Digital Subscriber Line
Digital Subscriber Line (DSL) is a family of technologies that provides digital data transmission over the wires of a local telephone
network. DSL originally stood fordigital
subscriber loop. In telecommunications marketing, the term
Digital Subscriber Line is widely understood to mean Asymmetric
Digital Subscriber Line (ADSL), the
most commonly installed technical variety of DSL. DSL service is
delivered simultaneously with regular telephone on the same telephone
line. This is possible because DSL uses a
higher frequency. These frequency bands are subsequently separated
Typical setup and connection procedures
Physical connection must come first. On the customer side, the DSL
Transceiver, or ATU-R, or more commonly known as a DSL modem, is
hooked up to a phone line. The telephone company(telco) connects the
other end of the line to a DSLAM, which concentrates a large
number of individual DSL connections into a single box. The location
of the DSLAM depends on the telco, but it cannot be located too far
from the user because of attenuation, the loss of data due to
the large amount of electrical resistance encountered as the data
moves between the DSLAM and the user's DSL modem. It is common for a
few residential blocks to be connected to one DSLAM.
When the DSL modem powers up it goes through a sync procedure. The
actual process varies from modem to modem but generally involves the
The DSL transceiver performs a self-test.
The DSL transceiver checks the
connection between the DSL transceiver and the computer. For
residential variations of DSL, this is usually the Ethernet (RJ-45) port or a USB port; in rare models, a FireWire port is used. Older DSL modems sported
a native ATM interface (usually, a 25 Mbit serial interface). Also,
some variations of DSL (such as SDSL) use synchronous serial
The DSL transceiver then attempts to synchronize with
the DSLAM. Data can only come into the computer when the DSLAM and
the modem are synchronized. The synchronization process is
relatively quick (in the range of seconds) but is very complex,
involving extensive tests that allow both sides of the connection to
optimize the performance according to the characteristics of the
line in use. External, or stand-alone modem units have an indicator
labeled "CD", "DSL", or "LINK", which can be used to tell if the
modem is synchronized. During synchronization the light flashes;
when synchronized, the light stays lit, usually with a green color.
Modern DSL gateways have more
functionality and usually go through an initialization procedure
very similar to a PC boot up. The system image is loaded from the flash memory; the system boots, synchronizes the DSL connection and establishes
the IP connection between the local network and the service
provider, using protocols such as DHCP or PPPoE. The system image can usually be
updated to correct bugs, or to add new functionality.
The customer end of the connection consists of a terminal adaptor
or in layman's terms "DSL modem". This converts data between the digital signals used by computers
and the voltage signal of a suitable frequency range
which is then applied to the phone line.
In some DSL variations (for example, HDSL), the terminal adapter connects directly to the computer via a
serial interface, using protocols such as ethernet or V.35. In other cases (particularly ADSL),
it is common for the customer equipment to be integrated with
higher level functionality, such as routing, firewalling, or
other application-specific hardware and software. In this case,
the equipment is referred to as a gateway.
Some kinds of DSL technology require installation of appropriate
filters to separate, or "split", the DSL signal from the low
frequency voice signal. The separation can take place either at
thedemarcation point, or with filters installed at the
telephone outlets inside the customer premises. Either way has
its practical and economical limitations. See ADSL for more information about this.
At the exchange, a digital subscriber line access
multiplexer (DSLAM) terminates the DSL
circuits and aggregates them, where they are handed off onto other
networking transports. In the case of ADSL, the voice component
is also separated at this step, either by a filter integrated in
the DSLAM or by a specialized filtering equipment installed
before it. The DSLAM terminates all connections and recovers the
original digital information.
Protocols and configurations
Many DSL technologies implement an Asynchronous
Transfer Mode (ATM) layer over the low-level bitstream
layer to enable the adaptation of a number of different
technologies over the same link.
DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers
effectively connect into a single subnet. The earliest
implementations usedDHCP to provide network details
such as the IP address to the subscriber equipment,
with authentication via MAC address or an assigned host name.
Later implementations often use Point-to-Point
Transfer Mode (ATM) (Point-to-Point
Protocol over Ethernet (PPPoE) or Point-to-Point
Protocol over ATM (PPPoA)), while authenticating with a userid and password and using Point-to-Point
mechanisms to provide network details.
Transmission methods vary by market, region, carrier, and
2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
CAP: Carrierless Amplitude Phase
Modulation - deprecated in 1996 for ADSL, used for
DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal