A Typical Page RequestThe download time for a typical web page, containing text, a background GIF, and 5 graphics, is shown below, assuming the user connects with a 14,400 bps modem. These modems do some compression of the data stream, but big files (such as graphics) are already compressed, so these modems don't deliver much faster than the advertised 14,400 bps. A byte is 8 bits, but by the time you consider starts bits, stop bits, and (possible) parity bits, a byte costs around 10 bits to send. Under ideal conditions, therefore, a 14,400 bps modem sends about 1,400 bytes per second.
This table reveals several things. First, nearly 17 percent of the total time spent at this example page is taken up by HTTP overhead. Second, over 92 percent of the download time is spent moving graphics. If you want the page to be downloaded faster, you can do three things:
Now, suppose that we add HEIGHT and WIDTH tags to each <IMG> tag. The download time stays the same, but the page is available for scrolling as soon as text is received - just 2.68 seconds. Suppose that the buttons are interlaced, and the larger graphics have a LOWSRC that is one eighth of the size of the original graphics. Further, suppose that the page is laid out so that the background graphic loads first, the logo, and the incidental graphic next, and the buttons load last. The new timeline is:
Even though the total loading process takes slightly longer than before, the user is able to do useful work on the page less than three seconds after the initial GET. Just 11 seconds after the page starts loading, the entire page is up (albeit at low resolution). Most industry experts believe that users start "timing out" or losing the focus of their attention, after about 12 seconds. If you make the page usable within that 12 seconds, most users will still be their browsing the page when the final graphics load at about 35 seconds.
 Return to main menu
If you have any problem browsing this web site, please contact stevegrn@attbi.com |