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A DSL filter blocks DSL frequencies from landline phones to reduce interference. It can be installed between the wall jack and phone, or a splitter can be used. Filters can be purchased for $3-$10 USD. Noise on the line may indicate other issues.

A DSL filter is a small inline device that blocks digital subscriber line (DSL) Internet frequencies from landline telephone equipment to reduce interference. It’s an inexpensive and optional addition to any home or business network.

DSL service works over traditional copper telephone lines, channeling incoming and outgoing Internet data at frequencies too high to interfere with traditional voice traffic. This allows you to use your phone for voice calls while simultaneously browsing the Internet, a feat not possible with dial-up Internet services. While the frequencies used for DSL Internet service shouldn’t interfere with voice calls, some people prefer to block these frequencies from their phone line to avoid losses. A DSL filter does just that.

There are several models of DSL filters, but generally the filter is a small rectangular device that fits easily in the palm of your hand. One end might have a tail with an RJ11 connector for plugging into the wall outlet. The other end, a female RJ11 port to receive telephone line. A DSL filter installed between the wall jack and the phone will cut off DSL frequencies from reaching the phone. Many people also choose to put a DSL filter on their answering machine and fax lines.

There are cases where DSL filters are not needed. When installing DSL service, a technician can optionally install a splitter on the incoming telephone line, before service enters the building. The splitter adds a second line of service, with DSL using one line and telephone service the other. The line with telephone service goes to the jacks used for telephone, fax, and answering machines, while the line with DSL service goes to the jack that will house the DSL modem. Separating the service this way is a cleaner solution, but more expensive, as it requires a technician to visit the premises. It also makes it more difficult to move the modem later, in case you rearrange your home or office.

In a DSL installation without a splitter, which is the most common situation, Internet signals go to all wall outlets and DSL filters can block out potential line noise. However, line noise can be caused by many factors, including old wiring, old jacks, faulty phone equipment, or line noise from the phone company itself. If you’ve already installed a DSL filter and are still having problems, contact your local telecommunications company and ask them to test the line for noise. This should be free and only take a moment. If the line is free of noise, the problem lies somewhere with the equipment or cabling and isn’t necessarily DSL related.
Many DSL service providers include one or more DSL filters with installation kits. For those who supply their own modems or need additional filters, they can be purchased at any electronics store where telephone or computer equipment is sold. Prices commonly range from $3 to $10 United States Dollars (USD). If there is no noise on the line and no problems with your fax or answering machines, then you probably don’t need filters.

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