A microfilter is a small, relatively inexpensive, inline device which is intended to filter the signal on your DSL-enabled phone line. The microfilter consists of a small box with two sockets - marked phone and DSL, and a short length of cable with a phone plug on the end.

DSL works by utilising the frequencies which can be carried by copper wiring, but which are not used by the POTS (Plain Old Telephone System).

Without filtering, the telephone system would experience high frequency interference, and the modem may be confused by the low frequency phone signals. The cut-off between the two is around 4kHz.

With supplier-installed systems, the filter is often installed in the master socket, and the consumer does not have to worry. There are two sockets on the front of the master, one for phone, one for DSL. This is why supplier-installed systems require the ADSL modem to be within 2 metres of the phone line.

Microfilters come in where consumers install their own system. There should be a filter between the main telephone line and any device on the system.

The following diagrams show possible configurations, with the phone company on the left side of the master socket, and the user's premises on the right.

Symbols: M=Master Socket, F=Filter, D=DSL Modem, P=Phone. L=Low Frequencies Present, H=High Frequencies Present.

1. Microfilter per device. The simplest, but more costly setup uses existing extension wiring, and is filtered at each extension socket.

                                       H
                                      +--D
                       L              |
                 +--|F|--P      +----|F|
             LH  |              |      
        --|M|----+---------+----+---
                     LH    |    
                           +--|F|--P
                                  L

2. Two circuits. A user-installation with two circuits might place a microfilter on the master socket, and then run any extensions from there.

                  H              L
                +---D          +--P
                |         L    |
         --|M|-|F|-----+-------+---
                       |
                       +--P
                        L

3. Master-socket filter. When installed by the supplier, the master socket will often be exchanged for one incorporating filters.

              H                L
            +---D            +--P
            |          L     |
         --|M|-------+-------+---
                     |
                     +--P
                      L 

4. Sharing a filter. If your phone and DSL modem are close to each other, they can both be plugged into a single filter.

                                   H
                                  +--D
                                  |
                            +----|F|--P
               HL           |       L 
        --|M|----------+----+---
                       |    
                       +--|F|--P
                             L

5. Internal exchange. A further possible problem which has come to my attention is if you have an internal exchange, which already uses microfilters on the extensions. In this case, you should apply a DSL microfilter on the master socket, splitting the DSL from POTS. Your phone system can then exist on the user-side of the first microfilter. In the diagram, E is your current exchange. The filter from the master socket will keep DSL signals from the exchange, and also keep exchange control signals away from the phone line.

                  H
                 +--D       
                 |          
         --|M|--|F|--- - - (E)
                     L   

Some DSL modems do not require filtering as long as all telephones on the network are filtered. The converse is NOT true, since the high frequencies would interfere with the phone line.

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