Internet Access Guide : ADSL

Picture: ADSL network
An ADSL modem at a subscriber site communicates with
the corresponding modem at the nearest operator site.

ADSL or Asymmetric Digital Subscriber Line is a type of copper cable enhancement technology (DSL) that delivers Internet traffic at different rates from the network to the user (downstream) and from the user to the network (upstream). ADSL uses existing telephone line to carry voice and data (Internet) signals simultaneously. But since data signal occupies higher frequencies in the telephone line spectrum, it doesn't disturb the telephone service.

To create ADSL, there must be a pair of ADSL modems, one at the subscriber site and the other at the network operator site (i.e. Central Office or nearest access node). During an Internet connection, the two ADSL modems communicates, converting signal to a format that can be transferred over telephone line.  DSLAM (Digital Subscriber Line Access Multiplexer) is a device at the operator site that contains ADSL modems and interfaces to backbone network.

ADSL signal from each subscriber is split into voice and data signals by a splitter (filter) that's usually contained in the same rack with the DSLAM. The voice (telephone) signal is forwarded to a telephone switch (Local Exchange) for further connection to the telephone network (PSTN). The data signal that carries Internet traffic is sent to the ISP over backbone network for connection to the Internet.

At a subscriber site, a splitter (filter) separates the telephone signal from the data signal. Therefore the subscriber can receive or make telephone calls during an Internet connection without interference. The presence of a separate splitter is required only in Full Rate ADSL. In the case of splitterless ADSL (also known as G.Lite, ADSL Lite, or Universal ADSL), an external splitter is not required.