How High-speed Dial-up Works

High-speed Dial-up: File Compression

The key element of high-speed dial-up Internet is file compression. If you've read How File Compression Works, then you know that there are two types of file compression: lossy and lossless.

Text and other files that need to remain perfectly intact during the compression process use lossless compression. Once they are uncompressed, the files return to their original state.

Photos and graphics can be transmitted using lossy compression. When these files are uncompressed, they are not exactly as they were before compression: They have lost some of the original data in the process. For example, a picture that originally had 2 million colors may only have 16 thousand after lossy compression. The loss in quality may not be important to the user when weighed against the increase in speed gained through the compression process. Companies like NetZero let the user control how much compression is used on photos and certain sites.

File compression is an evolving technology, and it doesn't work on every file type yet. The below information will help you understand what will and will not be accelerated by high-speed dial-up.


  • HTML/Java-based Web pages
  • Text
  • JPG/GIF-based graphics
  • E-mail

Not Accelerated

  • Streaming media (audio or video)
  • Secure Web pages
  • Music/photos sent as e-mail
  • Downloads

At this point, the on-the-fly file compression utilized in high-speed dial-up can't be added to the file types specified above because of the nature of the data. For instance, data on secure Web sites is encrypted. When it is transmitted, the code looks like a bunch of gibberish so that no one can read it. When this gibberish reaches the acceleration server, it can't compress the code: If the compression software were to change even one character in the encrypted transmission, that would render the data unusable.

Here is how a typical acceleration server compresses different file types:

  • For text files, including the HTML text of a Web page and the text in an e-mail message, the acceleration server compresses the text on the fly and sends it down your modem line. Typically, text will compress at least 50 percent using on-the-fly techniques.
  • For image files, including GIF and JPG images on Web pages as well as many banner ads, the acceleration server reads the image from the Web site and recompresses it to reduce its size. Typically, the image file size shrinks anywhere from 50 percent to 90 percent in the process.
  • In many other files, including video files, Zip files and MP3 music files, compression has already taken place. For example, an MP3 file is already one-tenth the size of the original track on the CD. It is not possible to compress the file any further in a quick way. In the case of secure Web pages, we already discussed why they cannot be compressed. The acceleration server will not touch these files -- it just passes them through as-is.

In the next section, we will learn how high-speed dial-up accelerators filter out useless data to increase speed.