Metalink Ltd., Globespan and Infineon all plan on near-term semiconductor support. Even the two companies most associated with HDSL-2, Intel Corp. and Conexant Systems Inc., concede that interest in a chip set that supports only a T1 (1.5-Mbit/second) rate is limited to transport-only functions for implementing T1 service, a factor that could affect the way future two-mode products are marketed.

G.SHDSL is a standards-based version of symmetrical rate adaptive DSL that can accommodate line rates of about 384 kbits/s to 2.3 Mbit/s off a single line card. Previous symmetrical schemes have necessitated multiple line cards to accommodate varying geographies and rates and have relied on proprietary coding schemes such as 2B1Q.

HDSL-2, meanwhile, was developed as a short-term solution to provide T1-only speeds over a single copper pair (as opposed to the dual copper pairs required by the original HDSL). But equipment vendors clamored for unified chip sets that could handle lower symmetrical rates up to the European E1 rate (2.048 Mbits/s), thus letting developers design one line card for symmetrical services worldwide.

Danny Gur, vice president of business development at Tel Aviv-based Metalink, said HDSL-2 "has only a small, hidden role to play in T1 infrastructure." Symmetrical chips supporting proprietary 2B1Q encoding, such as Conexant's ZipWire, will have a continued lifetime, Gur predicted, "but the future belongs to G.SHDSL." Metalink plans to be the first semiconductor company to sample a multichannel chip that puts G.SHDSL support directly into the telco central office.

Infineon already offers G.SHDSL support in a single-channel version for customer-premises routers and integrated access devices.

Ben Runyan, product marketing manager at Infineon, told a DSLcon panel that G.SHDSL used in a cabling binder group with other telephony services will have much less impact than 2B1Q, he said.

Legacy service

That claim sparked protest from several developers at Conexant Systems Inc., who said that 2B1Q still could offer long-reach services with acceptable noise margins. But Ron Cates, vice president of broadband access service at Conexant, emphasized that the company is pushing G.SHDSL in new designs.

The ZipWire 2B1Q chips will be used in legacy SDSL designs, he said.

The ITU has granted G.SH-DSL a new standard number, G.991.2, which was moved into a determination phase in April and is expected to be approved in February 2001.

Meanwhile, an official G.SH-DSL Consortium for interoperability testing was formed at the University of New Hampshire on Aug. 23. The four-company group of Adtran, Cisco, Globespan and Infineon has asked the forum to formally adopt the marketing and promotional work for G.SHDSL.

Cates said that new work in the ITU for adding analog POTS phone service to a G.SHDSL frequency band could lead to a day when asymmetric DSL, in both its full-rate and splitterless versions, becomes obsolete.

But Ahmed of Intel said that in the meantime, 2B1Q will be popular in the short term, while the current version of G.SHDSL will remain popular alongside ADSL for several years to come.