DSL (which stands for Digital Subscriber Line) is a type of technology that transfers high speed data on a standard 2-conductor common grade telephone line. DSL is usually added to an existing telephone line. When properly installed, the DSL (internet service) and phone can be used at the same time without interference. When improperly installed, telephones and other equipment using the lines (fax, computers, new phones) will cause problems with the DSL service.
First we will explain why improperly installed telephone equipment will cause interference. DSL can be used on an existing line because the frequencies of DSL are far above the frequencies used in voice conversations. This allows the DSL to function without interference with phones; however it works the other way around. The problem is that the circuitry in any device that connects to the phone line isn't designed to work with DSL frequencies. Telephone equipment is always "listening" to the line so the phone knows when to ring, the answering machine to pickup and your home alarm system needs to know when to arm remotely, check status, and dial 911. This circuitry often "shorts" across the DSL signal keeping you from getting a reliable connection, if not all the time guaranteed when you pickup the phone!
How to Properly Install DSL Service
There are two ways to keep your equipment from interfering with you DSL. The easiest is to put a "filter" between each piece of phone equipment and the line to keep them from interfering. The preferred method is to split the outside line, as it enters the house into a DSL line and a voice line and running a separate line for DSL. This is because surges in the line can cause the "filters" to go bad and then you have to one by one test for the bad one.
DSL has caught on in a few short years being affordable in the 1990's it has grown into almost every household today. This has dropped internet prices everywhere. Now that a human being doesn't come to your home to install things anymore, we provide DSL installation tutorials and offer our services for your benefit. Here is what a typical installer would do:
They would start at the "demarc box". They added a little box (alternatively called a splitter, filter, low-pass filter, or technically a "ASDL NID POTS Splitter") to the wall next to the demarc box. In the demarc box, they disconnect all wires that went to the phones inside the house and moved them over to the new little box. Then they run a jumper from the demarc box to the filter.
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Then they run a wire from the filter, around and through the house, to wherever your computer is.
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Next putting a jack on the wall there, marking it "DSL", plugging the DSL modem into it, connecting it to your computer, and even configuring your operating system with the proper addresses and options and such to get you connection up and make sure it was working.
That's the proper way, the common and way I suggest using is:
Now that it is more common to have DSL installation crews are becoming less likely so the service providers came up with a different way of doing things...one that didn't involve sending out an installer. The "self-install" kit contains several filters along with the DSL modem and an installation guide. The user is supposed to install a plug-in "filter" on all devices that connect to the phone line. Then plug the DSL modem directly into the phone line.
  The biggest problem with the self-install kit approach is the assumption that the person installing the kit can install filters on absolutely all equipment connected to the telephone line . This is usually not a problem ...except for one important device: the security system. As explained previously, your security system may be connected to your phone line, and may cause problems with your DSL connection. Security systems are often connected to the phone line in a different manner than ordinary telephones because they have the ability to "seize the line." This feature allows them to interrupt any current phone call in order to dial out to the alarm company. Without this feature, an intruder would be able to keep the alarm from dialing out simply by picking up any inside phone and dialing a digit.
Unfortunately, this line seizure feature makes it impossible to use a standard self-install filter on the security system the same way it is used on other telephone equipment. The bottom line is that people that have line seizing security systems will not be able to self-install their DSL without some extra work.
Note: Some service providers refer to DSL connections as ADSL, ASDL, or SDSL.
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