A better bandwidth booster: HDSL2 will further refine copper-enhancing HDSL for the competitive generation

When the American National Standards Institute's T-1/E-1.4 committee convenes in December in Sacramento, participants will try to work out some of the remaining kinks in reaching a standard for high bit-rate digital subscriber line transmission over a single copper pair. The resulting HDSL2 draft, likely to be finalized as a standard by next fall, will address long-apparent carrier needs and eventually reshape the market for HDSL and T-1.

That may sound like a big effect for a technology that is the less-famous member of the DSL clan, but the truth is that HDSL has been holding court over a solid market for years. In fact, as deployment of T-1 circuits has boomed during the last several years (see figure), HDSL, which enhances the ability to deliver T-1 over long distances without repeaters, also has grown. About 50% of T-1 circuits deployed today are deployed using HDSL.

"HDSL grew out of a need for line conditioning, to make T-1 cheaper and easier to deploy. It sparked explosive growth in T-1 deployment," said John Griffin, vice president of marketing at ADC Telecommunications.

However, while HDSL has made T-1 deployment more economical and has helped add value to deployed copper, technology developers also have continued to eye potential improvements.

"If you could put HDSL on a single pair of copper, you could save some loop and save money in the long run. This can position carriers to deploy second-line services like Internet to the home," said Tom Stanton, vice president of telco marketing at Adtran.

Developers also saw an opportunity to improve HDSL's signal-to-noise ratio and strive for lower-latency error correction. In response, Adtran recommended that HDSL2 offer strong forward error correction capabilities that the first generation technology does not, said Stanton.

Also, HDSL2 will meet carrier service area reach requirements of 12,000 feet from central offices and vendor interoperability. The T-1/E-1.4 committee also is moving to consensus over a final requirement-spectral compatibility with existing services-that is key to maintaining performance in today's complex loop environment.

"We [vendors] have all refined our state of understanding of HDSL," said George Zimmerman, chief scientist at PairGain. "We've found that some mixtures of small disturbers sometimes cause worse interference than one really big disturber. Now, with HDSL2, we're trying to push the limits of what we can do with HDSL."

That means initial HDSL2 systems will be assessed very carefully by the carriers that may have the most to gain from them. "Most HDSL2 interest is coming from competitive local exchange carriers with some congested copper. They pay by pair for copper capacity," Zimmerman said.

Meanwhile, "HDSL2 will be treated by incumbent local exchange carriers in the very same way they've looked at [asymmetrical DSL]. They will test it heavily at first to make sure it meets their infrastructure and performance requirements," said ADC's Griffin.

This somewhat delayed market development is one reason why carriers will maintain mixed HDSL and HDSL2 environments for the next several years, all vendors agree. HDSL is still good for deploying T-1. In the future, HDSL2 will help deliver more T-1 capacity for a wider range of high-bandwidth services under a competitive economic plan for those who need it.

INTEGRATION IS BLISS Premisys Communications has unveiled its new integrated bandwidth optimizer architecture. The solution combines the capabilities of T-1 integrated access devices, drawing bandwidth from an array of transport services and carrying them via time division multiplexing or ATM over Sonet.

PUSHING TO 92 KB/S

Multi-Tech Systems demonstrated 92 kb/s data transmission over two dial-up analog lines using its CommPlete Communications Server equipped with Lucent Technologies' K56flex 56 kb/s technology. The rate was attained by channel bonding, or splitting the data stream in half to travel over separate lines to two modems, where Lucent's networking software merges the streams.