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ADSL F.A.Q
What does ADSL stand for? : Asymmetric
Digital Subscriber
Line. ADSL is a modem technology
that transforms ordinary phone lines 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 and video on demand.
How it works ? : 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 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 benefits ? :
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.
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.
ADSL OR ISDN ?
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. 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.
Internet can handle this?
The Internet infrastructure is constantly being upgraded
to handle the rapidly increasing use of the Internet. Many servers operate
at 56 Kpbs. The Internet backbone has grown in an unplanned fashion and
a connection may see 25 or more routers, creating significant delay, and
for TCP connections, bandwidth throttling.
A great deal of work is underway to increase server access
speeds, improve backbone and NAP bandwidth, increase router speeds, and
introduce ATM into the backbone for much lower latency. Many Internet
service providers will implement proxy or cache servers for frequently
visited web pages, creating local access at least for these pages.
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 while 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 the controversy between CAP and
DMT ?
CAP and DMT are two "line codes" or modulation systems currently
on the market today for ADSL.
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).
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.
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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. 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.
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