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DMT gets the nod as next-gen VDSL spec

Posted: 14 May 2004 ?? ?Print Version ?Bookmark and Share

Keywords:dmt? dsl link? ikanos? stmicroelectronics? itu?

Discrete multitone technology has scored a major standards victory over an alternate line code scheme in the race to become the very high-speed DSL link.

The technology, known as DMT, was selected over quadrature amplitude modulation (QAM) during the latest meeting of an International Telecommunications Union (ITU) standardization group.

The ITU Study Group 1/15 has recommended that DMT should be the line code of choice for VDSL1. But as a compromise, and to assuage numerous chip and equipment makers and many operators in the Pacific Rim that have already deployed QAM based systems, QAM was also included as an annex to the main recommendations.

Still, the future VSDL2 standard will be based solely on DMT.

"To get agreement in what has been a long and often acrimonious process, we decided to document a common version for VDSL that has in the main part DMT as the line code scheme in the main body, but also includes, in an annex, a QAM implementation," Dick Stuart, rapporteur of Q.4/15 told

Stuart chairs all DSL-related standardization work within the ITU. He also serves as VP of standardization at Infineon Technologies AG, a backer of the QAM coding scheme.

"As part of the agreement, which is now out for comment over the next few weeks, we said the industry cannot afford this line war code continuing. So for anything pertaining to VDSL2 and any enhancement in the pipeline, only DMT will be considered, and we anticipate that will emerge as the single global standard."

This, he maintained, will benefit chip and line card makers based on the economies of scale that will be achieved. VDSL can be deployed from central offices or from optical fiber-fed cabinets located near customer premises.

The actual data rates for downlink and uplink depends on the distance between the two, but the ITU said the target is 50Mbps downstream, with 23Mbps a typical data rate and 4Mbps upstream. However, some chipmakers said 100Mbps will be possible, and devices capable of such data rates, for both upstream and downstream, are in the pipeline.

Stuart acknowledged there will be attempts by QAM proponents to enhance that technology. "I anticipate someone will present a paper which will be duly considered and reviewed, but when it comes to decision time it will not get consensus or support. It will die."

He anticipates that the latest agreement will receive few objections and that any changes during the approval process will be only editorial in nature. "I fully expect this to be a done deal."

Even though his employer, Infineon, backs QAM along with Metalink and several European operators, Stuart said "we have entered into a gentleman's agreement that Infineon will not block this or bring forward any objections or papers.

"Of course other people might," he added. "Nor can I tell how long it will take the proponents of QAM to get the message."

The IEEE and the T1E1.4 committee, both strong supporters of DMT as a unified coding scheme, are unlikely to dispute the ITU decision. "I doubt we'll see much opposition for this at [European Telecommunications Standards Institute] either, but you can never be sure," said Stuart.

Many large equipment vendors such as Alcatel are backing the DMT proposal, as are chipmakers such as Ikanos and STMicroelectronics.

- John Walko

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