A
D S L Frequently Asked Questions
What does ADSL stand
for?
Asymmetric
Digital Subscriber Line. ADSL is a modem technology
that transforms ordinary phone lines (also known
as "twisted copper pairs") into high-speed
digital lines for ultra-fast Internet access. ADSL
also
enables access to corporate networks for telecommuters, as well as exciting
new interactive multimedia applications such as multiplayer gaming, video on
demand and video catalogs.
How does ADSL work?
ADSL modems use digital coding techniques
to squeeze up to 99% more capacity out of a phone
line without interfering with your regular phone
services. That means you could be simultaneously
talking on the phone or
sending a fax - while surfing the World Wide Web.
How fast is ADSL?
ADSL provides speeds up to 8 Mbps
downstream (to the user) and up to 1 Mbps upstream,
depending upon line length and loop and line conditions.
What are the main benefits
of ADSL?
- Simultaneous Internet and voice/fax
capabilities over a single telephone line
- Uninterrupted, high-speed Internet access that's always on-line
- Cost-effective solution for residential customers, telecommuters and small
businesses
- Data Security that exceeds other technologies
What is the market
for ADSL?
ADSL enables two general types of
applications -- interactive video and high speed
data communications. Interactive video includes movies
on demand, other video on demand such as delayed
TV segments, video
games, video catalogs, and video information retrieval. Data communications
covers Internet access, telecommuting (remote LAN access), and specialized
network access. The strength of ADSL compared to other high speed transmission
alternatives (such as cable modems or Fiber To The Neighborhood (FTTN) lies
in the number of existing telephone lines -- now approaching 750 million --
compared to new cabling which has reached comparatively few homes and almost
no small businesses.
When is the market
for ADSL?
ADSL services are being deployed
in several areas around the world already. Millions
of people can now take advantage of this broadband
service. See the Point and Click Map on the DSLLife
site for more details.
What
will ADSL do to ISDN?
That
all depends upon the telephone companies offering both
services.
The two services are not the same -- ISDN provides two voice channels or a
128 Kbps data channel while ADSL is predominantly a data pipe providing an
asymmetrical bandwidth of up to 8 Mbps downstream and 1
Mbps upstream under good conditions. However, an ADSL access network will be
an overlay network and therefore will not require the expensive and time-consuming
switch upgrades that held ISDN back for so long. If ADSL service prices resemble
ISDN service prices, then one would expect ADSL to be favored for Internet
and video applications.
Can the Internet keep
up with so much speed?
The Internet infrastructure is constantly
being upgraded to handle the rapidly increasing use
of the Internet. Many servers operate at 56 Kpbs.
In addition, the Internet backbone has grown in an
unplanned fashion and a connection may see 20 or
more routers, creating significant delay, and for
TCP connections, bandwidth throttling. However, a
great deal of work is underway to (1) increase server
access speeds, (2) improve backbone and NAP bandwidth,
(3) increase router speeds, and (4) introduce ATM
into the backbone for much lower latency. Futhermore,
many Internet service providers will implement proxy
or cache servers for frequently visited web pages,
creating local access at least for these pages. At
the very least, ADSL will make many Internet experiences
far much better than voice band modems and the resulting
market pressures will inevitably lead to capacity
increases.
How does ADSL compare
to cable modems?
ADSL provides a dedicated service
over a single telephone line; cable modems offer
a dedicated service over a shared media. While cable
modems have greater downstream bandwidth capabilities
(up to 30 Mbps), that bandwidth is shared among all
users on a line, and will therefore vary, perhaps
dramatically, as more users in a neighborhood get
online at the same time. Cable modem upstream traffic
will in many cases
be slower than ADSL, either because the particular cable modem is inherently
slower, or becasue of rate reductions caused by contention for upstream bandwidth
slots. The big difference between ADSL and cable modems, however, is the number
of lines available to each. There are no more than 12 million homes passed
today that can support two-way cable modem transmissions, and while the figure
also grows steadily, it will not catch up with telephone lines for many years.
Additionally, many of the older cable networks are not capable of offering
a return channel;
consequently, such networks will need significant upgrading before they
can offer high bandwidth services.
What
is xDSL?
XDSL
is the name which as been coined for the family of
digital subscriber line technologies ranging from HDSL
through to VDSL (see accompanying Glossary with ADSL
firmly in the middle of the speed/capacity band.
What is the controversy
between CAP and DMT line codes?
CAP and DMT are two "line codes" or
modulation systems currently on the market today
for ADSL. The Forum has taken no position on the
merits or demerits of either. Each line code has
its own case to make. All major ADSL vendors belong
to the ADSL Forum and set aside their differences
while working together to create system guidelines
and market positions. As such, it can be fairly represented
that the line code issue will have little bearing
on the size, speed, or character of the ADSL market
as a whole.
What is CAP?
CAP stands for Carrier-less Amplitude/Phase
modulation, and describes a version of QAM in which
incoming data modulates a single carrier that is
then transmitted down a telephone line. The carrier
itself is suppressed before transmission (it contains
no information, and can be reconstructed at the receiver),
hence the adjective "carrier-less."
What is DMT?
DMT stands for Discrete Multi-Tone,
and describes a version of multicarrier modulation
in which incoming data is collected and then distributed
over a large number of small individual carriers,
each of which uses a form of QAM modulation. DMT
creates these channels using a digitial technique
known as Discrete Fast-Fourier Transform. DMT is
the basis of ANSI Standard T1.413.
What is DWMT?
DWMT stands for Discrete Wavelet
Multi-Tone, and describes a version of multicarrier
modulation in which each carrier is created by Wavelet
transform rather than Fourier Transform.
How can I find out
when I can have ADSL?
Please see the ADSL Service Deployments
table on this web site. You should contact your local
telephone service and/or Internet provider to ask
them when they will make such services available.
What does ADSL offer
the Internet service provider?
Today, high speed Internet access
is seen by many as the first "killer" application
of ADSL - there is a pent up demand for higher access
speeds and only ADSL can practically provide these
speeds. Today's analog modems routinely offer 28.8
Kbps or 33.6 Kbps and, in a few but growing number
of instances, up to 56kbps. However, 56 Kbps is probably
the practical limit for analog modems. ISDN can increase
this to 128 Kbps but this is still slow compared
to ADSL speeds of between 1.5 Mbps and 8.0 Mbps.
ADSL will open a whole new world of virtually instantaneous
downloading of massive graphics and even video applications
over the Internet.
What
speed range does DSL-lite support and is it compatible
with the regular
ADSL standard in terms?
At
present, the exact specifications of a splitterless
ADSL are still being discussed. The objective is to
define a service that makes installation as easy as
plugging an analog modem into a wall socket. DSL-lite
makes the installation of a splitter (a filter that
separates the frequencies normally reserved for POTS
service) device on the customer premises side
obsolete, and the service provider will not have to send out an engineer to
the customer site.
How compatible this service will be with other services, and under which speeds
it will operate, has yet to be defined by the UAWG. However, it is clear that
DSL-lite will not offer the full ADSL speeds, but operate at speeds of up to
1.5Mbit/s downstream.
If
I am on the phone, will that decrease my bandwidth
when downloading and by roughly how much?
Voice
communications over POTS (plain old telephone service)
operates at frequencies below 4KHz; all other services,
such as ADSL, will use the frequencies above that.
So no matter whether you are using the POTS frequencies
or not, the bandwidth available to ADSL is not affected.
What
do you mean by Asymmetric?
ADSL
is asymmetric in that it transmits data faster downstream
than it does upstream. This is appropriate for surfing
the Internet, for example, where large amounts of data
are transferred from the network to the user, and much
less data is passed back to the network. Traditional
systems, like most modem connections, have been symmetric.
ADSL usually has a downstream rate that is approximately
10 times faster than the upstream speed, but some ADSL
offerings today deliver the same speed bidirectionally,
e.g. 384kbps bidirectional.
ADSL boasts a great
bandwidth. Can it be shared among two or more PCs,
so that both can be connected to the internet at
the same time?
Yes. If the modem has one, or multiple
Ethernet 10baseT interfaced, it can connect an entire
LAN. That way, all users on the LAN can share the
higher connected bandwidth. If the modem has only
one 10baseT interface, you can connect it to a mini-Ethernet
hub which will distribute the bandwidth over a LAN.
What does the compression
of the ADSL do the resolution of the video images?
ADSL is simply a date pipe and can
be used to carry, amongst other things, video images
that are already compressed. Today's video compression
technology is such that it demands about 1 megabit
for VCR quality pictures and 2-3 Megabits for broadcast
(superb) picture quality.
(For example DirectTV satellite systems use about 3-4 Mbit/s.) ADSL can support
up to 6 Mbit/s per second and is therefore able to carry at least one such
video channel.
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