Asymmetrical Digital Subscriber Line (ADSL) service brings high-speed Internet service to homes and businesses via standard copper telephone lines. While voice traffic and Internet service occupy different frequency bands allowing both services to be used simultaneously, interference can occur. An ADSL line filter is a small device that plugs into the telephone wall jack, featuring a female RJ-11 port for a standard telephone line. This places the filter between the incoming telco signal and the telephone, filtering out ADSL service. An ADSL line filter should be installed on every line on the premises with the exception of the ADSL modem/router line.
The frequencies used by ADSL service are of a higher band than those used by telco voice traffic, but signals at the low end of the ADSL band and signals from the high end of the voice traffic band can interact, creating static, interference, echoes, and other anomalies. An ADSL line filter will keep telephone lines free of ADSL frequencies. Lines that run to analog modems and fax machines should also be used with an ADSL line filter.
In some cases ADSL providers that make a DSL modem available to clients for self-installation will also include one or more line filters with the installation package. If more filters are required, they can be purchased from most electronic stores and from online vendors. If a client opts to provide his or her own modem to use with ADSL service, he or she will have to purchase ADSL filters. The DSL modem itself will have a built in filter for the telephone port built into the back of the device. The telephone plugged into this port should not require an external filter.
In some cases people prefer to setup incoming ADSL service by using a line splitter on the incoming telephone line at the point of entry into the building. By splitting the incoming line into two separate legs, a single ADSL line filter can be placed on the leg running to the wall jacks. The remaining unfiltered line will run directly to a single jack where the ADSL modem will be installed. This setup has the advantage of only using one ADSL filter, but a telco employee must install the line splitter. Furthermore, the unfiltered wall jack is the only place the modem can operate, making it inconvenient if one decides to rearrange the household or office at a later date.
ADSL line filters range in price from about $4 US Dollars (USD) to $11 USD or more. A special design for wall-mounted phones includes an ADSL line filter integrated into the mounting plate.