What is ADSL
ADSL II is an advanced form of
ADSL that achieves the same throughput as regular ADSL, yet with
lower power consumption and other important technical
The typical fixed rate ADSL
can also be configured to be rate-adaptive. This means that the
modem can automatically adjust its transmission speed to the quality
and length of the line. If it is connected to a line of a long
length, it will automatically reduce the bit-rate in order to
provide a connection at the highest speed possible. This is useful
for lines that are longer than the typical 4 km supported by the 8
Mbps fixed rate ADSL modems. As you can see from the following
graph, fixed rate ADSL can reach about 90 percent of customers in
Europe. When you consider RADSL, telcos can reach nearly 100 percent
of their customers worldwide.
As telephone companies try to extend
their optical fiber backbone networks closer to the customer it
becomes prohibitively expensive to engineer a dedicated optical
fiber cable for the last few hundred meters into each customer
location. New communications infrastructure costs increase as they
are provided closer and closer to the customer, since the resources
become less shared. The last few hundred meters are the most
expensive, since they will often be dedicated to a single customer.
Economic reasons therefore dictate the consideration of a strategy
of fiber-to-the-neighborhood (FTTN) or fiber-to-the-curb (FTTC) or
for larger buildings fiber-to-the-basement (FTTB), combined with the
re-use of the existing copper cable for the last few hundred yards.
In these network architectures
an optical fiber cable would be installed to new remote stationed
equipment. The remote equipment will tap onto the existing copper
line (at frequencies above those already in use for POTS or even
ISDN) and transmit at very high bit-rates using VDSL modem
technology. Data rates of 13, 26 and even 52 Mbps are possible,
since only a short segment (the final few hundred meters) of the
cable is used.
The data rates and
transmission methods for VDSL have yet to be finalized by the
international standards bodies. Using today's 10:1 rule of thumb,
upstream data rates of typically 6 Mbps are being considered.
However, 13 Mbps symmetric is also available. The world's first
fully working VDSL prototype was demonstrated by Orckit at Telecom
Geneva in June 1995.
are the specific benefits of DSL technologies compared with
competitive technologies (HFC, fiber-optics,
The key benefit of DSL is that
by transmitting on an existing telephone line, it reuses the
existing infrastructure of installed copper cables. This saves the
costs (as much as $1000-1500 per home) of installing a new dedicated
wide bandwidth fiber optic cable. DSL is therefore very attractive
for those with access to existing copper telephone lines.
Unlike most alternatives, DSL
technology does not require a large upfront expenditure. Individual
modem links can be provided as new customers demand service, and
therefore costs are incremental. Some competitive methods such as
hybrid-fiber coax (HFC, or cable) demand a high-up front expenditure
to deploy head-end and cable infrastructure. This is often a high
fixed cost regardless of how many customers are initially connected.
Such systems are therefore highly sensitive to service penetration
(i.e. the percentage of homes that take a service).
From the technical standpoint,
HFC cable networks have high capacity, although this capacity is
shared by all the customers connected, meaning that the actual
datarate is much lower. Also, most cable networks are designed for
broadcast and are therefore one-way networks. For example, today
less than 10% of cable networks in the USA are able to provide
two-way communication, and extensive upgrades (such as exchanging
one-way amplifiers for two-way amplifiers) are needed.
How much do DSL
The costs to the telecom
operator depend greatly on the type and quantity of modems ordered,
and the features that are required. In the case of ADSL and VDSL,
the technology is only now reaching maturity and costs will continue
to fall over the next few years. With the expanding market volumes,
manufacturers are increasingly integrating the systems into smaller
chips and smaller systems, enabling lower costs. This is similar to
the trend that was seen with analog modems which rapidly evolved
from large, expensive boxes to very small units and PC card plug-ins
that are available from a variety of outlets at low cost. Since DSL
services are currently only available through the local or
competitive telephone company or Internet Service Provider, the cost
to the end-user is dictated by their service provider.
Are there any
problems with using DSL over old copper?
No, there are no problems with
using DSL over old copper - that's the beauty of DSL. DSL technology
is designed to cope with worst-case interference from adjacent
cables (crosstalk) and has spare margins in the design. However, a
telephone region may contain a percentage of very old, very poor
quality cables with poor connections and poor deployment practice.
Some DSL transceiver technologies, such as Discrete-MultiTone (DMT)
which was adopted as the Standard line code for ADSL, are able to
adapt the signals sent over the line. DMT type systems can therefore
avoid frequency regions that are not suitable for transmission. DSL
modems have integrated diagnostics and maintenance features which
provide useful indicators to the operators of line conditions and
trends in cable loss. These features help with operations and
management of the link. If a geographic region is of particularly
poor quality it may already be affecting the quality and reliability
of the regular telephone service and will often be targeted for
rehabilitation with new cables.
How effective are
DSL technologies over ATM and IP networks?
ATM and IP are types of data protocol. A
protocol is an agreed set of rules for passing and transferring
pieces of data. IP is the basic protocol used by the Internet. ATM
is a new protocol that will enable different types of service such
as voice, data and video, to be integrated onto the same link, and
has begun to be deployed for large high-speed backbone networks. ATM
and IP do not alone provide a network, since the protocols have to
apply to a communications link. DSL technology is one way of
providing that transport link. Both ATM and IP protocols can
therefore be used to set the rules for transporting data over a DSL
Does the US
Telecom Bill and the deregulation in European telecoms market affect
The passing of the Telecom Bill and
European deregulation open up intense competition by enabling
service providers to compete with each other's existing services.
Cable companies are able to compete with local telephone companies
by offering telephone and data services over the cable network.
Telephone companies can reciprocate by offering video services over
the copper telephone wires. In adding these new services, both
players are seeking to exploit the massive investment already made
in their existing installed infrastructures, and make incremental
upgrades as needed.
Telephone companies currently have access
to more than 600 million existing copper lines worldwide. These are
already connected into homes and business locations. By applying DSL
technology these lines can be upgraded to high-speed digital links
that can carry such new services.
Will DSL become
No, DSL will not become outdated for
quite some time. As long as copper telephone wires are used for
transporting data, and voice , DSL will be a viable technology.
Although advances in digital signal processing research and chip
fabrication continually increase the power of DSL chips and make new
features possible, the real world useful life-cycle of the
end-products is much slower and longer. It often takes years from
concept and prototype demonstration, to reach high volume network
deployments of approved products, and once approved for use, such
products typically have useful lifetimes of more than a decade.
Furthermore as the overall costs reduce during the product lifetime,
new applications of the mature technology can sometimes emerge. For
example, when copper telephone lines were installed decades ago, no
one ever dreamed that these lines would remain in use for more than
just voice service, and yet the services that they can carry have
been continuously and incrementally upgraded over the
How broad is
Orckit's DSL product line compared to
Orckit is currently active in many
flavors of DSL technology, including HDSL, SDSL, ADSL and VDSL.
Orckit develops and manufactures complete systems for sale to
telephone companies. Orckit is unique in that it also develops the
core modem chip-set technology for the equipment systems, and is
therefore able to provide flexible and rapid customization of
equipment if requested. Orckit equipment and system integrators are
intimately familiar with the internal operation of the modem
chip-sets and can provide highly flexible and rapid technical
modifications during the testing and equipment trial process, which
is necessary as telephone companies complete their deployment and
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