Interoperability steals the show: Sizing up HDSL2 and SDSL

After two years in standards development, high bit-rate digital subscriber line 2 equipment is breaking out for carrier installation and testing. Carriers' need to conserve limited copper resources and to reduce installation intervals have caused an industry migration from two-copper-pair solutions-like the earlier version of HDSL-to single pairs. The plan was that the single pairs would offer the same data rates as two-pair technologies and take up half of the copper.

This is a similar goal vendors achieved in developing symmetrical DSL, which like HDSL2, can operate bidirectionally at 1.54 Mb/s over a single pair. With SDSL already available, why the need for HDSL2?

HDSL2 overcomes some of SDSL's limitations, including potential interference problems and reach, said Sean Martin, vice president of marketing business for ADC Telecommunications' broadband group.

HDSL2 meets a 5 dB noise margin in worst-case crosstalk and has a latency of less than 500 msec.-significantly better performance than SDSL can provide, according to ADC sources.

Bettina Tratz-Ryan, an industry analyst for Dataquest, concurred: "[HDSL2] encompasses spectral compatibility with other services, which are delivered in the same binder group, such as asymmetrical DSL or ISDN, without signal degradation due to crosstalk."

While HDSL2 has an unrepeatered carrier serving area of 12,000 feet, SDSL data rates drop when it is used over distances greater than 10,000 feet.

"When people discuss SDSL's reach of 20,000 feet, that is the result of scaling down the speed to enable it to go farther. This is achieved through two-bit, one-quaternary modulation, which is a four-level signal that reduces the bandwidth," said Claude Romans, a senior analyst with RHK.

However, SDSL's ability to operate at distances up to 20,000 feet-albeit at a slower data rate-will likely cause it to stick around, even after HDSL2 is widely available.

"SDSL really fills a different role [than HDSL2]. Incumbent carriers use it in rate adaptive services, for example," said George Zimmerman, chief scientist at PairGain Technologies.

The most powerful advantage of HDSL2 over SDSL may be interoperability.

"SDSL is not a standard. The industry has come to recognize a number of ways to offer symmetrical service over a single pair," said Martin.

One version, which offers data rates up to only 768 kb/s, basically takes one loop from the existing two-loop HDSL approach, reducing the four-wire data rate by half. Another version uses that approach, but then speeds up the clock at least two times, enabling T-1 transport over the span. A third implementation uses carrierless amplitude/phase modulation to achieve T-1 rates over a single loop.

HDSL2 is standards-driven. HDSL2 vendors ADC and PairGain have designed and tested their products to work together, enabling a transceiver from one vendor to exchange data with the other vendor's product. And Level One recently completed interoperability tests between the central office and the remote end of PairGain's HiGain System and ADC's Soneplex.

The stinger with HDSL2 is that the hardware may be ready now, but much of the standard has yet to be ironed out.

"The physical layer was approved last year, but the remaining elements still to be determined are operational support, maintenance, billing, administration and power issues," Tratz-Ryan said. "But those components can be reprogrammed later."

"The standards [for HDSL2] should be completed by late fall," added Zimmerman.

FORCE FOR CONVERGENCE Taqua Systems is introducing the Open Compact Exchange architecture that converges voice, data and enhanced services. The OCX system is a programmable Class 5 switch that supports subscriber services; enhanced services; access, concentration and routing functionalities; and multiple voice and data protocols.

O CANADA Bell Canada is testing Nortel Networks' OPTera DWDM solutions, which provide end-to-end managed optical networking of voice and native IP, ATM and multimedia protocols. The 10 Gb/s long-haul device supports 320 Gb/s per fiber. The 32-channel metro ring solution supports 80 Gb/s per fiber.