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.
Most DSL technologies require that a signal splitter be installed at a customer premise. However, it is possible to manage the splitting remotely from the central office. This is known as splitterless DSL, "DSL Lite," G.Lite, or Universal ADSL.
Protocol Structure
The following table provides a summary of various DSL specifications:
| Type | Description | Data Rate | Mode | Distance | Applications |
| IDSL | ISDN Digital Subscriber Line | 128 kbps | Duplex | 18k ft on 24 gauge wire | ISDN service Voice and data communication |
| HDSL | High data rate Digital Subscriber Line | 1.544 Mbps to 42.048 Mbps | Duplex | 12k ft on 24 gauge wire | T1/E1 service Feeder plant, WAN, LAN access, server access |
| SDSL | Single Line Digital Subscriber Line | 1.544 Mbps to 2.048 Mbps | Duplex | 12k ft on 24 gauge wire | Same as HDSL plus premises access for symmetric services |
| ADSL | Asymmetric Digital Subscriber Line | 1.5 to 9 Mbps 16 to 640 kbps | Down Up | Up to 18k ft on 24 gauge wire | Internet access, video on-demand, simplex video, remote LAN access, interactive multimedia |
| DSL Lite (G.Lite) | Splitterless DSL | 1.544 Mbps to 6 Mbps 16 to 640 kbps | Down Up | 18k ft on 24 gauge wire | The standard ADSL; sacrifices speed for not having to install a splitter at the user premise. |
| VDSL | Very high data rate Digital Subscriber Line | 13 to 52 Mbps 1.5 to 2.3 Mbps | Down Up | 1k to 4.5k ft depends on data rate | Same as ADSL plus HDTV |
Related protocols: ISDN, DSL, ADSL, HDSL, VDSL, SDSL, G.Lite, IDSL, ATM
Sponsor Source: DSL is defined by ITU-T (http://www.itu.org) and DSL Forum (http://www.dslforum.org)
Reference:
http://www.dslforum.org/: Educate yourself about DSL
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/adsl.htm: Digital Subscriber Line