Fact Checked

Should I get a DSL Filter?

R. Kayne
R. Kayne

A DSL filter is a small in-line device that blocks Digital Subscriber Line (DSL) Internet frequencies from landline telephone phone equipment to decrease interference. It is an inexpensive, optional addition to any home or office network.

DSL service runs on traditional copper telephone lines, channeling incoming and outgoing Internet data on frequencies too high to interfere with traditional voice traffic. This makes it possible to use the phone for voice calls while simultaneously surfing the Internet – a feat not possible with dial-up Internet services. Although the frequencies used for DSL Internet service should not interfere with voice calls, some people prefer to block these frequencies from the phone line to avoid bleed-through. A DSL filter does just that.

DSL filter.
DSL filter.

There are several models of DSL filters, but generally the filter is a small rectangular device easily fitting in the palm of your hand. One end might have a tail with an RJ11 connector for plugging into the wall jack. The other end, a female RJ11 port for receiving the phone line. An installed DSL filter sitting between the wall jack and telephone will eliminate DSL frequencies from reaching the phone. Many people also opt to place a DSL filter on answering machine and fax lines.

A DLS filter blocks DSL Internet frequencies from landline telephone equipment to decrease interference.
A DLS filter blocks DSL Internet frequencies from landline telephone equipment to decrease interference.

There are cases in which DSL filters are unnecessary. At the time DSL service is installed, a technician can optionally install a splitter on the incoming phone line, prior to service entering the building. The splitter adds a second service line, with DSL using one of the two lines, and phone service, the other. The line with phone service runs to jacks used for phone, fax and answering machines, while the line with DSL service runs to the jack that will accommodate the DSL modem. Separating the service in this manner is a cleaner solution, but more costly, as it requires a technician visit the premises. It also makes moving the modem later more difficult, should one rearrange the house or office.

In a splitterless DSL installation, which is the more common situation, Internet signals runs to all wall jacks, 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 telephone company itself. If you have already installed a DSL filter and are still having problems, contact the local telco and request they test the line for noise. This should be free and only takes a moment. If the line is noise-free on their end, then the problem lies somewhere with your equipment or wiring, and is not necessarily DSL related.

Many DSL service providers include one or more DSL filters with installation kits. For those who provide their own modems or who need extra filters, they can be purchased at any electronics store where telephone or computer equipment is sold. Prices commonly range from $3 - $10 US Dollars (USD). If you do not have line noise and are not having problems with your fax or answering machines, you probably do not require filters.

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Discussion Comments


@Melonlity -- Most of those DSL providers may give DSL filters to their customers, but how many of those customers actually install them? Sure, you might have a technician show up and install those, but we were just sent a package of equipment back when we had DSL and were told to set up everything ourselves. In that environment, I bet a lot of people don't bother with the filters.

And they are very useful. We kept having Internet trouble after we installed DSL and trying to diagnose it drove me nuts. I figured out the problem was our phone in the garage. I missed that one when setting up those DSL filters, and the interference from that line caused all kinds of trouble with our Internet connection. Once I put a filter on the line, that took care of the problem.


Most DSL companies provide these filters to all customers and advise them to install them on every phone jack in the house. In my experience, those filters prevent line interference, dropouts and all sorts of things. Should you get a DSL filter? Absolutely.

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      DSL filter.
    • A DLS filter blocks DSL Internet frequencies from landline telephone equipment to decrease interference.
      By: Alex
      A DLS filter blocks DSL Internet frequencies from landline telephone equipment to decrease interference.