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VoDSL is a local access technology that enables service
providers to add voice capability to data services over
DSL lines. DSL lines are typically used today for
higher-speed Internet access and business data
connectivity, as they are priced reasonably and can be
quickly installed. VoDSL is a new type of access
technology for making a standard phone call. It promises
to deliver equal or better quality voice services than
currently provided by the local phone company, through
the standard PSTN. It is also a major revenue
opportunity for data service providers to offer an
entirely new service to their customers over the same
line, which enables them to match the offerings of
cable-based Internet service providers who are rapidly
expanding their voice services.
The battle for the local loop is driving change at a
phenomenal rate. Current economics notwithstanding, the
Telecommunications Act has given competitors equal
access to the local loop as the local phone company.
This has leveled the playing field and unleashed a
frenzy of activity as new companies vie with one another
to deliver services. The implementation of VoDSL is one
result.
A derivative of voice over network technology that
enables the sharing of a common data stream for both
voice and data access, VoDSL routes the phone call data
over the same phone line as the Internet data stream or
other data services. VoDSL not only eliminates the need
for additional lines, but can also deliver a very high
level of voice quality with a minimal impact on data
rates.
VoDSL is based on asynchronous transfer mode (ATM), a
very high-speed transmission technology. A
high-bandwidth, low-delay, connection-oriented,
packet-like (referred to as "cells") switching and
multiplexing technique, its small fixed-length cells
require lower processing overhead and allow lower
latency than other packet switching methods. In
addition, ATM allocates bandwidth on demand and adjusts
network capacity automatically to meet system needs,
making it highly suitable for high-speed converged
connection of voice, data, and video services.
Additional hardware for implementation of VoDSL is
required. At the end user location an integrated access
device (IAD) supports voice, data, and in some cases,
video. The IAD allows the user to have standard access
to services through industry standard ports such as
Ethernet for data services or a RJ-11 jack (the most
common telephone jack in the world) for telephone
applications. With increased bandwidth functionality,
each IAD can support multiple voice channels, typically
up to 16.
VoDSL does not present the quality issues common in
voice over IP. It is a local loop connection using an
ATM-based network that typically spans a single metro
area. With ATM, transmission problems are few, and loss
or mis-ordering of cells is minimal. ATM-based networks
use standard G.726 and G.728 voice codec protocols that
guarantee very high voice quality.
At the provider location, a VoDSL gateway connects
calls from the PSTN received from anywhere in the world,
into the DSL-based link. Both the IAD and the gateway
serve similar functions. They are bi-directional, as
both must encode and decode signals to accommodate the
signal transport media to which they are connected.
However, the gateway terminates calls from many clients
on one piece of equipment and could interface with a
very large communications network. It must have the
capability to process a potentially huge number of
channels.
Since VoDSL is a relatively new technology, the
hardware that accommodates flexibility through software
is crucial. While the use of fixed function ASICs can be
less expensive at installation, they are severely
limiting. As businesses grow and require more voice
channels, providers can incur significant costs to
replace IADs and gateways designed around ASICs rather
than more flexible software-driven digital signal
processors (DSPs).
Interoperability is also important. There have been a
number of proprietary implementations of VoDSL that
create interoperability issues between the IAD and the
gateway. Software-based systems facilitate modifications
to facilitate interoperability between new classes of
gateways designed to support evolving standards.
Finally, as these systems mature, flexible software
driven solutions can provide the opportunity to advance
well beyond the voice quality of today�s PSTN with
high fidelity audio services. As voice and data networks
converge, a major advantage is the improvement of voice
quality by using different software, higher data rates
or both, without the need to replace the whole network.
In addition, expanded business management services such
voice messaging, call following, and a host of new
applications can be easily realized.
Scot Robertson is product line manager at Analog
Devices, Inc. Analog Devices is a semiconductor
company that develops, manufactures, and markets
high-performance integrated circuits (ICs) used in
signal-processing applications.
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