Incompatibility woes drive DSL compromise

By Tim Greene
Network World Fusion, 3/16/98

The makers of digital subscriber line gear are finally waking up to the fact that the technology lacks one major feature: interoperability.

Seeking a remedy, a newly enlightened group of vendors is meeting this week at the University of New Hampshire to run interoperability tests on their asymmetric DSL (ADSL) modems. These devices can turn regular phone lines into broadband links to access the Internet or corporate networks.

The vendors' effort has been a long time coming in a crowded and confusing field. "ADSL has not kept up with interoperability," said Brett Azuma, a chief analyst at Dataquest, Inc., in San Jose, Calif.

There are at least eight different flavors of DSL, each suited to its own purpose and none compatible with another. A high-bit-rate DSL (HDSL) modem, for example, cannot talk to an ADSL modem. And a rate adaptive DSL (RADSL) modem cannot talk to a symmetric DSL (SDSL) modem.

Even within one DSL technology, vendors have not made their products interoperable. So a PairGain Technologies, Inc. HDSL modem does not work with an Adtran, Inc. HDSL modem, even though both meet the HDSL standard. For some DSL flavors, there is no standard at all. But now, with cable modems threatening to capture the consumer broadband access market, the DSL world is caught up in a frenzy of cooperation.

DSL sees the lite

Microsoft Corp., Intel Corp., Compaq Computer Corp., the major regional phone companies and virtually every DSL hardware vendor are working frantically on a standard for yet another flavor, DSL Lite, an inexpensive, easy-to-install version of DSL.

Known as the Universal ADSL Working Group (UAWG), the groups goal is to ship PCs equipped with DSL Lite modems by Christmas. That is a very ambitious goal, but some believe it can be met. The UAWG also is committed to interoperability, although that could take two to three years to achieve, according to Robert Weiner, vice president of sales for DSL products at Paradyne Corp. The compatibility lag could hurt sales.

Currently, the UAWG is writing a proposed DSL Lite standard that it plans to present to the International Telecommunication Union (ITU) sometime this spring. With major UAWG members also sitting on the ITU, fast action is expected. ``The idea of a standards battle is not making anyone feel warm and fuzzy," Azuma said.

Many vendors don't plan to wait for a standard. They say they will make DSL Lite modems based on the UAWG proposal as soon as the proposal is ready. In many cases, that just means rewriting the firmware that runs their existing modems.

"Three to six months after the UAWG makes its proposal, we will have first samples of these [DSL Lite] modems," said Robert Rango, general manager of the modem and multimedia group at Lucent Technologies, Inc. Once a standard is formally set, Lucent, Alcatel Telecom and other vendors said they will upgrade to it.

In addition, vendors plan to make the DSL Lite modems so they can also act as 56K bit/sec analog modems. That way customers can buy the modems and use them immediately over any phone line. When a DSL service becomes available in their area they can upgrade to DSL Lite without buying a new device.

A refined standard for ADSL, which will push interoperability by clarifying some areas in the current standard that are open to interpretation, is expected later this year. When different vendors build modems based on different interpretations, the modems tend to not work with each other.

The refined standard also will make DSL Lite a subset of ADSL. That means an ADSL modem will also be able to act as a DSL Lite modem. A service provider then could install a single DSL modem that could support full ADSL service or DSL Lite service, depending on what the customer wanted.

The ITU also is working on a standard for a handshake between DSL modems that will let them reveal to each other what DSL technologies they support. That way the modems can negotiate how they will speak to each other, according to Ken Krechmer, principal at Communications Standards Review, who is a member of the ITU.