Tutorials - ADSL > Wiring and Filters

Tutorials & FAQs: ADSL: Wiring and Filters

Some people find it very confusing when trying to work out how many filters / splitters are needed and how to put them in circuit. This tutorial is intended to shed some light on this and help those who are new to the setting up of ADSL.

Why do we need the filter(s) at all?

Basically your telephone line was originally designed to carry "Commercial Speech" between your home and the telephone exchange. This uses a band of frequencies from 300 to 3000 hertz.

ADSL uses frequencies very much higher than this speech band so you now finish up with two different systems on the one line. In order to keep these systems apart and stop them interfering with each other it is necessary to separate the two components from the telephone line in your home. This is where the Filter / Splitter comes in. It is normally a small plastic box with a short lead that plugs into your phone socket and two outputs, one for your ADSL Modem and another for a telephone (or multiple telephones on this output, but more of that later)

Inside this box are the filters that select the band of frequencies for each of the outputs, phone or ADSL, and send just the correct band to the appropriate socket.

So lets now look at how it fits together.

First a simple line with just one telephone socket.

OK, nothing complicated here: just plug the filter / splitter into the phone socket then the modem and phone into the outputs. These are usually labelled and are of different types so it's impossible to get this wrong.

Next a line and extension(s)

Still nothing earth-shattering here but it is very important that all your other telephony equipment is separated from the ADSL signals by the use of a splitter / filter -- this equipment includes Telephones, Answering machines, Sky and/or Freeview boxes, Diallers on Burglar Alarm systems, "normal" computer modems, etc, etc.

The above has looked at simple setups where the phone wires are "hard wired" with permanent sockets. However, life is often not that simple and some people have self-installed extension cables coming out of socket doublers and the like. So let's now have a look at how you could make just one filter cope with this sort of complexity.

Home installed extensions from the Master Socket.

The above diagram show exactly how my wiring is at home and as you will see, you only need one filter / splitter in this layout. All the phones are still separated from the ADSL part of the line because signals from them all have to pass through the filter before mixing with the ADSL signals on the phone line.

How to get it Wrong

If you follow the route to the ADSL modem you will see that you have passed through 2 Filter / Splitters. This will cause you severe problems because at the first Filter / Splitter at the master socket you are passing through the Low Frequency part. Remember that the ADSL part of the system is at a much higher frequency than the phone system.

What will happen here is that the high frequencies of the ADSL signal will be severely attenuated by at least 30dB with a good filter and the signal reaching your modem will be very weak -- in most cases too weak to work at all -- although, if you are close to the telephone exchange you may find it works but you suffer from dropouts.

To correct the error above

As you will see from the diagram, this is the same setup as the incorrect one. The only difference is that the positions of the first Filter / Splitter and the Double Adapter have been swapped around.

This now means that the ADSL signal is only passing through the high frequency section of one Filter / Splitter and each of the phones is filtered.

Testing if you have problems

If you do have problems with cutting-off or dropping-out, then you can always set things up as above.

This shows the Modem or Router connected directly to the Master Socket on the phone line. As there are no phones connected, a Filter / Splitter is not needed and if all is Ok when you set up like this then you will know that either you have wired something wrong or you had a faulty Filter / Splitter.

To sum up

It is not possible to cover every conceivable combination of house wiring for phones but hopefully the above drawings will help you come up with what you need for your own system and as long as you follow the guiding principles below it should function correctly.

All phones or other equipment must pass through a filter.
Make sure that the ADSL signal is only passing through one Filter / Splitter.
It can be the same Filter / Splitter for all of the phones.

If you do have a complex phone system then sit down with paper and pencil to draw up the wiring as above and see if it follows the the guidelines outlined here.

Once you have a clear idea of the rules for ADSL wiring you may want to visit the following sites for:-
Cable and Hardware available and Issues with Alarms

Original Article by: johnessex - Edited by: johnessex