A DSL filter, also known as a micro-filter, is a small in-line device sometimes required in DSL installations. It filters line interference to standard telephone phone equipment when the equipment lines share the same line as the DSL service.

DSL stands for Digital Subscriber Line, an always-on Internet service provided through in-place telephone lines. The ISP providing DSL service uses these standard telephone lines to provide a high-bandwidth channel that can deliver downstream speeds up to 3mbps. A special DSL modem is required and DSL filters may also be required, depending on the installation method.

There are two methods for installing DSL service: split, or splitterless. A DSL filter is not required if the split method is used. In this case a technician must visit the premises and install a splitter in the main telephone cable that feeds service to the premises. From the splitter two lines emerge. One line feeds your standard RJ11 phone jacks, while the other line is used exclusively for the DSL modem. Filters are not required in this case because the splitter has separated out the line feeding the DSL signal.

The disadvantage to this method is that it requires a technician visit the premises to install the splitter.

The second method has gained popularity because it does not require a technician and can be user-installed. In this case the ISP sends a DSL modem directly to the end-user, along with several micro-filters.

The DSL service is sent through the main service line so the end-user only has to connect the DSL modem to any existing RJ11 wall jack. However the signal will be feeding to all jacks, creating possible bleed-over noise on telephone, answering machine and fax machine lines. Filters are provided for these lines to block any interference that might be created by the DSL service.

A DSL filter is a very small rectangular device that has RJ11 connectors on both ends. The subscriber simply unplugs the telephone line from the wall jack, inserts the filter into the RJ11 jack, then plugs the telephone line into the filter. Fax machine lines and answering machine lines will also require DSL filters.

DSL has three great advantages over dial-up service: it's always on, it's up to 50x faster, and it does not tie up your phone line, even though it uses the phone line. Your standard telephone equipment uses an analog signal, while DSL communicates on a higher digital frequency. Therefore you can cruise the Internet using DSL service, while talking on the phone at the same time.

DSL is sometimes called ADSL. The "A" stands for asymmetric, meaning downstream data does not flow at the same rate of speed as upstream data. Home users don't usually require fast upstream rates because most of what is sent upstream, i.e. from your computer to the Internet, are small bits of code requesting Web pages. Where the fast rate is required is downstream, from the Internet to your computer, to load pages faster.

In the case of businesses, however, where high upload rates might be required for network applications, there is SDSL, or symmetric DSL. SDSL offers matching datastream flow rates. If the download rate of the SDSL service is 3mbps, the upload rate will be 3mbps as well. The high bandwidth demands of SDSL preclude the line from being used for anything but SDSL service.