DSL modem

A DSL router consists of a box with an RJ11 jack to connect to a standard subscriber telephone line.

Upon powering the router it may take several minutes for the local network and DSL link to initialize, usually indicated by the status lights turning green.

Before DSL, voice-band modems transmitted information through the telephone network with audio frequencies within that bandwidth, which limited them to a data rate of about 56 kbit/s.

The device at the local switching center which communicates with the DSL modem is called a Digital Subscriber Line Access Multiplexer (DSLAM), which is connected directly to the Internet.

With ADSL, the modem and the DSLAM communicate by a protocol called discrete multitone modulation (DMT), which is a form of frequency division multiplexing.

Each data stream is sent using an error-correcting code to allow minor bit errors due to noise to be corrected at the receiving end.

[2] Thus interference or poor quality lines will generally not interrupt transmission, but only cause the data rate of the modem to degrade.

The modem demodulates the carrier, extracting the data stream from each carrier signal, performs error correction, puts the data together again in the proper order, and sends it to the computer over the Ethernet line, or for a wireless (Wi-Fi) network by radio signals.

Because the telephone lines were never designed to carry such high frequency signals, DSL is distance-sensitive.

It serves fundamentally the same purpose as the voice-band modem that was a mainstay in the late 20th century, but differs from it in important ways.

Most of these differences are of little interest to consumers, except the greater speed of DSL and the ability to use the telephone even when the computer is online.