Getting ANSI with HDSL2: Vendors aim for T-1 market

The ANSI T1E1.4 standards committee responsible for defining HDSL2 has reached a conclusion after nearly two years of progress. Although it won't go to ballot until the spring of 1999, the HDSL2 specification is solid enough to begin product development.

The committee defined some key differences between HDSL2 and high bit-rate digital subscriber line. First is that HDSL2 can deliver a full-duplex T-1 over a single copper pair instead of the two pairs required with HDSL. It targets the T-1 market, which is growing at a clip of 30% per year.

As T-1 service became more popular, carriers began seeing exhaustion in the local loop, said Sean Martin, vice president of marketing for ADC Telecommunications' loop transport division. HDSL2 was designed to increase bandwidth in the local loop. According to ADC, at least 60% of T-1 customers can be served by HDSL2.

"The primary application is for T-1s in the local loops," Martin said. "T-1 as an access means is a key enabler for a number of services," he said. "We see that trend continuing, and we see HDSL2 adding to that trend as T-1s become more cost-effective." The services once supported by T-1 can now be supported by HDSL2.

The ANSI committee also defined an open interface for HDSL2. The new standard gives way to vendor interoperability, unlike today's proprietary HDSL solutions. Although products are still in development, interoperability looks promising. Level One's HDSL2 chips are being tested for use with ADC's Soneplex platform and PairGain's HiGain T-1 access and Avidia product lines.

In addition, HDSL2 is "spectrally friendly," Martin said. "Spectral friendliness is our ability to be in a binder group with other services that may interfere with us, [but HDSL2] still performs to the full specification."

That means HDSL2 won't impair or be impaired by adjacent transmission technologies in a copper binder group, he explained. "We didn't want to introduce service that interferes with the existing service or future service. It was a key challenge with HDSL2 [to make] sure that [asymmetrical] DSL or T-1 service would not be impaired by HDSL2." It also works with ISDN.

After weighing the performance/interference trade-offs, ANSI opted for a 16-level pulse amplitude modulation (PAM) coding scheme with overlapped PAM transmission and interlocking spectra signaling. HDSL2 features low-latency forward error correction.

The HDSL2 standard supports a loop reach of 12,000 feet and has a 5 dB noise margin in worst-case scenarios.

"There wasn't a huge debate over the line code," Martin said. "We selected PAM because it had the best performance" in an HDSL environment.

"HDSL was introduced to the [local] loop eight years ago, but forward error correction was not used," he continued. "Subsequent DSL technologies [led to an] improvement in performance, but at the expense of delay." HDSL2 set out to improve the coding gain, "without introducing unbearable delay in the local loop," he added.

Power might be the only hitch, said Claude Romans, senior analyst for Ryan Hankin Kent. With HDSL, power runs down a separate wire pair, but with the single pair HDSL2, power travels on the same wire as data. However, Romans doesn't expect that to limit HDSL2 deployment.

"HDSL2 will supplant most HDSL sales in the future," he said. "Saving pairs is going to become more important with the [competitive local exchange carriers] wanting pairs. Anytime [carriers] can defer putting more copper in the ground, the better off they are."

The ANSI committee completed its mission, Romans added, but the proof is in the products. Interoperability and competition are at stake.

"The market is dominated by PairGain, ADC and Adtran, but their products don't interoperate," he said. The companies' eagerness to conform to an open interface could be motivated by an urge to maintain market share, Romans said.

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