VDSL offers new potential

VDSL offers new potential
Among a raft of papers on symmetric and asymmetric digital subscriber lines at the recent DSLcon in Framingham, Mass., at least five focused on the mother of all DSLs: the very high-bit-rate, or VDSL, service. VDSL has been moribund for two years, since its 51-Mbit/second downstream speed requires short copper loops and the type of infrastructure with fiber in the neighborhood usually seen only in switched digital-video schemes.

Switched digital video never got very far as a market, but VDSL is finding new potential on the wings of alternative carriers who are laying fiber out to residential neighborhood nodes and want a simpler place to access local copper other than through a phone company central office.

Jim Szeliga, general manager of Orckit Communications Inc., made a strong case for Internet2 as a driver for home broadband access within five years. VDSL previously was seen as driven by one-way video delivery from a central server farm, Szeliga said, but the real driver will be the demand by residential customers for streaming multimedia traffic from Internet2 gigaPOPs. Depending on what speed of downstream VDSL is desired and the types of passive optical networks used, VDSL can be provisioned for significantly less than $400 per user. While still high, it's still less than true fiber-to-the-home architectures, which are unlikely to be cost-effective for residential use for several decades, Szeliga said.

Siemens Semiconductor Inc. already is sampling the first VDSL chips in an anticipated onslaught for higher speeds, which telecom marketing manager Uwe Hering said will be driven by ADSL customers desiring much higher (and in some cases, symmetrical) speeds, but with virtual parity in chip-set costs. Depending on installation details, POTS splitters can be much less complex with VDSL, and power dissipation can be 40 percent less than ADSL.

But those figures are based on quadrature-amplitude-modulation line codes. Jacky Chow, director of VDSL development at Texas Instruments Inc., provided details at DSLcon of yet another line-code war emerging in the DSL camps. Broadcom Corp. has lined up several companies behind QAM, including Analog Devices, Aware, Rockwell and Orckit. But Alcatel, TI, Nortel, Cadence Design Systems and an assortment of other companies have chosen to back a discrete-multitone derivative, SDMT/Zipper. The ANSI T1E1.4 committee will start work on a standard for VDSL this month, which will initially include both line codes. Soon, however, the camps represented by VDSL Coalition and VDSL Alliance will need to come to terms on line codes, if the vision of a near-term VDSL market is to become a reality.