Like HowStuffWorks on Facebook!

How High-speed Dial-up Works

        Tech | Connectivity

High-speed Dial-up: Filtering and Caching

When you type a URL (like into the address bar of your browser, you are sending a request for a specific page. If that page uses pop-up advertising, there are pop-up parameters hiding in its programming code. When the information is sent back to your machine, the hidden code executes a program that launches the advertisement. In order for the pop-up to pop, that hidden code must display parameters that tells your machine what size the ad is, where on the screen it should appear, and other details about the ad. These ads take up valuable bandwidth, slowing down the transmission of data to your machine.

To combat this drag, high-speed dial-up providers have bundled a pop-up blocker into the software they send to subscribers. This pop-up blocker is programmed to recognize those lines of code that spell out the ad parameters. When it sees those tell-tale lines of code, it rejects the ad's request to be displayed. What this amounts to is less information being sent across the phone line to your machine. The less data that is sent, the faster the load time.


The first time your browser loads a Web page, it has to load the entire thing, along with all of the images it displays. If the browser saves the images and text, then the second time it loads the same page it can check for duplicates. If an image has not changed, there is no need to download it again. This process of saving a file in the hopes of reusing it in the future is called caching. For a complete explanation of the caching process, see How Caching Works.

High-speed dial-up uses a similar system for commonly requested Web pages. Instead of constantly requesting the same page, the acceleration server takes note of which Web pages are being commonly asked for by all subscribers. So instead of asking the HowStuffWorks server thousands of times a day if it can see the homepage of, it just asks once. Then it stores the page in its memory, and every time another subscriber asks to see HowStuffWorks, it simply transmits the page out of its memory to the user. This is called server-side caching, and it saves time by eliminating redundant requests.

There is a second side to caching -- client-side caching. Internet browsers like Explorer or Netscape are made to cache frequently viewed pages to cut down on load times.

Client-side caching
Client-side caching

The browser stores the cached pages on your computer's hard disk. High-speed dial-up software enhances this feature. In addition to storing frequently viewed pages, it also looks for elements in those pages that remain constant. For instance, instead of caching the entire HowStuffWorks homepage, most of which changes every day, it looks for things that don't change. On our homepage, the logo, the header, the navigation, and the search bar stay the same every day. The software makes note of that consistency, saves those elements, and then only loads what has changed every time you come to the HowStuffWorks homepage.

You can see how caching saves time by avoiding unnecessary data transmission. The most amazing thing about this tool is that with the combination of server-side caching and client-side caching, the system learns about your surfing habits. It uses what it learns to streamline your connection process as much as possible. So the more you use it, the faster it gets.

Compression, filtering and caching are the three key steps in dial-up acceleration. But what actually happens when you put all three techniques together? Does performance really improve? And is the improvement enough to be noticeable?

The answer is yes, and in the next section we will try out NetZero to see how well it actually works with real-world Web pages.