26-12-2012, 02:05 PM
Digital subscriber line
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INTRODUCTION
Digital subscriber line (DSL, originally digital subscriber loop) is a
family of technologies that provide internet access by transmitting digital
data over the wires of a local telephone network. In telecommunications
marketing, the term DSL is widely understood to mean asymmetric
digital subscriber line (ADSL), the most commonly installed DSL
technology. DSL service is delivered simultaneously with wired
telephone service on the same telephone line. This is possible because
DSL uses higher frequency bands for data separated by filtering. On the
customer premises, a DSL filter on each outlet removes the high
frequency interference, to enable simultaneous use of the telephone and
data.
The data bit rate of consumer DSL services typically ranges from 256
kbit/s to 40 Mbit/s in the direction to the customer (downstream),
depending on DSL technology, line conditions, and service-level
implementation. In ADSL, the data throughput in the upstream direction,
(the direction to the service provider) is lower, hence the designation of
asymmetric service. In symmetric digital subscriber line (SDSL) services,
the downstream and upstream data rates are equal.
History
Theory behind DSL, like many other forms of communication, can be traced back to Claude Shannon's
seminal 1948 paper: A Mathematical Theory of Communication. An early patent was filed in 1987 for the
use of wires for both voice phones and as a local area network.[1]
The motivation of digital subscriber line technology was the Integrated Services Digital Network (ISDN)
specification proposed in 1984 by the CCITT (now ITU-T) as part of Recommendation I.120, later reused as
ISDN Digital Subscriber Line (IDSL). Employees at Bellcore (now Telcordia Technologies) developed Asymmetric Digital Subscriber Line (ADSL) and filed a patent in 1988,[2] by placing wide-band digital
signals above the existing baseband analog voice signal carried between telephone company telephone
exchanges and customers on conventional twisted pair cabling facilities.[3] Consumer-oriented ADSL was
designed to operate on existing lines already conditioned for Basic Rate Interface ISDN services, which itself
is a switched digital service (non-IP), though most incumbent local exchange carriers (ILECs) provision
Rate-Adaptive Digital Subscriber Line (RADSL) to work on virtually any available copper pair facility
—whether conditioned for BRI or not. Engineers developed higher-speed DSL facilities such as High bit rate
Digital Subscriber Line (HDSL) and Symmetric Digital Subscriber Line (SDSL) to provision traditional
Digital Signal 1 (DS1) services over standard copper pair facilities.
A DSL circuit provides digital service. The underlying technology of transport across DSL facilities uses
high-frequency sinusoidal carrier wave modulation, which is an analog signal transmission. A DSL circuit
terminates at each end in a modem which modulates patterns of bits into certain high-frequency impulses for
transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a
corresponding bit pattern that the modem retransmits, in digital form, to its interfaced equipment, such as a
computer, router, switch, etc. Unlike traditional dial-up modems, which modulate bits into signals in the
300–3400 Hz baseband (voice service), DSL modems modulate frequencies from 4000 Hz to as high as
4 MHz. This frequency band separation enables DSL service and plain old telephone service (POTS) to
coexist on the same copper pair facility. Generally, higher bit rate transmissions require a wider frequency
band, though the ratio of bit rate to bandwidth are not linear due to significant innovations in digital signal
processing and digital modulation methods.
Operation
Basic technology
Telephones are connected to the telephone exchange via a local loop, which is a physical pair of wires. Prior to the digital age, the use of the local loop for anything other than the transmission of speech, encompassing
an audio frequency range of 300 to 3400 Hertz (voiceband or commercial bandwidth) was not considered.
However, as long distance trunks were gradually converted from analog to digital operation, the idea of
being able to pass data through the local loop (by utilizing frequencies above the voiceband) took hold,
ultimately leading to DSL.
For a long time it was thought that it was not possible to operate a conventional phone-line beyond
low-speed limits (typically under 9600 bit/s). In the 1950s, ordinary twisted-pair telephone-cable often
carried four megahertz (MHz) television signals between studios, suggesting that such lines would allow
transmitting many megabits per second. One such circuit in the UK ran some ten miles (16 km) between
Pontop Pike transmitter and Newcastle upon Tyne BBC Studios. It was able to give the studios a low quality
cue feed but not one suitable for transmission.[citation needed] However, these cables had other impairments
besides Gaussian noise, preventing such rates from becoming practical in the field. The 1980s saw the
development of techniques for broadband communications that allowed the limit to be greatly extended.
The local loop connecting the telephone exchange to most subscribers has the capability 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 higher frequencies used by DSL travel only short
distances). The pool of usable channels is then split into two different frequency bands for upstream and
downstream traffic, based on a preconfigured ratio. This segregation reduces interference. 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 they are usable.
Naked DSL
A naked DSL (a.k.a. standalone or dry loop DSL) is a way of providing DSL services without a PSTN
(analogue telephony) service. It is useful when the customer does not need the traditional telephony voice
service because voice service is received either on top of the DSL services (usually Voice over IP) or
through another network (mobile telephony).
It is also commonly called a "UNE" for Unbundled Network Element, in the USA. It has started making a
comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of
AT&T's merger with SBC,[5] and Verizon's merger with MCI,[6] those telephone companies have an
obligation to offer naked DSL to consumers.
Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of
telephone landlines in the US dropped from 188 million in 2000 to 115 million in 2010, while the number of
cellular subscribers has grown to 277 million (as of 2010).[7] This lack of demand for landline voice service
has resulted in the expansion of naked DSL availability.
Typical setup
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.
DSL technologies
The line-length limitations from telephone exchange to subscriber impose more restrictions on higher
data-transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method
of delivering "triple play" services (typically implemented in fiber to the curb network architectures).
Technologies like GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that
allow the conversion of copper based ISDN, ADSL and DSL over fiber optics.