The ABC of ADSL
Summary Description How does ADSL work, and is
it better than cable? 5 Minute Guides looks at the new breed of
high-speed data connections.
Author
Publication
Roullas Top10 Simon Vandore
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No
Editorial InformationArticle Location
Article Topic 5 Minute Guides
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Story Group 000806
Post Date 31/07/2000 08:14 AM Status Posted Entered by Simon
Vandore on 26/07/2000 02:11 PM
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What is ADSL?
It's a high-bandwidth technology for transmitting digital data
over everyday copper telephone wiring, while still carrying
analog phone calls. The letters stand for asymmetric (or
asynchronous) digital subscriber line. By upgrading telephone
exchanges, the existing twisted-pair copper telephone network can
be used to provide similar data speeds to cable and satellite
Internet. Like cable Internet, ADSL has a high download bandwidth
and a comparatively low upload bandwidth, meaning it is aimed at
Internet users rather than service providers. The catch is that
ADSL only works over relatively short ranges of several
kilometres between the user and the telephone exchange (under
5.5km, depending on the variety of ADSL), so it's not for
everybody.
How fast is it exactly?
There are various forms of ADSL offering from 512Kbps to 6.1Mbps.
Nowadays when people say 'ADSL' they are generally referring to
the International Telecommunications Union (ITU-T) standard known
variously as G.Lite, DSL Lite, Universal ADSL, or more correctly
as ITU-T standard G-992.2. This was developed with Internet
industry input and does not require the installation of a 'signal
splitter' at the receiving end to distinguish between analog and
digital transmissions. This means it sacrifices a little speed to
make it much easier and cheaper for customers to hook up. ADSL
can theoretically be offered at up to 6Mbps, but it is more
commonly delivered at about 1.5Mbps.
What's the difference between DSL and ADSL?
You might see ADSL referred to as xDSL, because it is one of
about five or six variants of the digital subscriber line
technology. For example, Consumer DSL (CDSL) is a proprietary
variant owned by chipset maker Rockwell. Rate-adaptive DSL
(RADSL) was developed by Westell in the US to gauge the line
quality between the user and the telephone exchange and adapt the
data speed to suit conditions. High bit rate DSL (HDSL) was the
original DSL, and unlike ADSL is symmetric, offering the same
speeds upstream and downstream. It is offered to businesses in
Australia by Telstra. HDSL has a lot in common with the later
Symmetric DSL (SDSL). Over very short distances, Very high speed
DSL (VDSL) can theoretically use the technology to provide about
10 times the speed of ADSL.
Who's offering ADSL in Australia?
Telstra is deploying its ADSL technology as you read this,
commencing service in a range of metropolitan areas in August
2000. However, the company was ordered by the Australian
Competition and Consumer Commission (ACCC) last July to allow
rival phone companies to use its local call network for ADSL.
Primus also has an ADSL service for residential users called
Jetstream, priced at $75 per month (after the purchase of a $400
modem), having beaten Telstra to market. In partnership with
Cisco, Request DSL (part of resources group Pilbara Mines) is
spending $2.85 million to enter the same market, but its service
will be rolled out by Telstra's Network Design and Construction
company (NDC). Request DSL will be available in Perth, Adelaide,
Brisbane, Sydney and Melbourne in that order.
Does ADSL work better than cable Internet?
It depends. Cable TV companies designed their networks to carry
digital television, not Internet data. Telephone companies
designed their networks to carry voice calls, not digital
connections. Both have been adapted to suit the Internet age and
each makes sacrifices in its delivery of high bandwidth
connections. ADSL is likely to offer more consistent, reliable
speed as you're not sharing your connection with your neighbours
-- on the other hand, you've practically got to have the
telephone exchange next door to ensure the service will work! The
difference may come down to upload speeds, which are slightly
higher for cable.
Fortunately, both technologies are 'always on', which means you
are always connected and you do not have to open a 'call' to your
provider. Reports from the US indicate users often prefer ADSL to
cable, but cable Internet is still very popular. In Australia,
with our relatively low population and high distances, it's
probably best to take whatever bandwidth you can get!
What about regional areas?
As with cable Internet, it looks like most rural areas are going
to miss out on ADSL. No doubt the Universal Service Obligation
(USO) required of Telstra (and soon to be tendered out to rival
phone companies) by the government will be applied to broadband
Internet, but regional users may have to content themselves with
Big Pond Advance's satellite Internet service. ADSL will probably
be offered in large regional towns when local telephone exchanges
are upgraded, but due to distance limitations it is unlikely to
ever reach outlying farms and smaller towns. Satellite offers
high download speeds, but also high ping times (more milliseconds
between sender and receiver) and relies on an analog modem
connection for upstream data. On the other hand, Telstra is
maximising transmission distances in its rollout of ADSL and the
technology can theoretically be extended through the use of
retransmitters to renew the signal after 5km.
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How does ADSL work, and is it better than cable? 5 Minute Guides
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