This document is to assist in understanding and correcting issues with ADSL wiring within the customer premises.  For general information on ADSL, visit How ADSL Works in our support pages.


ADSL Wiring – Businesses

There are three common situations for company ADSL wiring:  security systems, POTS splitters, and line filters.

Please note that in these diagrams, the lines leading to telephones imply a digital and/or multi-line system, and the lines leading to fax machines and modems imply an individual separate line operating on it’s own telephone number.  Also, modems do not need to be wired directly to the line as shown in some examples; the line can be wired to an RJ11 wall-jack and the modem connected to that jack.

Security & Alarm Systems

Alarm systems often require a the same kind of copper connection as ADSL connections, which often results in both services being attached to the same line – usually a fax line.  This can interrupt the ADSL freqency band or an entire line for many reasons (alarm tests, etc).  Alarm companies often insist that the alarm be the first device on the line, however this is known to cause problems.  This image shows the most common wiring situation, which is often problematic:

Security - improper

This setup can occur even without the ADSL filter by the fax machine, as that’s not the real source of concern.  Problems occur because the security/alarm system have the power to break the connection, leaving anything connected above it on the same line, like the fax machine and ADSL modem in this diagram, at its mercy.

The ideal configuration looks like this:

Security - proper

The POTS splitter divides the connection, leaving the security/alarm unit in-place on the same line, but separates the modem so that it doesn’t rely on the unit to pass the signal through.

POTS Splitters

A POTS splitter is designed to separate the ADSL signal range from the telephony signal range.  This has several benefits.

  • First, as above, it allows separation from security type systems.
  • Second, it allows for the ADSL signal to be run to another destination, using another cable pair.  For example, if your fax machine is in a busy central area and you’d like the ADSL modem in a small server-room or computer area, you could use a POTS splitter at the demarcation point, and run the fax machine to the central area phone-jack, and the ADSL signal to a phone-jack in the computer room.
  • Third, it helps to attain a cleaner signal (and therefore better performance) on both devices attached.  The fax machine will not have any ADSL signal approaching, and the modem will not have any telephony signal coming its way.

Businesses with a plain line but no security system attached to it are usually best to use a POTS splitter in this configuration:

Business - POTS Splitter

 Line Filters

A line filter is designed to block out the ADSL signal so that it doesn’t bother another device.  In a business where the ADSL connection is tied to the fax line, the fax machine can often complain and/or show poor performance from the extra frequencies it’s getting. By attaching a line filter to the phone cable that runs into the fax machine, this can be resolved.  This method will work, often reliably, but it is not ideal.

Business - Filter

This doesn’t, change that the modem is still getting the telephony/fax signals from the splitter, as drawn above. We always recommend that businesses consider POTS splitters over the regular splitter/filter combination.


ADSL Wiring – Residential



ADSL Wiring Legend

Description Diagram Example
Actual Example
Demarcation Point – where the  telephone wiring enters the premises   Demarcation point  Demarcation point
ADSL Modem – creates an internet link between the telephone infrastructure and the customer premises  ADSL Modem  ADSL Modem
Standard RJ11 (telephone plug) wall-jack RJ11 wall-jack  Telephone wall-jack
ADSL filter – installed on telephone lines to filter out the ADSL frequencies and improve phone audio quality ADSL Filter ADSL Filter #2 ADSL Filter #1
Security alarm system Security alarm  Security alarm
POTS Splitter – separates the phone frequency range and the ADSL frequency range so they can be run on separate wires POTS splitter  POTS Splitter
Phone splitter – small, plastic & copper unit that plugs into a single wall-jack and gives two or more RJ11 plugs Phone splitter    RJ11 splitter