DSL Technology Tutorial

DSL delivers broadband to more people today than any other technology. Roughly two-thirds of all broadband subscribers are DSL subscribers, and there are more new DSL subscribers each month than new subscribers for all other broadband access technologies combined.

DSL, which stands for Digital Subscriber Line, is a technology that delivers broadband speeds over distances of miles or kilometers via copper wiring. DSL was originally delivered over the same wires that are used to provide traditional voice telephony services. These wires run from a telephone company’s Central Office (CO), the location where voice switching and other traditional telephony functions are performed, to a subscriber’s home or business. Increasingly, DSL is delivered from a device situated closer to the subscriber’s home or business that is connected to a CO via an optical fiber link, and then to the subscriber’s premises via copper wires. In all cases, however, DSL delivers broadband over the copper connections that exist already in almost every residence and business in the developing and developed worlds.

This architecture is depicted in the figure below. At the CO, or at a remote location typically connected to the CO via fiber optics, there is a DSL Access Multiplexer (DSLAM) that sends and receives broadband data to many subscribers via DSL technology. At each subscriber’s location, there is a modem (modulator-demodulator) that communicates with the DSLAM to send and receive that subscriber’s broadband data to and from the Internet and other networks. A DSLAM communicates with many individual subscriber modems. Each subscriber’s modem is dedicated to that subscriber’s broadband connection.

Voice services utilize only a small fraction of the total information carrying capacity of copper connections. In an analogous manner to Ethernet technology, which can transmit a Gigabit (more than one billion bits) per second of data over copper connections or the equivalent of tens of thousands of simultaneous phone conversations, DSL exploits the information carrying capacity of copper lines to deliver broadband services over long distances.

To engineers, “DSL” means a set of formal standards for communicating broadband signals over copper lines. It also means equipment that complies with those standards. The principal DSL standards are published by the International Telecommunications Union (ITU), a standards body based in Geneva, Switzerland, that establishes standards for communications systems. Within the ITU, there is a division responsible for communications over copper, named the ITU-T, and a division responsible for communications using radio technology, known as the ITU-R. The ITU has several other divisions including one devoted to telecommunications in the developing world, known as the ITU-D.

DSL standards have evolved significantly since the first DSL standards were established in the early 1990’s. The DSL standards have evolved to support higher data rates, to take advantage of advances in equipment technologies, and to ensure that DSL can coexist on copper lines with other communications standards such as Integrated Services Digital Network (ISDN), an early digital voice and data service that is still in use in many countries. The table below lists some of the principal DSL standards in use today.

Common Name Peak Speed Standard Deployment Status
ADSL1 8 Mbps ITU-T G.992.1 Pervasive
ADSL2+ 24 Mbps ITU-T G.992.5 Pervasive
VDSL2 50-75 Mbps ITU-T G.993.2 Pervasive
Vectored VDSL2 120+ Mbps ITU-T G.993.5 Standard complete, field use by 2011

1) “Mbps” means Megabits per second.  A Megabit is a million bits.

2) DSLAMs and subscriber modems are capable of the peak speeds listed in the table. Lower speeds may be delivered depending on the service packages offered by a subscriber’s DSL provider, and also on the provider’s network design and management practices.

3) There are several variants of the VDSL2 standard. The peak speed is dependent on the particular VDSL2 variant implemented by the DSL service provider.

With few exceptions, DSL technology is unique among broadband access technologies in that subscribers do not compete with one another for broadband access. Because each subscriber has their own copper connection to the DSLAM, all subscribers can achieve the peak speeds listed in the table above so long as the connection from the DSLAM to the Internet or other networks has adequate capacity. This is a significant advantage of DSL relative to other broadband access technologies where subscribers share a single physical connection, such as in a cable network, or a limited allocation of radio frequencies, such as in a 3G or 4G wireless network.