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July 2001

Scot Robertson

VoDSL: What It Is & How It Works

BY SCOT ROBERTSON


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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