A DSL filter is an analog low-pass filter

Low-pass filter

A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...

 installed between analog devices (such as telephones or analog modems) and a plain old telephone service

Plain old telephone service

Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

 (POTS) telephone line, in order to prevent interference between such devices and a digital subscriber line

Digital Subscriber Line

Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...

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

Typical installation for an existing home involves installing DSL filters on every telephone, fax machine, voice band modem, and other voiceband device in the home, leaving the DSL modem as the only unfiltered device. For wall mounted phones, the filter is in the form of a plate which hangs on the standard wall mount, and upon which the phone hangs in turn.

In cases where it is possible to run new cables, it can be advantageous to split the telephone line after it enters the home, installing a single DSL filter on one leg and running it to every jack

Telephone plug

A telephone plug is a type of male connector used to connect a telephone to the telephone wiring in a home or business, and in turn to a local telephone network. It is inserted into its female counterpart, a telephone "jack", commonly affixed to a wall or baseboard...

 in the home where an analog device

Analog device

Analog device is an apparatus that measures continuous information. The measured analog signal has an infinite number of possible values. The only limitation on resolution is the accuracy of the measuring device. Such apparatus operates with variables which are constituted by continuously measured...

 will be in use, and dedicating the other (unfiltered) leg to the DSL modem. Some devices, such as monitored alarms and Telephone Devices for the Deaf

Telecommunications device for the deaf

A telecommunications device for the deaf is a teleprinter, an electronic device for text communication over a telephone line, that is designed for use by persons with hearing or speech difficulties...

, mainly certain older models using an acoustic coupler

Acoustic coupler

In telecommunications, the term acoustic coupler has the following meanings:# An interface device for coupling electrical signals by acoustical means—usually into and out of a telephone instrument....

, may be hardwired and may not easily accept a DSL filter. Some of these devices can be successfully filtered with a DSL filter or splitter, especially if the hardwired connection is converted into a jacked connection.

Some DSL modems have filtering circuitry built-in, to which the telephones and fax machines can be connected.

Modulation Techniques and specifications

• ITU G.992.1
  ITU G.992.1
  In telecommunications, ITU G.992.1 is an ITU standard for ADSL using discrete multitone modulation. G.DMT full-rate ADSL expands the usable bandwidth of existing copper telephone lines, delivering high-speed data communications at rates up to 8 Mbit/s downstream and 1.3 Mbit/s upstream.DMT...
  
   (G.DMT),
• ITU G.992.2
  ITU G.992.2
  In telecommunications, ITU G.992.2 is an ITU standard for ADSL using discrete multitone modulation. G.Lite does not strictly require the use of phone line splitters, but like all ADSL lines generally functions better with splitters.G.lite is a modulation profile which can be selected on a DSLAM...
  
   (G.Lite)
• ITU G.992.5
  ITU G.992.5
  ITU G.992.5 is an ITU standard, also referred to as ADSL2+ or ADSL2Plus. Commercially it is notable for its maximum theoretical download speed of 24 Mbit/s.-Technical information:...
  
  (G.DMT2+)