Front Page - A New Standard for DSLA New Standard for DSLDSL

Posted 02/2001

Front Page

A New Standard for DSLA
ITU Expected to Approve G.SHDSL This Month
By Paula Bernier

Yet another flavor of DSL is coming down the pike. G.SHDSL (symmetrical high-data-rate DSL) can support various transport protocols and has better reach and spectral compatibility as compared to other DSL options. It also offers the promise of interoperability--something that has evaded DSL to date.

This month, the International Telecommunication Union (www.itu.org) is expected to approve the standard for G.SHDSL, which offers data at 192kbps to 2.31mbps over a single pair. Early beta testing of G.SHDSL products was expected to begin at the end of 2000. Commercial availability of equipment was planned for sometime during the first quarter of 2001 and volume shipments are slated for the second quarter of this year.

Metalink Ltd. (www.metalinkinc.com), a fabless semiconductor company, in November announced that it had started alpha tests with Gryphon, its G.SHDSL chip for use in CPE. Also in November, Celsian Technologies Inc. (www.celsian.com) announced its Momentum line of products--including a modem, PCI card and router--that support G.SHDSL and are used to connect small and medium-sized businesses, as well as SOHO and residential users to a variety of symmetrical DSL (SDSL) and asymmetrical DSL (ADSL) services.

Chart: PSD: G.SHDSL vs. SDSL

Cahners In-Stat Group (www.instat.com)believes that standards-based G.SHDSL will be a significant factor in the rapidly growing worldwide marketplace for business-class and residential SDSL. According to the research firm, the worldwide demand for business-focused SDSL will grow at a combined annual growth rate of over 76 percent, reaching 9.7 million units to be deployed in 2004.

"There is a lot of demand not being addressed today between the small and medium-sized business markets. G.SHDSL provides a beautiful fit. And some say it will make a move into the residential space," says Mark Peden, a member of the DSL Forum's (www.dslforum.org) board of directors. Peden adds that the G.SHDSL standard does not currently address support for embedded POTS, noting that that will probably be addressed in an extension to G.SHDSL.

G.SHDSL has been developed from the ground up as an international standard and that will help facilitate interoperability, adds Peden, who is also vice president of technology and marketing at vendor Simpler Networks Inc. (www.simplernetworks.com). "So it's really seen as a global standard, and you have both a vendor and carrier community with a lot of support behind it." With ADSL, Peden says, there were one or two vendors that were so far ahead of the others early on that they didn't care as much about interoperability, he says, "but no one yet owns G.SHDSL."

Chart: SDSL Data Rate vs. Reach

Adds Keith Atwell, director of business development at ADTRAN Inc. (www.adtran.com): "SDSL is not standardized. And ANSI [American National Standards Institute] was developing HDSL [standards], but then people jumped the gun and never finished that standards work."

Of course, interoperability means that service providers won't have to be married to a single vendor to offer G.SHDSL. Interoperability also sets the scene for customer premises equipment to be sold through retail channels since any standards-based G.SHDSL CPE would theoretically be able to work with any vendor's network-based G.SHDSL equipment (although Peden believes it will probably take a few years before the CPE is sold retail).

The University of New Hampshire staged the first G.SHDSL interoperability event Nov. 6, 2000, and another such "plugfest" was scheduled for the last week of January, says Atwell. The success of the November event was limited because not everyone was able to interoperate, and none of the vendors' equipment interoperated at all levels, Atwell reports, "but we're early in the development of product sets."

Beyond potential standards-based interoperability, Peden says another key benefit of G.SHDSL is its flexibility to support a variety of services, such as native IP or frame relay and ATM--and without running a frame relay or ATM software stack. "That is something that's very intriguing," he says. ADSL, by comparison, was developed at a time when the industry thought "everything had to do with ATM," he adds. With G.SHDSL, says Peden, "we didn't have all of the entanglements of ADSL."

G.SHDSL is also more spectrally compatible with other services such as ADSL, whereas its predecessor SDSL generates more interference (see "PSD: G.SHDSL vs. SDSL"chart), says Peden. That's because it uses less power to transmit data over the loop--a measure known as power spectral density (PSD). "G.SHDSL is more efficient and lower power than other technologies, certainly lower than T1 and other legacy technologies," he says, adding that that enables better densities and better scalability.

ADTRAN's Atwell says that G.SHDSL uses trellis-coded PAM (pulse amplitude modulation) to enable 5 decibels (dB) of gain, resulting in a higher bit rate and farther reach (see "SDSL Data Rate vs. Reach" chart).

"From a range standpoint you get a 20 to 35 percent increase in range using G.SHDSL vs. 2B1Q, [the line code used with] SDSL. Bandwidth increases 15 to 20 percent," Atwell says.

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