Move over, T-1 and HDSL, Vendors continue to evolve HDSL2 standard

Cover your ears - another potential digital subscriber line fight is on the horizon at the American National Standards Institute, this time involving HDSL2 proposals.

The first camp comprises Adtran and Siemens, which advocated partially overlapped echo transmission (POET) a few months ago. Now, PairGain Technologies, ADC Telecommunications and Level One Communications have announced they are advancing pulse amplitude modulation (PAM).

Fortunately, this battle should not be as noxious as some, including the carrierless amplitude/phase (CAP) modulation vs. discrete multitone (DMT) line code war for asymmetrical DSL or the 56 kb/s standards haggling between x2 and K56Flex technologies. The reason for the civility is simple: The PairGain/ADC/Level One proposal is basically an improvement and advancement of the original POET, which is a transmission technology not dependent on modulation.

"There are differences. I would call it an evolution of POET, and we'll continue to evolve that," said Tom Stanton, Adtran's vice president of telco marketing. "The PairGain/Level One changes are still just changing the original proposition. We've also made changes to POET.

The HDSL working group has been lucky that the HDSL2 standards process has not been marred by the "religious warfare" and "nastiness" of the CAP/DMT manueverings, said George Zimmerman, PairGain's chief scientist. Because HDSL is already an existing technology, working from within and between certain parameters prevents games of "specmanship," as he called it.

"Like most other standards procedures, it's going to depend on compromise by one or both parties to get this standard out," said Kieran Taylor, broadband consultant with TeleChoice.

Last week, the ANSI's T1E1.4 working group interim meeting heard several HDSL2 improvements and suggestions. While nothing has been officially decided or voted on, most agree that a standard by the end of the year would serve the best interests of all HDSL vendors.

The committee had already outlined the specifics that PAM, POET and any other future suggestions should provide: full carrier-serving area reach of 12,000 feet, spectral compliance with ANSI ADSL standards, and interoperability between gear from different vendors.

But whichever technology or blend is chosen, the result is only good news for HDSL in general. HDSL2 has the ability to run 1.5 Mb/s in each direction over one single copper pair, as opposed to the two pairs now needed for both HDSL and T-1. That becomes significant for customers who may only have one pair now or need several 1.5 Mb/s connections.

Another reason for pushing the HDSL2 standard forward quickly is emerging competitive pressure from ADSL technology, Taylor said.

While HDSL2 promises some efficiencies and may eventually replace HDSL, there will still be room for both in the market in the short term, Zimmerman said. The incumbent HDSL's advantages include its lead position on the cost curve as a well-understood technology, and the simpler powering issues when using one copper pair instead of two, he said.

"From a technology standpoint, a fair amount of performance gains in HDSL2 are gotten by methods that involve frequency planning. One direction uses a different wavelength. HDSL2 on one pair would be constrained by powering doublers," Zimmerman said.

ATM CHIP FORMS GOLDEN ARCH Telogy Networks has added inverse multiplexing to its portfolio of products with the announcement of the Golden Gateway ATM InverseMux chip. The chip is based on ATM Forum standards and designed for use by wide area network switch vendors.

ISP SUPPORTS SHARED ACCESS CAIS Internet is offering a high-speed solution for some apartment dwellers and hotel customers. The Internet service provider plans to trunk T-1 or T-3 lines to a building and then use patented wiring technology to create shared access, similar to an Ethernet LAN in a business environment. Overvoice is now available to residents at the Arlington Courthouse Plaza in Virginia, with the Washington Marriott hotel to follow soon.

TODAY'S OPTICAL AMPLIFIER An exclusive Web site feature by Thomas Fuerst. The erbium doped fiber amplifier not only plays an important role in today's network, it also will be a critical piece of the all-optical network of the future. www.internettelephony.com