by Jeff Fisher
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.
Note that your service provider may refer to your DSL connection as ADSL, ASDL, SDSL, VDSL, or XDSL. This application note applies to all known forms as of this writing.
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 signalkeeping 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.
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 early days 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 ten 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:
- He would start at the "DMARC Point" (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 DMARC box. In the DMARC 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 DMARC box to the filter.
- He would then run a wire from the filter, around and through the house, to wherever your computer lived.
- He would put a jack on the wall there, mark it "DSL", plug the DSL modem into it, connect it to your computer, and even configure your operating system with the proper addresses and options and such to get you connection up and make sure it was working.
Of course, in my case, I wanted the DSL modem to go at my "head-end" in my garage where my hub and router already lived. I had the cables already installed and labeled, and the installer just plugged in the jumpers and turned on the modem. Still, it was nice to have a human being present to verify my wiring and make sure everything worked before the billing began.
So what I'm calling a proper installation involves actually "intercepting" the outside phone line before it enters the house, splitting it into a DSL side and a voice side, connecting the voice side to the inside phones, and then connecting the DSL side directly to one and only one device: the DSL modem.
The "Self-Install" Installation
As service providers ramped up installations of DSL, they realized that the limiting factor was the number of installation crews they could put on the road. 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.
In this scheme, the "raw" DSL/telephone line travels throughout the home. Telephone equipment doesn't interfere with the DSL line due to the filters that were added by the user. The DSL modem plugs into the raw DSL/telephone line, but doesn't interfere with telephone communications because it is designed not to.
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. (For more information on line seizure click here.)
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.
When Self-Install Doesn't Work
Users faced with the problem of a line-seizing security systems have several choices:
- Option 1
- The simplest option is to install a special version of the DSL filter that is designed for security systems at the RJ31X jack.
- Option 2
- If you have more than one telephone line coming into your home, have the service provider change the DSL to the line without the security system and install the self-install kit on phones that use that line. Alternatively, you could have the security company change the security system to the other, non DSL, line.
- Option 3
- If the security system is connected to the phone line via a RJ31X jack, and you do not have monitoring or use any telephone access to your security system, simply unplug the modular plug from the RJ31X jack. This will disconnect the alarm from the phone line and pass the phone line, unmolested, through to rest of your phones. The self-install kit should then be adequate. Warning: Doing this will prevent your security system from calling out in the event of an alarm. Any local bells or sirens should still work normally although you should test the system to make sure. Also, the alarm keypad may indicate a phone line problem. If your security system does not have an RJ31X jack, you can have your security company install one and then unplug the security system from it. (The above warning still applies.)
- Option 4
- The preferred method involves running new wire from the DMARC Point to the location where DSL is required. See "A Proper DSL Installation" above. For step-by-step instructions, see "Running a New Wire For DSL" below.
- Option 5
- Your last option will only work if you have only one phone line, but four wires (not just two) connected throughout your house. It may be a bit crude, but it should work, doesn't interfere with the operation of your security system, and should be fairly easy. See "Using the Second Pair for DSL" below.
Running a New Wire For DSL
For this method, you will need the following:
- A weatherproof DSL splitter..
- Some telephone wire, either the old quad wire (Belden 1242A) or Category 5 style is fine. Get enough to run from your DMARC box on the outside of your house to the location of your DSL modem.
- A wall-mounted jack (Leviton 41089-2WP) and voice-grade insert (Leviton 41106-RW]) for the DSL jack. (Although there are other methods to mount a jack. See the Vanco surface-mount jack CMTJ4WX.
- For tools, you'll need a flat blade screwdriver, pliers, and a 110 style punch-down tool.
Using The Second Pair For DSL
If your home is wired with four-conductor wire (the Red, Green, Yellow, Black stuff), and if you only have one phone line in use, and IF all four conductors are connected throughout all the intervening jacks (that's a big "if"), you can use the second pair to carry the DSL signal from the splitter to your DSL modem.
Please don't call or email us for help with this one, there is no way that we can know how your jacks are wired. Sometimes is takes a little of what I call "residential archeology". Go around with a flashlight and a medium straight blade screwdriver. Pull the plates and jacks of a few telephone jacks and see how they're wired. If there are two phone cables going into the jack, are all four wires from both cables connected to the screw terminals? If the yellow and black wires aren't, you'll have to connect them.
You'll need the following:
- A weatherproof DSL splitter. (Suttle 649A1.)
- A two-line adapter (APM2P12).
Connect the splitter out at your DMARC box. The connections are as follows:
|Network Tip||Incoming line Green|
|Network Ring||Incoming line Red|
|Voice Tip||To House line Green|
|Voice Ring||To House line Red|
|Data Tip||To house line Black|
|Data Ring||To house line Yellow|
The black and yellow wires should now be carrying the DSL signal to the second line of your jacks. At the jack where the DSL modem is to be connected, plug in the two-line breakout plug and plug the DSL modem into line two.
I take great care in writing these tutorials, but I do make mistaakes sometimes. If you find one, please let me know and I'll correct it.
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