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and needs of the marketplace. Some forms of xDSL are proprietary,
some are simply theoretical models and some are widely used
standards. They may best be categorized within the modulation
methods used to encode data. Below is a brief summary of some of the
known types of xDSL technologies.
Asymmetric Digital Subscriber Line (ADSL) is the most popular form
of xDSL technology. The key to ADSL is that the upstream and
downstream bandwidth is asymmetric, or uneven. In practice, the
bandwidth from the provider to the user (downstream) will be the
higher speed path. This is in part due to the limitation of the
telephone cabling system and the desire to accommodate the typical
Internet usage pattern where the majority of data is being sent to
the user (programs, graphics, sounds and video) with minimal upload
capacity required (keystrokes and mouse clicks). Downstream speeds
typically range from 768 Kb/s to 9 Mb/s Upstream speeds typically
range from 64Kb/s to 1.5Mb/s.
ADSL Lite (see G.lite)
Consumer Digital Subscriber Line (CDSL) is a proprietary technology
trademarked by Rockwell International.
Globespan's proprietary, splitterless Consumer-installable Digital
Subscriber Line (CiDSL).
EtherLoop is currently a proprietary technology from Nortel, short
for Ethernet Local Loop. EtherLoop uses the advanced signal
modulation techniques of DSL and combines them with the half-duplex
"burst" packet nature of Ethernet. EtherLoop modems will only
generate hi-frequency signals when there is something to send. The
rest of the time, they will use only a low-frequency (ISDN-speed)
management signal. EtherLoop can measure the ambient noise between
packets. This will allow the ability to avoid interference on a
packet-by-packet basis by shifting frequencies as necessary. Since
EtherLoop will be half-duplex; it is capable of generating the same
bandwidth rate in either the upstream or downstream direction, but
not simultaneously. Nortel is initially planning for speeds
ranging between 1.5Mb/s and 10Mb/s depending on line quality and
A lower data rate version of Asymmetric Digital Subscriber Line
(ADSL) was been proposed as an extension to ANSI standard T1.413 by
the UAWG (Universal ADSL Working Group) led by Microsoft, Intel,
and Compaq. This is known as G.992.2 in the ITU standards
committee. It uses the same modulation scheme as ADSL (DMT), but
eliminates the POTS splitter at the customer premises. As a
result, the ADSL signal is carried over all of the house wiring
which results in lower available bandwidth due to greater noise
impairments. Often a misnomer, this technology is not splitterless
per se. Instead of requiring a splitter at customer premises, the
splitting of the signal is done at the local CO.
G.shdsl is an ITU standard which offers a rich set of features (e.g.
rate adaptive) and offers greater reach than many current
standards. G.shdsl also allows for the negotiation of a number of
framing protocols including ATM, T1, E1, ISDN and IP. G.shdsl is
touted as being able to replace T1, E1, HDSL, SDSL HDSL2, ISDN and
High Bit-rate Digital Subscriber Line (HDSL) is generally used as a
substitute for T1/E1. HDSL is becoming popular as a way to provide
full-duplex symmetric data communication at rates up to 1.544 Mb/s
(2.048 Mb/s in Europe) over moderate distances via conventional
telephone twisted-pair wires. Traditional T1 (E1 in Europe)
requires repeaters every 6000 ft. to boost the signal strength.
HDSL has a longer range than T1/E1 without the use of repeaters to
allow transmission over distances up to 12,000 feet. It uses pulse
amplitude modulation (PAM) on a 4-wire loop.
High Bit-rate Digital Subscriber Line 2 was designed to transport T1
signaling at 1.544 Mb/s over a single copper pair. HDSL2 uses
overlapped phase Trellis-code interlocked spectrum (OPTIS).
ISDN based DSL developed originally by Ascend Communications. IDSL
uses 2B1Q line coding and typically supports data transfer rates of
128 Kb/s. Many end users have had to suffice with IDSL service
when full speed ADSL was not available in their area. This
technology is similar to ISDN, but uses the full bandwidth of two
64 Kb/s bearer channels plus one 16 Kb/s delta channel.
Usually this stands for multi-rate Digital Subscriber Line (MDSL).
It depends on the context of the acronym as to its meaning. It is
either a proprietary scheme for SDSL or simply a generic
alternative to the more common ADSL name. In the former case, you
may see the acronym MSDSL. There is also another proprietary scheme
which stands for medium-bit-rate DSL. Confused yet?
Rate Adaptive Digital Subscriber Line (RADSL) is any rate adaptive
xDSL modem, but may specifically refer to a proprietary modulation
standard designed by Globespan Semiconductor. It uses carrierless
amplitude and phase modulation (CAP). T1.413 standard DMT modems
are also technically RADSL, but generally not referred to as such.
The uplink rate depends on the downlink rate, which is a function
of line conditions and signal to noise ratio (SNR).
Symmetric Digital Subscriber Line (SDSL) is a 2-wire implementation
of HDSL. Supports T1/E1 on a single pair to a distance of
11,000 ft. The name has become more generic over time to refer to
symmetric service at a variety of rates over a single loop.
Universal DSL. See G.lite.
Very High Bit-rate Digital Subscriber Line (VDSL) is proposed for
shorter local loops, perhaps up to 3000 ft. Data rates exceed 10
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