DSL Information

About DSL

Digital Subscriber Line, or DSL, is a technology for
bringing high-bandwidth connectivity to homes and
small businesses over ordinary copper telephone lines.
xDSL refers to the family of digital subscriber line
technologies, such as ADSL, HDSL, and RADSL.
Connection speeds for DSL typically range from 1.544
Mbps to 512 Kbps downstream and around 128 Kbps
upstream. In addition, a DSL line allows for one line to
carry both voice and data signals, and for the data part
of the line to be continuously connected. DSL achieves
higher data transfer rates by utilizing more of the
available bandwidth spectrum. Ordinary telephone
service only makes use of the 0 – 3400 Hz frequency
range, which accounts for the 56 Kbps speed limit on
standard analog modems. DSL eludes the 3400 Hz
frequency boundary by outmoding the digital-to-analog
conversion that modems perform and connecting both
ends digitally. Hence, larger bandwidth is available,
allowing higher transfer rates. 

DSL's use of the existing twisted pair infrastructure
makes it cheaper to install than other services which
require additional cable to be laid. And unlike cable
modems, DSL is not a bus technology, so the
bandwidth available to the end user is more consistent.
However, despite all of its positive attributes, DSL is
not without flaw. For instance, in order to be eligible for
DSL, the end user must be geographically within a
certain distance from the central telephone office,
otherwise the signal degradation is too great and DSL
is unfeasible (for ADSL that distance is two miles.) In
addition, numerous standards still exist for DSL,
hardware is still comparatively pricey, and service is
available only in limited areas. Despite these
drawbacks, DSL is still a faster alternative to analog
modems and ISDN, and should rival cable modems as
far as actual bandwidth offerings

What is DSL?

DSL is the acronym for Digital Subscriber Line
technology. DSL is a relatively new technology that
makes use of the old copper phone lines to
significantly increase bandwidth between the phone
companies and their customers. DSL is a service
that everyone should have if they are using a
standard dial-up connection and want faster
internet. Remember DSL uses your existing phone
line and does not require an additional phone line.
The DSL service uses your existing phone line. This
gives you 24 hour access and does not tie up your
line and cause it to have a busy signal if you’re on
the web. In its various forms -- DSL offers users a
choice of speeds ranging from 32 Kbps to more
than 50 Mbps. These digital services can and will
be used to deliver bandwidth-intensive applications
like video on demand and distance learning. Today
DSL is putting high-speed Internet access within
the reach of homes, small and medium-size
businesses. DSL takes existing voice cables and
turns them into a high-speed digital link. Over any
given link, the maximum DSL speed is determined
by the distance between the customer site and the
Central Office. Most ISPs offer a range of speeds so
customers can choose the rate that meets their
specific business needs. At the customer
premises, a DSL modem connects the DSL line to
a local-area network (LAN) or an individual
computer. Once installed, the DSL modem provides
the customer site with continuous connection to the
DSL will let you use the Internet as it was meant to
be. Web pages will load onto your computer
instantly, files will download with amazing speed
and you'll be able to play network games with
relative ease. Soon streaming audio and video will
be a common place application for DSL. 

Types of DSL

What types of DSL are available?

ADSL - (Asymmetric Digital Subscriber Line) is the most popular
form of DSL technology.

ADSL Lite - A lower data rate version of Asymmetric Digital
Subscriber Line (ADSL)

CDSL - Consumer Digital Subscriber Line (CDSL) is a proprietary
technology trademarked by Rockwell International.

EtherLoop - EtherLoop is currently a proprietary technology from
Nortel, short for Ethernet Local Loop.

HDSL - High Bit-rate Digital Subscriber Line (HDSL) is generally
used as a substitute for T1/E1.

IDSL - ISDN based DSL developed originally by Ascend

RADSL - Rate Adaptive Digital Subscriber Line (RADSL) is any
rate adaptive DSL modem, but may specifically refer to a
proprietary modulation standard designed by Globespan

SDSL - Symmetric Digital Subscriber Line (SDSL) is a 2-wire
implementation of (HDSL).

VDSL - Very High Bit-rate Digital Subscriber Line (VDSL) is
proposed for shorter local loops, perhaps up to 3000 ft.

DSL, Analog, Cable, or ISDN?

Analog Modems


Analog modems use a telephone network as is. That is, there are
no special provisions that are required to use analog modems in
today's telephone networks. Analog modems simply allow digital
data to flow over the telephone company's already analog network
by performing a digital to analog conversion for transmission onto
the network and vice versa on the receiving end. The only
necessity for analog modems is that each end of the call must
have a compatible modem. In essence, this makes analog
modem connections the most ubiquitous form of data
communications available today. However, analog modems are
thus limited by the telephone company's voice bandwidth service.
Current analog modems are struggling to achieve rates of only
56Kbps. With only a bandwidth of about 3,000 Hz, there is a
extremely small finite limit on the amount of data that may be
encoded and sent reliably through this network. User
requirements far outstrip what analog modems can obtain today.



ISDN is a telephone company technology that provides digital
service typically in increments of 64Kbps channels. ISDN has
been around for many years, but it's popularity is now only
beginning to increase due to the limitations of analog modems
and the rise of Internet usage. ISDN requires the phone company
to install services within their phone switches to support this
digitally switched connection service. Roll out of this service
initially got off to a slow start and was stalled by high costs, lack
of standards and low acceptance rate by consumers.



DSL is technology backed by telephone companies to provide
next generation high bandwidth services to the home and
business using the existing telephone cabling infrastructure. DSL
to the home over existing phone lines promises bandwidths up to
9Mbps, but distance limitations and line quality conditions can
reduce what will actually be achievable. DSL technologies will use
a greater range of frequencies over the cable than traditional
telephone services which in turn allow for greater bandwidth with
which to send and receive information. This technology is still in
the early stages of roll out with standards and products just
getting under way. Driving this market is the competition from
competing access providers and the pursuit of your Internet
access dollar.

Cable Modems


Cable modems are devices that attach to the cable TV network
connection in a home. This broadband technology is being driven
by the cable companies to provide services beyond traditional
broadcast cable TV such as Internet access. Along with xDSL, it
is still in the early stages of development. There are a number of
challenges faced by this industry, including return path
capabilities, customer service issues and standards. However,
potential bandwidth estimates range upwards of 30Mbps from the
service provider to subscriber. Cable networks are inherently
different in design than telephone networks. Cable networks are
broadcast oriented, with each subscriber in an area receiving the
same signals as all others in that area. xDSL is circuit oriented
so that each connection is independent of all others. Cable
networks are inherently hierarchical in nature and thus require two
paths, one for downstream and one for upstream. This requires
either a second cable plant for upstream or a second frequency
band allocated onto the existing system.



There are a number of different wireless schemes proposed,
planned and implemented throughout the world. Wireless access
technology takes shape in a number of different forms such as via
a satellite TV service provider or a cellular phone network.
Wireless systems can provide ubiquitous access to a large
number of subscribers in a relatively large area. Bandwidth can
range from a few kilobits a second to many megabits and be
either symmetrical or asymmetrical. Like all other technologies,
there can be deployment issues which may include spectrum
licensing, interference and noise problems, or bandwidth


What type of DSL is best for me?

ADSL is generally best for home and small
business use. This is because of its larger data rate
downstream. This means that the typical user will
be able to download faster than they can send data.
Which, is very typical of most home users.
Customers who require larger bandwidth might do
better with SDSL or HDSL.

How close to the phone company must I be?

Three miles is the longest acceptable distance from
the Central Office. Beyond that, the data rates DSL
offers begin to degrade. Usually this distance is
large enough to include most people in any
particular area.

Can the phone company use my existing wires, or do
they need to install new copper wire?

This question all depends on the results of the line
test. If the location fails to support an acceptable
data rate, this means that your existing lines may
not work with DSL. However, new lines can be
installed just like an ISDN installation.

How fast is DSL?

DSL speeds have a wide rang. Typically ADSL
users can expect maximum downstream rates of
1.5Mbps. Other DSL standards have their own
specific speeds. 

I already have ISDN; can I use DSL also?

Yes, If you go through the steps mentioned above.
But, your ISDN line will not support both DSL and
ISDN. Meaning that you must have one set of wires
for each service. Now, if you want to drop ISDN
service in favor of DSL you can use the wires that
were installed for the ISDN line. In fact, this is
preferable because the ISDN lines are newer
installations and more able to support DSL

Can I purchase my own DSL modem?

Yes, but the cost is prohibitively expensive for the
home user. It is wiser to rent the modem from your
Telco. They will be responsible for maintaining it.
Also, you are garanteed that it will interface with the
DSLAM on their end. Larger businesses might be
interested in their own DSL equipment in order to
cut out monthly charges.

Can noise on the line affect DSL?

Noise may be defined as the combination of unwanted interfering
signal sources whether it comes from crosstalk, radio frequency
interference, distortion, or random signals created by thermal
energy. Noise impairs the detection of the smallest analog levels
which may be resolved within the demodulator. The noise level
along with the maximum clip level of an analog signal path set the
available amplitude dynamic range. The maximum data rate of a
modem is limited by the available frequency range (bandwidth)
and signal-to-noise ratio (SNR) which is amplitude dynamic
range. If more of either is available, more bits may be transferred
per second. The information carrying limit was discussed
theoretically by Claude Shannon and is known as Shannon's
limit, or information theory. Because modems run close to
Shannon's limit today, no further advances will be made to
traditional telephone line modems other than incremental
improvement of V.90. The frequency range of the audio channel is
very limited at about 4KHz. V.34+ modems are limited to a
maximum data rate of 33.6Kbps by an SNR of about 36dB
caused mostly by network PCM quantization noise. While V.90
improves the SNR by utilizing the network PCM levels directly, it
is still subject to Shannon's limit. xDSL modems take advantage
of the spectrum above the telephone audio channel. While
operating with somewhat less amplitude dynamic range they
increase data rates by greatly increasing the frequency range of
the communication signal (from about 10KHz to over 1.0MHz). To
do this they require the installation of special equipment at the
central office and customer premise.


Source for information: DSL.com