Digital Subscriber Line, or DSL, is a family of technologies that provide digital data transmission
over the wires used in the “last mile” of a local telephone network. Typically, the download speed of
DSL ranges from 128 kilobits per second (kbit/s) to 24,000 kbit/s depending on DSL technology
and service level implemented. Upload speed is lower than download speed for Asymmetric
Digital Subscriber Line (ADSL) and equal to download speed for Symmetric Digital Subscriber Line (SDSL).
The origin of DSL technology dates back to 1988, when engineers at Bellcore (now Telcordia Technologies)
devised a way to carry a digital signal over the unused frequency spectrum available on the twisted
pair cables running between the telephone company’s central office and the customer premises.
Implementation of DSL could permit an ordinary telephone line to provide digital
communication without interfering with voice services. However, incumbent local e
xchange carriers (ILEC) were not enthusiastic about DSL, since it was not as profitable as
installing a second phone line for consumers who preferred simultaneous dial-up internet
and voice connections, and the broadband data connection would cannibalize existing ISDN customers.
This changed in the late 1990s when cable television companies began marketing broadband Internet access.
Realizing that most consumers would prefer broadband Internet to dial-up Internet,
ILECs rushed out the DSL technology, which they had delayed implementing, as an attempt to
win market share from the broadband Internet access offered by cable television operators.
DSL is the principal competition of cable modems for providing high speed Internet access
to home consumers in Europe and North America. Older ADSL standards can deliver
8 Mbit/s over about 2 km (1¼ miles) of unshielded twisted pair copper wire.
The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM.
 Some customers, however, are located farther than 2 km (1¼ miles) from the central office, which significantly
reduces the amount of bandwidth available (thereby reducing the data rate) on the wires.
The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication
and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy,
the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible.
This is known as commercial bandwidth. Dial-up services using modems are constrained by the Shannon capacity of the POTS channel.
At the local telephone exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a
64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the
Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).
The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS.
Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused
bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured.
Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL)
until new channels are deemed unusable.
Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection.
More usable channels equates to more available bandwidth, which is why distance and line quality are a factor.
The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio.
Once the channel groups have been established, the individual channels are bonded into a pair
of virtual circuits, one in each direction.
Like analog modems, DSL transceivers constantly monitor the quality of each channel
and will add or remove
them from service depending on whether or not they are usable.
The commercial success of DSL and similar technologies largely reflects the fact that in recent decades,
while electronics have been getting faster and cheaper, the cost of digging trenches in the ground for new cables (copper or fiber) remains expensive.
All flavors of DSL employ highly complex digital signal processing algorithms to overcome
the inherent limitations of the existing twisted pair wires.
Not long ago, the cost of such signal processing would have been prohibitive but because of VLSI technology,
the cost of installing DSL on an existing local loop, with a DSLAM at one end and a DSL modem
at the other end is orders of magnitude less than
would be the cost of installing a new, high-bandwidth fiber-optic cable over the same route and distance.
Most residential and small-office DSL implementations reserve low frequencies for POTS service,
so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service.
Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL.
Only one DSL modem can use the subscriber line at a time. The standard way to let multiple
computers share a DSL connection is to use a router that establishes a connection between
the DSL modem and a local Ethernet or Wi-Fi network on the customer’s premises.
Once upstream and downstream channels are established, they are used to connect the subscriber to
a service such as an Internet service provider.