The terms streaming media and webcasting often are used synonymously. In this blog I refer to webcasting as the equivalent of television broadcasting, but delivered over the Web. Live or prerecorded content is streamed to a schedule and pushed out to the viewer.

The alternative is on-demand delivery, where the user pulls down the content, often interactively. Webcasting embraces both streaming and file download. Streamed media is delivered direct from the source to the player in real-time. This is a continuous process, with no intermediate storage of the media clip.

In many ways this is much like conventional television. Similarly, if the content has been stored for on-demand delivery, it is delivered at a controlled rate to the display in real-time as if it were live.

Contrast this with much of the MP3 music delivery, where the file is downloaded in its entirety to the local disk drive before playback, a process called download-and-play. True streaming could be considered a subset of webcasting.

But streaming does not have to use the Web; streams can be delivered through wireless networks or over private intranets. So streaming and webcasting overlap and coexist. Streaming media has been around for 70 years.

The conventional television that we grew up with would be called streaming media if it were invented today. The original television systems delivered live pictures from the camera, via the distribution network, to the home receiver.

In the 1950s, Ampex developed a means of storing the picture streams: the videotape recorder. This gave broadcasters the option of live broadcast (streaming), or playing prerecorded programs from tape.

The television receiver has no storage or buffering; the picture is displayed synchronized to the emissions from the transmitter. Television normally is transmitted over a fixed bandwidth connection with a high quality of service (QoS).

Today, streaming media is taken to mean digitally encoded files delivered over the World Wide Web to PCs, or IP broadcasting. Whereas television has a oneway channel to the viewer, Internet Protocol (IP) delivery has a bidirectional connection between the media source and the viewer.

This allows a more interactive connection that can enable facilities just not possible with conventional television. The first of these new facilities is that content can be provided on demand. This often has been promised for conventional television, but has not yet proved to be financially viable.

Streaming also differs from television in that the media source (the server) can adapt to cope with varying availability of bandwidth. The goal is to deliver the best picture possible under the prevailing network conditions. A normal unicast stream over IP uses a one-to-one connection between the server and the client (the media player).

Scheduled streaming also can be multicast, where a single IP stream is served to the network. The routers deliver the same stream to all the viewers that have requested the content. This allows great savings in the utilization of corporate networks for applications like live briefings or training sessions.

As a single stream is viewed by all, it cannot be used for on-demand delivery. Like subscription television, streaming media can offer conditional access to content using digital rights management.

This can be used wherever the owner of the content wants to control who can view; for example, for reasons of corporate confidentiality, or for entertainment, to ensure that the viewer has paid for the content.

What is real-time?

Streaming often is referred to as real-time; this is a somewhat vague term. It implies viewing an event as it happens. Typical television systems have latency; it may be milliseconds, but with highly compressed codecs the latency can be some seconds.

The primary factor that makes a stream real-time is that there is no intermediate storage of the data packets. There may be some short buffers, like frame stores in the decoder, but the signal essentially streams all the way from the camera to the player.

Streamed media is not stored on the local disk in the client machine, unless a download specifically is requested (and allowed). Just because streaming is real-time does not mean it has to be live. Prerecorded files also can be delivered in real-time.

The server delivers the packets to the network at a rate that matches the correct video playback speed.