G.shdsl reaches out with speed

One of the main hiccups in widespread DSL deployment is distance, but with the development of a new standard for symmetrical DSL, data-focused competitive local exchange carriers may be able to solve the distance problem.

Called G.shdsl, the proposed standard can offer symmetrical high bit-rate DSL that supports multiple bit rates over one pair of copper wires and could increase reach and data rates by at least 25% to 35%.

The standard will allow service providers to offer data rates from 192 kb/s to 2.3 Mb/s to customers at least 20,000 to 5700 feet, respectively, beyond the central office. The distance is a significant increase when compared to the 1 to 1.5 Mb/s DSL can offer today with a maximum reach of 18,000 feet.

Besides allowing for additional reach and bandwidth, G.shdsl also should reduce the need for repeaters because the signal is stronger and can travel longer distances. "The data CLECs are going to be all over this," said Keith Atwell, product line manager for loop technologies at Adtran. The new standard ultimately will lead to more vendors, more competition and better products, he added.

G.shdsl is the next generation of SDSL with a different line coding, called trellis-coded post amplitude modulation (TC-PAM). The trellis-coding technology is the same technology used in 56 kb/s modems and was adapted to TC-PAM for HDSL2, which enables HDSL services across a single pair of copper wires.

"We took the trellis coding used in the modem world and applied it to the digital world," Atwell said. "We changed the line code and more than doubled the distance. It's a better signal."

Although there are many ways to boost a DSL signal, some solutions increase the interference level. G.shdsl was developed to be compatible with other technologies. "We wanted to increase the distance and lower the interference. [G.shdsl] is the best of both worlds," Atwell said.

The other benefit of G.shdsl is its flexibility in supporting different data rates on the same copper pair, which allows for a range of DSL services.

"Service providers are basically able to offer different types of services using the same copper pair instead of leasing a T-1," said Bettina Tratz-Ryan, an industry analyst for Dataquest.

Considering the high price of a T-1 line, the new standard enables service providers to use half the copper to bring T-1, DSL and ISDN services to their customers, she said. "[Carriers] can slash costs and put more and differentiated services over the same line."

Because of G.shdsl's additional reach, a service provider's addressable market increases. The standard should be especially effective for reaching the metropolitan area market, Tratz-Ryan said.

Providers such as Rhythms NetConnections welcome any new standard that reduces distance concerns, said Michael Sabo, director of innovation for Rhythms. "It's a good sign for DSL in general," he said.

Rhythms' multivendor strategy, which includes using DSL access multiplexers from Cisco Systems, Paradyne and Copper Mountain, guarantees that Rhythms can limit the risk if one of the equipment providers won't support any new standard rolled out. This ensures its ability to offer greater reach and bandwidth capabilities.

"We can play one against the other. That's what happened with G.Lite," Sabo said.

In addition, any DSL technology that doesn't require repeaters is a good thing, he said. "It reduces cost and increases reliability."

G.shdsl currently is under review by the American National Standards Institute and the International Telecommunications Union. Proponents hope it will become a nationally and internationally accepted DSL standard.

Equipment following the standard will not come to market before the end of the year, Tratz-Ryan said. Other vendors working on the standard include ADC, Alcatel, PairGain Technologies and Level One Communications.

But vendors looking at HDSL2 already will have the basic technology to adopt the G.shdsl standard easily, she said. "They'll be able to address the growing international market as well."