DSL

However, the local loop of copper telephone connections to individual subscribers can usually communicate using a much wider range of frequencies, ranging from these low frequencies up to 200 - 800 kHz, depending on the quality of the circuit and the sophistication of the equipment.

DSL services were devised as a means of overcoming the restriction on the amount of data that could be transmitted over the "voiceband" (see Shannon capacity), by using these additional frequency ranges.

While DSL services often reserve the 0.3 - 4 kHz band for voice calls over the "plain old telephone service" (POTS), the frequencies above and below this range can be used to transmit data.

A DSL connection takes place between equipment of the subscriber and the telephone exchange, with some other protocol used between the exchange and whoever the subscriber really wants to connect to, typically an Internet service provider. This differs from a normal telephone connection, where the public telephone network links subscribers to each other.

In addition the subscriber may need to install a passive electronic filter (known variously as a "splitter" or "filter" or "micro-filter") if using the POTS service on the same line (and possibly also to improve the DSL termination and prevent echoes). This ensures that the DSL modem and the telephone only receive the frequencies they are designed to handle. Subscribers can plug a filter into an existing telephone socket when using a "wires-only" service; or alternatively the DSL provider may install it.

At the exchange a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. It also separates out the voice component.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over ethernet or ATM (PPPoE or PPPoA, also known as PPPoATM), with authentication with a userid and password and using PPP mechanisms to provide network details.

Example DSL technologies (sometimes called xDSL) include:

• ISDN (pre-dates the use of the term DSL)
• IDSL (ISDN Digital Subscriber Line, a data-only ISDN variant)
• ADSL (Asymmetric Digital Subscriber Line)
• HDSL (High Bit Rate Digital Subscriber Line)
• RADSL (Rate Adaptive Digital Subscriber Line)
• SDSL (Symmetric Digital Subscriber Line, a standardised version of HDSL)
• VDSL (Very high speed Digital Subscriber Line)
• G.SHDSL (ITU-T Standardised replacement for early proprietary SDSL)

See also

• Modem
• COFDM

External links