This page presents alternative wiring for KPN ADSL in The Netherlands. It may be valid in other countries as well.
If you subscribe to ADSL at KPN in The Netherlands, you are required you to install a splitter at the point where your telephone line enters your house. This splitter splits the incoming signal into a pure ADSL signal and a pure telephony signal.
Fig.1: Wiring recommended by KPN.
There are several disadvantages for you to the installation required by KPN:
The alternative installation method on this page overcomes all the above problems. You can continue using all existing wiring in your house, and you can place the ADSL splitter next to your ADSL modem.
An analogue telephone line uses frequencies upto 4 kHZ, as well as DC. ADSL operates on frequencies above 26 kHz. So the splitter does nothing more than blocking all frequencies above 26 kHz to the telephone and blocking all frequencies below 4 kHz and DC to the ADSL modem. It is a simple frequency splitter.
Fig.2: Block diagram of ADSL splitter.
Most ADSL modems have internal DC blocking and high-pass filter. This makes the DC blocking and high-pass filter in the splitter redundant.
If you put an in-line low-pass filter in the cable of a telephone you can connect it without problems before the ADSL splitter.
Fig.3: Alternative wiring.
I have used D-Link Inline ADSL Filters DSL-10MF. My experience with this filter is good. Everything operates as it should. One telephone showed some high frequency intermodulation noise while the ADSL modem was active. I solved this by putting two filters in series with that telephone. Later I replaced this filter with an Excelsus Z-BLOCKER Filter Z-200SM. This filter blocks the high frequencies better. Of cause other brands of filters can be used as well.
I have asked KPN for a reaction to this article. KPN admits in-line low-pass filters are a good alternative to the central splitter. They do not like the combined use of in-line low-pass filters and their splitter. According to KPN there is a special circuit in their splitter, which enables KPN service personel to detect whether the splitter is present or not. They did not tell me what circuit they are using. KPN did perform a measurement on my connection. Their measurement tool was unable to determine whether a splitter was present or not.
From KPN's reaction I conclude that it is desirable to unplug all equipment connected via in-line low-pass filters in case KPN is called for service. Unplugging all equipment connected via in-line low-pass filters makes the installation identical to the installation required by KPN, so they must be able to perform service according to their standard procedures.
 ETSI TR 101 728 V1.2.1 (2002-05); Access and Terminals (AT); Study for the specification of low pass filter section of POTS/ADSL splitters; Free downloadable at http://www.etsi.org/