Rate-adaptive digital subscriber line
(R-ADSL)
R-ADSL operates with the same transmission
rates as ADSL, but the modem adjusts dynamically to varying lengths and
quantities of the twisted-pair local lines. It makes possible to connect over
different lines at varying speeds. Connection speed is negotiated by the
end-points when the line synchronizes up or as a result of a signal from the
central office.
R-ADSL is designed to increase range and
noise tolerance by sacrificing upstream speed. The modem automatically creates
a greater frequency band for the downstream than the upstream band. If line
noise or signal degradation is large, the upstream bandwidth is decreased and
may fall up to 64 kbps, which is equal to the speed of ISDN.
Uni-digital subscriber line (UDSL)
Uni-DSL technology was originally developed
by Texas Instruments and is meant for one DSL for universal service. Thus all
discrete multi-tone services can be provided from one line card or home
gateway, which yields in ease of deployment and more affordability. Further,
Uni-DSL is backward- compatible to ADSL, ADSL2, ADSL2+, VDSL and VDSL2. The
aggregate downstream and upstream speed provided by UDSL is at least 200 Mbps.
Etherloop
Ethernet local loop is the next-generation
DSL technology that incorporates the features of Ethernet and DSL. It is
capable of delivering speeds up to 6 Mbps over a reach of 6.5 km on a
moderate-quality copper line. Etherloop uses half-duplex transmission and is
almost unaffected by interference caused by poor line quality. So it is
possible to provide Internet services up to long distances. Etherloop modems
can also be used as a LAN extension in a situation where direct Ethernet is not
possible due to distance limitation.
Very high-speed digital subscriber line
(VDSL)
VDSL is the fastest xDSL technology over a
single copper-pair wire, supporting downstream rates of 13 to 52 Mbps and
upstream rates of 1.5 to 2.3 Mbps. It was standardized by ITU-T recommendation
G.993.1 in November 2001. The standard VDSL specified both quadrature amplitude
modulation (QAM) and discrete multi-tone (DMT) modulation systems. Seven
different frequency bands are used in VDSL, which enable customization of
upstream and downstream data rates. Further, this technology provides a
cost-effective alternative to fiber-to- the-home (FTTH). This asymmetric
technology operates over 300 meters to 1.4 km only from the central office.
VDSL
is the fastest xDSL technology over a single copper-pair wire, supporting
downstream rates of 13 to 52 Mbps and upstream rates of 1.5 to 2.3 Mbps.
One method of VDSL deployment is to run a
fiber up to an optical network unit in the neighborhood and then run copper
pair to the customer's premises within 1.2 km of the optical network unit. The
high bit rate support makes VDSL ideal for services like high-definition
television (HDTV), as well as voice-over-Internet protocol (VoIP) and general
Internet access.
In February 2006, ITU-T recommended
discrete multi-tone for an advanced version of VDSL called 'VDSL2' and the
recommendation was defined in G.993.2. This new and advanced version is capable
of supporting triple-play services such as data, voice, video, HDTV and online
gaming. Data rates of more than 100 Mbps simultaneously in both the downstream
and upstream are achievable up to about 300 meters.
VDSL2 utilizes bandwidth of up to 30 MHz.
The performance greatly depends on the loop attenuation and degrades quickly
from 250 Mbps at source to 100 Mbps at 0.5 km and 50 Mbps at 1 km. Its
performance is equal to ADSL2+ for distances more than 1.6 km. ADSL-like
long-reach performance is one of the key features of VDSL2.
Gigabit digital subscriber line (GDSL)
Gigabit digital subscriber line technology
is based on binder multiple-input and multiple-output (MIMO) technology.
Channel matrices generated from a binder MIMO channel model use transmission
methods that can yield more than 1Gbps symmetric data rates over four twisted
pairs of copper wire for a 300m range. In practice, extra copper pairs exist
(usually two to six pairs) in the final drop segment to connect a subscriber,
but there is almost no extra copper pair (from drop point near the subscriber
to central office) to connect all these pairs for each subscriber. So the
information-carrying capacity of a fiber can be exploited to connect back to
central office. Using this arrangement, all the unused copper pairs can be
properly vectored and bonded to be treated as a single transmission path and
can be utilized to provide high-speed data rates.
Gigabit
digital subscriber line technology is based on binder multiple-input and
multiple-output (MIMO) technology.
To sum up
DSL is drawing significant attention from
service providers because it has the ability to deliver high-speed data
transmission over existing infrastructure with relatively small changes. xDSL
family of technologies provides data and voice services at the same time and on
the same copper line, as it uses high-frequency bands for data services and
low-frequency band for regular voice services.
A
comparison of downstream data rate offered by different DSL technologies
