Microfilter Tutorial

This page attempts to give an overview of the use of ADSL microfilters and splitters to help you plan your broadband ADSL connection.

A normal UK telephone line has two copper wires, loosely twisted together, that connect from your local telephone exchange to the master socket in your house. Usually, you have two pairs coming into your house, the second pair can be used to give you a second telephone line.

The copper pair is connected to your master socket, and is converted into 3 wires that go to your telephones and extension sockets around your home. The third wire is used to carry a ringing signal in the UK network.

Most BT master sockets in use today are of the "NTE5" variety - they have a removable front plate that can be used to separate the BT-provided line from the user's own internal wiring. If you remove the front plate then you are disconnecting all the internal wiring and you have access to the BT line directly. This can be useful for testing purposes - to make sure that a fault is not being caused by your internal wiring.

So, a typical set-up looks something like this, before ADSL is installed:

When ADSL was first launched in the UK, the process involved a BT visit to your home to install a splitter in the master socket (this was before the introductions of "self-install") and to connect your ADSL modem. This splitter was built-in to a new NTE5 faceplate, providing 2 sockets - 1 for the telephone and 1 for the ADSL modem, and at the same time filtering all the extension wiring. This kind of installation is also being used for high-attenuation lines where noise introduced by intenal wiring can cause problems maintaining a reliable ADSL connection.

Here's a picture of that kind of installation:

This is fine, but it does mean that your ADSL equipment has to be close to your master socket, or you'd have to run an extension cable. (It is possible to buy NTE5 faceplates with a built-in ADSL splitter to have this kind of installation now if that's what you prefer, see here.)

Today, the most common home ADSL service does not require a BT visit to your home, and you are no longer restricted to using a BT-provided ADSL modem. Instead, you install your own modem (or router, or whatever ADSL equipment you prefer) and you provide your own splitter, using so-called microfilters.

The simplest configuration is shown in the following picture. Here a microfilter is used for each telephone socket where equipment is connected. The microfilter provides 2 outputs - 1 for normal telephones, fax machines etc. and the other for the ADSL equipment. The telephone socket is a standard BT socket, the ADSL equipment socket is RJ11.

An advantage of this solution is that the ADSL equipment can be connected to any telephone extension on the same line anywhere in your house and not just the master socket, e.g. ...

Another point to note is that a microfilter is not needed just for the ADSL equipment - i.e. if you are connecting your ADSL modem and no telephone to one of your extension sockets, then that can connect directly to the telephone socket without a filter (using the correct cable - usually the same cable you use for connecting a dial-up modem to the telephone socket - RJ11 to BT plug). Like this ...

Finally, it is also possible to use a microfilter in the same way as the splitter described earlier - to filter all extension wiring from the master socket. However this requires you to change your internal wiring so that extensions are connected to the telephone port of the microfilter. This configuration is shown here ...

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