Advanced Streaming Format

What is the role of ASF in the multimedia world?

Until very recently, digital multimedia was available for playback only from local storage -- usually a hard disk or a CD-ROM. The Internet is changing all of that. Multimedia is now available on remote servers residing on a network. Clients connect to the server across the network, and receive the multimedia content from the server in the form of a multimedia data stream.

In the network scenario, rules are needed every step of the way. When the multimedia data is created, content creation rules are needed for describing how to organize that data and lay it out on the disk of the server. When the data is streamed by the server across the network to the client, content delivery rules are needed for describing how the server and client communicate, and how the data is transferred across the network. Finally, when the data reaches the client and is played back, content playback rules are needed for describing how the multimedia will be rendered, or presented, on the client display.

ASF is a set of content creation rules. It is neither a set of delivery rules, nor a set of playback rules.

Delivery rules are handled by network protocols. Network protocols are like the rules of the road. Autos (the packets) contain passengers (the data) and travel on the road (the network pipes). The autos must travel in their own lane, no faster than the speed limit, lane changes must be signaled, passing is permitted only on the left, emergencies are signaled by raising the car hood, and so on. In the multimedia world, important network protocols include:

RTP (Real-Time Protocol) - rules describing how data is to be streamed, in other words delivered in "real time"
RTSP (Real-Time Streaming Protocol) - rules under which the client communicates with the server to start, stop, and control the multimedia stream
IP Multicast - rules describing how a single stream of data makes its way around a network, being delivered to clients who signed up to receive it

ASF works with these network protocols. But it also works with other network protocols, since ASF is independent of the rules of delivery. That being said, ASF was designed with an eye toward delivery. Data in the ASF format is quickly and efficiently placed in the "wire formats" required by the standard network protocols for network delivery.

Similarly, ASF is optimized for efficient playback on the client end. For example, a prevalent set of playback rules is embodied in Microsoft's DirectShowTM framework. DirectShow can easily and efficiently take ASF formatted data and render it on the client display. However, as with the network protocols, ASF is independent of specific playback rules. ASF is designed to be readily acceptable to any and all multimedia playback rules.

As a format for content creation, ASF falls squarely in the camp of presentation formats. Content creation formats come in two primary flavors: editing formats and presentation formats. An editing format is an arrangement of data that is optimized for high-end editing. In the case of video, an editing format for the video would allow the video to be easily clipped, cut and pasted, and enhanced with special effects. A presentation format, by contrast, allows the video to be efficiently played.

This is not to say that ASF files cannot be edited. Indeed they can, and content creation tools will be available for editing ASF files. The point here is that ASF is not specifically optimized for high-end editing. Rather, it is a presentation format specifically optimized for network delivery and playback. So under one scenario, ASF files will be created and edited using an ASF editing tool. Under another scenario, other editing tools and formats will be used to create and edit a multimedia presentation. When that presentation is complete and ready for network delivery, it will be saved as an ASF file.

So ASF is a presentation file format for multimedia data. As such, ASF answers a crying demand of the multimedia world. But why is the multimedia world demanding ASF?