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Voice over DSL sounds promising

August 5, 1999
Web posted at: 4:26 p.m. EDT (2026 GMT)

by Greg Langdon

Network World Fusion

(IDG) -- Digital subscriber line (DSL) service to date has been used for moving data over the Internet at high speeds through existing copper links.

However, a new class of equipment will broaden DSL's usefulness by allowing the movement of voice and data simultaneously over a single copper link, without architectural changes to existing networks.
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The payoff: Smaller organizations will soon be able to buy integrated, richly featured voice/data services in a way previously available to only the largest firms.

All the major telephone companies have standardized on ATM as the Layer 2 DSL transport protocol. In DSL data applications, stand-alone or networked PCs connect to a DSL modem, bridge or router, which function as network endpoints and provide a high-speed interface to the DSL service.

The DSL modem, bridge or router encapsulates the PCs' IP-based data into ATM and transmits the resulting cell flows as ATM over DSL to the carrier's central office. At the central office, ATM traffic from multiple DSL links is aggregated and multiplexed onto a common upstream link, and each cell flow is directed toward its destination by one or more ATM switches.

DSL links are ready-made for voice/ data integration. ATM is designed to simultaneously transmit diverse traffic types over a common net and does an exceptional job differentiating traffic into distinct classes of service.

What's needed to enable integrated voice/data over DSL is equipment that supports voice over ATM at each end of the local loop. The types of equipment needed for customer premises and central offices are the next-generation integrated access device (NG-IAD) and voice gateway, respectively.

The NG-IAD eliminates the DSL modem, bridge or router for data communications by interfacing PCs or PC networks to the DSL service, encapsulating IP-based data into ATM for DSL transmission and handling functions such as routing and IP address management. At the same time, the NG-IAD provides the DSL interface for voice equipment such as telephones, fax machines, key systems and PBXs, and sends and receives voice over ATM on the same DSL line.

Because ATM excels at simultaneous transmission of voice and data, the result is toll-quality telephone service with enhanced calling features intact, and continuous, high-speed Internet access or remote LAN access over a single twisted copper pair.

At the carrier's central office, a voice gateway completes the picture for integrated voice/data over DSL. Encapsulated voice traffic received at the carrier's DSL Access Multiplexer (DSLAM) is sent to the voice gateway, where it is converted to conventional voice signals and sent to a Class 5 voice switch. Data received at the DSLAM is carried as packet or cell traffic to its destination, typically an ISP or corporate network, just as in current DSL service.

This service can be readily and seamlessly integrated into existing nets. The use of a NG-IAD at the customer premises ensures that the impact on the voice and data equipment is small. Also, a voice gateway converts voice traffic between voice over ATM and the traditional formats used in existing phone nets.

One of the primary benefits of integrated voice/data over DSL is the ability to purchase all voice and data services from a single provider while gaining very high-speed data communications. Any business of any size will be able to enjoy the simplicity of a single point of contact for customer service, billing, expansion of services and management.

Another benefit is NG-IADs can dynamically make under-used voice-traffic bandwidth available to data traffic.

The market for NG-IADs has just begun to emerge as vendors explore the requirements for offering bundled services using DSL and a single copper pair. Technology demonstrations of NG-IAD and voice gateway products working together have provided proof of concept.

Although there are limited equipment offerings that can provide this functionality today, a number of options will be available by next year and widespread adoption should follow.

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Integrated Services from ATM and DSL–Shared Access for Voice and Data (white paper)
ATM over DSL technical recommendations (ADSL Forum)
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