The download speed reflect the visitor bandwidth in bytes/second. This information can be detected using:
You can use this information to adapt your web site to the visitors capabilities.
The download speed is computed for each visit by using a specific download tracker script. Because a large amount of data is required in order to make a good estimation of the available bandwidth (let's say 100 KB), the script should be used at most one time during the visit. When possible, such tests should be made with usefull data (for example applet ressources like images and sounds).
Download speed can be wrong in several cases and should be considered with care. If the test data is already in the browser or useragent cache, the reported download speed will be very high (about 40 MB/s). If the client internet connection is very busy (e.g. parallel downloads), the reported download speed will be very low.
The "unknown download speeds" come from visits in which the download speed couldn't be computed. Two possible reasons:
A second method to estimate the download speed is to compute it from the log file information. The concept is the following: a visitor comes to the web site and see one page and the associated ressources (e.g. images); the download speed is the number of bytes in the first page seen (including ressources), divided by the total loading time of the page (including ressources). This usually gives a less precise download speed than the applet method because of the caching mechanisms used by some browsers. However, the estimation is good enough to determine which kind of internet connection your visitors do have.