DSL, xDSL
DSL (Digital Subscriber Line) is a modem technology for broadband data access over ordinary copper telephone lines (POTS) from homes and businesses. xDSL refers collectively to all types of DSL, such as ADSL (and G.Lite), HDSL, SDSL, IDSL and VDSL etc. They are sometimes referred to as last-mile (or first mile) technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations.
xDSL is similar to ISDN in as much as both operate over existing copper telephone lines (POTS) using sophisticated modulation schemes and both require the short runs to a central telephone office (usually less than 20,000 feet). However, xDSL offers much higher speeds - up to 32 Mbps for upstream traffic, and from 32 Kbps to over 1 Mbps for downstream traffic.
Several modulation technologies are used by various kinds of DSL:
- Discrete Multitone Technology (DMT)
- Simple Line Code (SLC)
- Carrierless Amplitude Modulation (CAP)
- Multiple Virtual Line (MVL)
- Discrete Wavelet Multitone (DWMT)
To interconnect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer (DSLAM). The DSLAM aggregates data transmission from all access DSL lines and then connects to an asynchronous transfer mode (ATM) network. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections.