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ADSL: Wiring and Filters

Community Veteran
Posts: 6,983
Thanks: 8
Registered: 10-04-2007

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.[/listShocked]
    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
  • 52 REPLIES

    ADSL: Wiring and Filters

    My comments:

    1 The ADSL connection on a splitter is just a pass-through so if you don't want a telephone where your ADSL device is then you can use a "Modem" lead to connect your ADSL device to a phone socket.

    2 Things hanging out of phone sockets are a nuisance. A "Faceplate" splitter is often a great improvement.

    3 In my experience a faceplate splitter improved the line quality for voice calls. Two plugin splitters were enough to degrade the echo-cancelation.
    Community Veteran
    Posts: 14,469
    Registered: 30-07-2007

    ADSL: Wiring and Filters


    1 The ADSL connection on a splitter is just a pass-through so if you don't want a telephone where your ADSL device is then you can use a "Modem" lead to connect your ADSL device to a phone socket.

    This may not be true for all filter / splitters . There have been several members who have had disconnection problems where they had their ADSL modem plugged in without a filter / splitter. Fitting a filter / splitter solved the disconnections which may indicate some kind of filtering is occuring for the ADSL connection as well as the phone connection.

    So to play safe I would always recommend an ADSL filter / splitter be used to plug your ADSL modem into the wallsocket. And if you are having problems try using the filter / splitter supplied with the ADSL modem.

    ADSL: Wiring and Filters

    But, logically, if it is true of some filters, doesn't it mean that plugging in one of these filters will make no difference?

    ADSL: Wiring and Filters

    Cleanly illustrated and explained, however I feel that installing or using an existing BT NTB5 box is often the easiest and neatest way.

    The NTB5 is in fact a master socket, and can be used to replace your master socket. If (as is often the case) all the extensions are wired from this socket then replacing the lower portion of the NTB5 with the relevant ADSL adapter automaticaly filters all extensions and has both a filtered and unfiltered socket on its face.

    This has also the advantage of getting rid of all those dangling filters.

    ADSL: Wiring and Filters

    Alternaticly, you can purchase one of the modified version of the NTe5 ADSL faceplate (It is NTe5, not NTB5).

    Using the spare pairs within extention cabling, you can then have both filtered and unfiltered traffic on a single line.

    It does take a bit of managment to remember which cables you have used, but you can then designate a specific socket in the house as the ADSL socket.

    ADSL: Wiring and Filters

    The diagram and advice worked for me .I guess when we all stop believing the claims and promises of the computer industry and its offshoots(LOOK NO HARDWARE HASSLES!!!) and realise that computing is like other so called easy enter hobbies like gardening or diy, anybody CANT just do it or there wouldnt be so many people making a living fixing my house and garden.Just dont believe the hype and teach the kids to prepare to fix stuff. P.cs are not like t.vs and radios (which I am close to mastering)they are more like women,just when you think you,ve got ,em sussed.......high maintianance prone to breakdowns and crash a lot.Yes I have a daughter.Still just think how they benifit mankind ,if I wasnt typing this at 12.30 am I would probably be wasting time drinking my money away in a pole dancing club .........wonder if I can still get in ? Bye

    ADSL: Wiring and Filters


    This may not be true for all filter / splitters .

    I just pried open a "Dynamode" DYMS 603UK splitter as supplied by Plusnet and it's a pass-through.

    One thing that might confuse things though, for direct connection the lead must connect the voice pair of the BT socket ( wires 2 and 5) to the voice pair of the RJ11 (wires 3 and 4). A direct lead would connect one side of the ADSL signal to the ring circuit and probably leave the other side floating, which might occasionally get connections where the signal is very strong.

    Leads sold as replacement modem leads will work. Leads sold as replacement telephone leads will be straight through and will not work. The leads can handmade with difficulty, the trick is getting the middle two wires into the right positions on the BT plug.

    For what it's worth the filter in the Dynamode doesn't look all that impressive. It's mostly there to stop the phone interfering with ADSL service and not the other way round.



    Ok I am confused how to get my connection working.

    The phone socket is downstairs, my bedroom is upstairs.

    I am using one of the long wind up phone extension leads.

    I have setup :

    downstairs in bt wall socket :

    adsl splitter going to a Telephone Doubler

    (main telephone socket >> splitter >> telephone doubler)

    then I plug my extension cable bt socket into one of the telephone doubler sockets

    upstairs I have the filter plugged into one of the bt sockets and the rj11 cable coming out of the filter to my usb modem.

    this does not synch

    from what I have been reading here it should go like :

    main telephone socket >> telephone doubler >> adsl splitter

    or would I just be better with a really long rj11 to rj11 cord and setup :

    main telephone socket >> adsl splitter >> rj11 cable?


    ADSL: Wiring and Filters

    The setup your have sounds OK.

    However, do you have anything plugged into the other part of the socket doubler downstairs?

    If you have a phone, a second filter should be used there.

    ADSL: Wiring and Filters

    If I read the layout correct, There is a DSL filter plugged into the main socket and then a phone splitter is plugged into that. The extension cable is plugged into the phone splitter.
    If this is the way it is configured, it is WRONG.
    The extension will be going through the low pass side of the filter and stopping the DSL signal.
    You will require another DSL filter if you have a phone upstairs, then do this:-
    Plug phone splitter into master socket. Plug DSL filter into one side of phone splitter and plug phone into that. Plug extension into other side of phone splitter and run that upstairs. Plug DSL filter into end of extension cable and then modem & phone as required.

    this is what i have


    Ok I do not have a phone upstairs, just the dsl modem, but need an extension cable to get upstairs.

    I am thinking of just getting a rj11 to rj11 extension cable, as I only need the rj11 cable to run upstairs.

    For now though I need to get a horizontal telephone doubler or a bt plug to rj11 convertor.



    ADSL: Wiring and Filters

    Phone extensions are cheaper than RJ11 to RJ11, even taking into account that you will need a DSL filter on the end.

    what about this - does this apply


    Just wondering if this applies :

    Note that you don't need a splitter at the point where your modem is connected. The modem can live quite happily with the low-frequency telephone signal. In fact the wires in the splitters American Style ADSL connection are just connected directly to the BT plug that goes into the wall. This means that if you don't have a phone plugged into the same point as your modem, you could save some money and just use a lead with a BT plug on the end to connect your modem. We have leads like this, (Stock Code RJP/BTPxx) on our

    ADSL: Wiring and Filters

    Yes, you do not need a filter if ONLY the modem is connected at the end of your phone extension lead. However, the phone extension lead will have a BT socket at the end and you will need a BT to RJ11 convertor to connect to your modem, or the easier to obtain method, a DSL filter !!