The ASF and ASX File Formats

Windows Media Player plays ASF and ASX files. ASF is a compressed file format that can store audio and video information and is designed to run over the Internet. ASF may also include slide shows and synchronizing events. The big advantage of this file is that it is delivered continuously and starts playing almost without a delay. Your users don't need to wait any more for your audio and video files to fully download before starting to view them. For example, a 40 seconds-long AVI file can take more than 40 minutes to download over a 28.8Kbps modem. Because of this long download time, it would be impossible to deliver Internet-based training class that is 20 minutes long. When converted to ASF file, this class begins playing after just a few seconds. Practically, the file's size has no relevance to the problem.

ASX files are textual command files that manage streaming of ASF files. They are very small in size (about 1K) because they contain no data, just instructions. When a user clicks a link to an ASX file, the browser downloads it to the cache directory, launches the Windows Media Player, and then starts the streaming. The advantage of using ASX files over ASF files is their server location. ASF files can reside on different server types:

• Windows Media Services Server. File names will start with mms://.
• HTTP Server. File names will start with http://.
• Local or network drive. File names will start with file://.

ASX files, on the other hand, are small text files that can always sit on an HTTP server. When the browser interprets the ASX file, it access the streaming media file that is specified inside the ASX file, from the proper HTTP, mms, or file server. The following is an ASX file:

<ASX VERSION="3">
   <ENTRY>
      <REF HREF="mms://netshow.microsoft.com/ms/sbnasfs/wmt/turtle28.asf/">
      <REF HREF="mms://netshow.microsoft.com/ms/sbnasfs/wmt/wmt.asf/">
   </ENTRY>
</ASX>

The ASX file format is very rich. You can add banners, icons, images, and watermarks underneath the streaming media. We will revisit ASX files later in our series. The purpose of this page is to give you a brief overview on what is ASX files and how they are different from ASF file and other audio and video formats.