DSL Installation

Digital Subscriber Line (DSL) is a technology that puts high speed digital data on standard two-conductor voice-grade telephone wiring. DSL service is often added to an existing telephone line. When properly installed, the DSL service and phone can be used simultaneously without interference. When improperly installed, telephones and other equipment connected to the telephone line can cause problems with the DSL service. This application note describes why this happens, and how to fix the problem.

Why Telephone Equipment Interferes

The DSL signal can be "superimposed" on an existing phone line because the frequencies DSL uses are far above the frequencies used in voice conversations. Telephones won't even reproduces these frequencies. So the DSL signal doesn't interfere with phones...it's the other way around. The problem is that the circuitry inside virtually any device that connects to the phone line isn't designed with DSL frequencies in mind. DSL wasn't even around when most of these products were designed! If you think about it, telephone equipment is always "listening" to the phone line...after all, the phone needs to know when to ring, right? The answering machine needs to know when to pick up, and even your alarm system may have the capability to receive incoming calls (to arm the alarm remotely, check on status, etc.) Even if these features aren't enabled, the circuitry is still connected to the phone line. This circuitry often "shorts out" the DSL signal--keeping you from getting a reliable DSL connection. Even if it doesn't, I'll guarantee it will as soon as you pick up a phone! So something must be done to keep the telephone equipment from interfering with the DSL signal.

How To Properly Install DSL Service

There are two ways to keep your telephone equipment from interfering with your DSL signal: The easiest method is to put something between each piece of telephone equipment and the line to keep the equipment from interfering. The preferred method is to split the outside phone line, as it enters the house, into a DSL line and a voice line, and run separate wires to each.

A Proper DSL Installation

Way, way back, in the beginning of DSL. I was one of those first pioneering soles (and luck enough to be near a central office) that subscribed to a DSL service. It must have been...gosh, let me think...over two years ago now! (Isn't it amazing how fast DSL has caught on in a few short years?) Back in those "good old days", an actual human being would come to your house to perform the installation. The process is easy to describe. But, depending on the layout of your house, where the phone line came in, and where you wanted the DSL jack, the actual installation could be quite difficult. In essence, here's what the installer would do: 1. He would start at the "demarc box" (a little box where the phone line comes into the house, see Network Interface Device Boxes.) He 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, he disconnected all wires that went to the phones inside the house and moved them over to the new little box. Then he ran a jumper from the demarc box to the filter