DSL Filter
DSL (Digital Subscriber Line) is an unlimited access, high-bandwidth, Internet service, provided through telephone lines. DSL requires a special modem and, depending on the installation method, may also require DSL filter. 

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There are two methods for installing DSL service i.e. split, or splitter-less connection. A DSL filter is not required if the split method is used. In this case, a splitter has to be installed in the main telephone cable that feeds service to the premises. Two lines emerge from the splitter, out of which one line feeds the standard RJ11 phone jacks, while the other line is used for the DSL modem. DSL filters are not required, as the splitter separates out the line feeding the DSL signal.

In the splitter-less connection method, the ISP sends a DSL modem directly to the end-user, along with several micro-filters. 

The end-user only has to connect the DSL modem to any existing RJ11 wall jack as the DSL service is sent through the main service line. To protect against any interference that might be created by the DSL service, DSL filters are provided for these lines.

A DSL filter has RJ11 connectors on both ends and is a very small rectangular device. The subscriber simply inserts the dsl filter into the RJ11 jack, and then plugs the telephone line into the dsl filter. The DSL filters are also required for the fax machine lines and answering machine lines.