate URL must be published for each file.

Encoded bit rate can differ from delivery bit rate

Content authors normally encode streaming media content into different bit rates to meet the needs of the different speeds of Internet access--modem, ISDN, DSL, and LAN. In contrast, the delivery bit rate is the actual speed at which the content is delivered to the client. For example, a stream encoded for playback at 56 kbps must be delivered to clients at a bit rate of 56 kbps or higher. A client with enough bandwidth might ask the streaming server to send the 56-kbps encoded stream at 220 kbps; the data is buffered locally and played back at 56 kbps. The playback experience of 56-kbps stream delivered at 220 kbps would be better at 220 kbps than at 56 kbps. The reason is that more time is available for the client to request packets to be retransmitted if packets are dropped.

Clients and servers actively respond to changing network conditions

During streaming media presentations, the client and server communicate quality of service (QOS) information to maintain an acceptable level of playback quality. A variety of QOS mechanisms are used by streaming media vendors:

If data is arriving too slowly, clients might request a higher delivery bit rate.
If data is arriving too quickly, clients might request an entirely different bit rate to match the current network conditions. This process is called stream switching.
If clients perceive network congestion, they might request thinned streams.
Note
Streaming media vendors support varying combinations of the preceding QOS mechanisms.

Thinning and stream switching can occur several times during playback.