- DSL uses the same copper telephone lines that have been bringing you voice traffic for 80 years. This line type that you have always had is called Plain Old Telephone Service (POTS) by professionals. Voice traffic takes up a small part of the frequencies available over these lines. That means there is lots of room for other signals like the digital Internet. Voice uses zero to 3,400 hertz. The copper lines are capable of up to several million hertz depending on the length.
- The filter separates the low-level voice signal and the high-level digital signal, so both stay clear and pure. If the DSL signal were allowed on the same line as a normal voice or modem device, it could damage the phone or modem. They are separated from each other by bandwidth, but copper lines are prone to crosstalk interference where signals propagate, like echoes, into nearby frequencies, so a filter is needed.
- If you have ever lived close to a radio station, or had someone using a high power CB radio pass the house while transmitting, you may have heard the user's voice on the radio breaking over the television set signal. This is a result of resonant frequencies. Resonant frequencies work the same way over a telephone line; if your DSL modem is not separated from other devices, breakthrough can happen and you are not guaranteed reliable Internet service.
- If you have a computer modem, credit card machine or fax machine in your home office, you will need the filters there, also, to keep the DSL signal from interfering with them. These types of machines use a Modulation/Demodulation (Modem) protocol that happens at the top end of the voice frequencies.
- This simple little filter plugs directly into your wall socket and has two ports on it that correspond to telephone and DSL plugs. The telephone side is usually filtered by a low pass filter that allows the voice frequencies through and stops all those above. The higher DSL signal is not allowed to pass through the telephone port, and instead travels through the DSL cable straight to your computer. This is important in keeping your phone signal clear, and your Internet signal turned on; both would be in danger without the filter.
