In a move that will have big impacts on the deployment of VDSL in the United States, and around the world, ANSI’s T1E1.4 committee has opted for discrete multitone (DMT) technology over quadrature amplitude modulation (QAM) as the line coding scheme for VDSL systems deployed in the US. DMT provides backward compatibility with existing DSL modems and greatly increases speeds.
The T1E1.4 North American Standards Working Group held a “VDSL Olympics” and DMT modulation won. The Alliance for Telecommunications Industry Solutions (ATIS) announced that its Standards Committee will specify only Discrete Multi-Tone (DMT) in its American National Standard for VDSL.
A competing modulation standard, Single Carrier Modulation (QAM), was backed by Metalink and others and is currently used in Asia.
The VDSL Alliance backed the standard. Telephone carriers may now provide up to 50 Mbps capacity to the home enabling a triple play of telephony, Internet and video service. Korea Telecom, for example, delivers 50 Mbps (VDSL-DMT) services over a single copper pair.
VDSL (Very-High-Bit-Rate Digital Subscriber Line), is a supercharged DSL service. It provides asymmetric and symmetric speeds of 52, 26, 13, 6.5 Mb/s for distances of 100 meters, 1000 meters and 1500 meters. But standards have been problematic.
The VDSL Olympics pitted two DMT chip set developers, Ikanos and STmicroelectronics, against two QAM chip vendors, Infineon and Metalink. All four agreed to have two independent test labs perform tests. Chip makers Ikanos and STmicroelectronics would appear to be the big winners.
VDSL lets phone companies use twisted pair phone wiring. But range is a limiting factor. In South Korea and Japan, where shorter loops dominate, VDSL is used to bring connectivity to campuses and apartment complexes. VDSL performance suffers in the range of 5,000 to 12,000 feet, dropping below even ADSL.
ADSL2 and ADSL2+ push the downlink performance and the range of an ADSL from 6 Mbits/s into the 24-Mbit/s range. When coupled with emerging compression schemes, this could be good enough to support broadcast video services. “Unless you’re doing HDTV, ADSL2 links can support multiple video streams,” said Peter LeBlanc, vice president of marketing at Aware Inc.
Ethernet in the First Mile (IEEE 802.3ah), is a related broadband technology. It may use GigE metropolitan networks for distribution. EFM earlier this year also chose DMT modulation and were waiting for “VDSL Olympics” results.
Ethernet in the First Mile will operates over fiber optic cables to the home. A box outside the house or on a utility pole, converts the optical signal to an electrical signal which can then be distributed via twisted pair or coax inside the home.
Wireless users could break the “last mile” bottleneck. IP-TV Settop boxes, VoIP phones and multi-media networking may be fed by VDSL, ADSL2, Ethernet in the First Mile, DOCSIS-2 Cable Modems or “Wireless DSL” (using 802.16a).
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