A Basic Guide to POTS/DSL Filters

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What is a POTS filter and why do I need one?

A DSL filter or POTS filter is an analogue low-pass filter installed between analogue devices (such as telephones or analogue modems) and a plain old telephone service (POTS) line, in order to prevent interference between such devices and a digital subscriber line (DSL) service operating on the same line.

Without DSL filters, signals or echoes from analogue devices at the top of their frequency range (low end frequency) can result in reduced performance and connection problems with DSL service, while those from the DSL service at the bottom of its range (high end frequency) can result in line noise and other issues for analogue devices.

DSL filters are passive devices, requiring no power source to operate. A few high-quality filters may contain active transistors to refine the signal.

Below is an example of a type of POTS filter.

POTS Filter

Maintenance and Failure

POTS filters are produced cheaply and en masse and as a result are often the cause of a number of issues with DSL connections.

These seemingly innocuous little devices separate the low-end frequencies required for voice calls from the high-end frequencies needed for your ADSL connection. However, the generally cheaply made devices are prone to fail, leading to excessive noise interfering with the speed of your connection. A tell-tale sign that one of your filters has gone faulty is noise on the line when you're making voice calls. Another symptom of a failed filter is frequent DSL disconnects or slow internet speed. The usual procedure to test for failed filters is to remove all filters from the telephone line leaving only the telephone line connected to the wall socket and router, then checking to see whether the DSL speed increased.

It is highly recommended that you replace your filter after a year of use, anything beyond that can lead to the filter being the cause of slow or intermittent connectivity.

Where can I get one?

POTS/DSL filters are cheaply and readily available from most ISPs, Telkom outlets and most computer hardware / IT stores.


Installing a POTS filter is a fairly easy procedure.

Most POTS filters have labels above or below each socket, this is typically shown as DATA or ADSL and the VOICE or PHONE. These two sockets are also typically next to each other on the POTS filter. The third socket normally has a telephone cable on it as seen in the example above.

The diagram below, illustrates how a POTS filter is typically installed.

POTS/DSL filter installation

Should you need assistance with the installation, please feel free to contact our Support Team on 010 555 0650.

The iSAT Team

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