WAN Services /

DSL: Can't we all get along?

DSL-Lite spec does not ensure interoperability.

By Tim Greene
Network World, 06/14/99

ATLANTA - More than 30 digital subscriber line (DSL) modem companies last week demonstrated compatibility among their products. But that doesn't mean you can buy the modems and expect them to work with everyone else's - yet.

Interoperabilty results

The modems on display at last week's SuperComm '99 show in Atlanta met all the basic specifications outlined by the pending standard for DSL-Lite. That DSL flavor is seen by many as the best hope for widespread DSL deployment and as a top challenger to cable modems.

But because of variations in how vendors implemented the specifications, the modems could talk to some of the other modems in the demo, but not to all of them.

Modem makers and service providers at SuperComm say they are still on track to deliver DSL-Lite services and interoperable equipment late this year or early next year. But vendors and providers also acknowledge that they have to do more to show that the modems can work over long enough distances and recover quickly after DSL connections fail.

DSL-Lite, along with other flavors of the high-speed access technology, were the talk of SuperComm '99, a show geared mainly toward service providers and their suppliers.

Unlike other DSL flavors, DSL-Lite, also known was G.Lite, can be used in modems installed by customers and can support 1.5M bit/sec downloads. The modems are suitable for branch- or home-office connections to corporate networks via the Internet. The modems also support a regular voice channel over the same phone line.

Still, the Universal ADSL Working Group (UAWG), which organized the showcase, declared it a success.

"We thought if we could get six vendors here, this would be a successful event," says Mark Peden, the UAWG marketing committee coordinator.

But others were less enthusiastic because the tests were done under laboratory, not real-world, conditions.

"They can test the bejesus out of the technology, but they'll still have some problems when it's deployed,"says Mike Lutz, director of product evaluation for DSL provider Darwin Networks. "At least for a while, it's going to be like analog modems were. There are going to have interoperability problems, and that will give the technology a black eye."

"These are not the last tests that will be needed to assure compatibility," says Ken Krechmer, editor of the "Communications Standards Review" and one of the authors of the DSL-Lite standard.

But Krechmer notes that the showcase overall drove vendors to hasten interoperability.

Although the demonstration was held at SuperComm, the actual testing was conducted last month at the University of New Hampshire. During the tests, DSL modem makers had their equipment pass data over wire that was 6,000 feet long, according to participants. Typically, carriers hope to be able to deploy DSL-Lite over wires that are 12,000 feet or longer.

Also, the tests didn't require modems to perform fast retrain, which involves re-establishing a link quickly when a DSL connection is disrupted.

While insisting the demonstration was successful, the UAWG at the same time refused to publish the results of the interoperability tests. The only way to get the information was to travel from demonstration station to demonstration station and write the results down.

Even then, the results were partial. Vendors said they could demonstrate more compatibility, but could not get enough connections at their demonstration stations to do so.

Peden says that by summer's end, a test bed group will be appointed to certify whether DSL-Lite gear is interoperable. The testing group will be similar to CableLabs, the cable TV industry group that developed a standard for cable modems.

Joining the interoperability parade, purveyors of highbit-rate DSL2 (HDSL2) technology - which essentially turns a regular phone line into a T-1 line- have established the HDSL2 Interoperability Consortium.

The group will fund a lab at the University of New Hampshire where HDSL2 vendors can test their gear for compatibility with other vendors' equipment.

Voice over DSL was also demonstrated widely at SuperComm, but it comes with the interoperability problems that all other flavors of DSL face. Vendors are starting to address interoperability issues, but there is no over-riding interoperability scheme yet.

Also at SuperComm, Adtran introduced yet another flavor of DSL - multi-bit-rate DSL (MDSL). This technology is said to work on phone wires as long as 30,000 feet without a repeater in the middle of the line to boost the signal.

The speed of the connection drops from 1M bit/sec on a line that is 15,000 feet long to 144K bit/sec on a 30,000-foot line. One problem with DSLdeployment is that some sites are too far from phone company switching gear for DSL to carry over the line. Installing repeaters jacks up the price of service.

G.Lite interoperability results

These are the results of interoperability tests that were demonstrated at SuperComm last week.

Each participating vendor of customer site modems is listed, followed by the names of vendors whose switching office modems they could talk to.

Note: Some vendors claimed they could interoperate with more than they were actually demonstrating at the show, but this could not be verified by the group running the showcase. The Universal ADSL Working Group would not release the results of its interoperability tests.

3Com: Alcatel, Cisco, ECI Telecom, Ericsson, Hyundai, NEC, Newbridge, Nortel, Pairgain, Samsung, Siemens

Alcatel Access Systems: Alcatel, Cisco, Ericsson, NEC, Nortel, Siemens

Analog Devices: Alcatel, Ericsson, Cisco, Hyundai, Newbridge, Samsung, Siemens

Ascend: ECI Telecom, Hyundai, NEC, Next Level Communications, Samsung

Cayman Systems: Cixco, Ericsson, NEC, Next Level Communications, Nortel, Siemens

Centillium: Centillium, Fujitsu

Cisco: Alcatel, Cisco, ECI Telecom, Hyundai, NEC, Samsung

Compaq: Alcatel, Cisco, NEC, Nortel, Samsung, Siemens

Conexant: Hyundai, Pairgain

Efficient Networks: Alcatel, NEC, Newbridge, Nortel, Siemens

Flowpoint: Alcatel, Cisco, Ericsson, NEC, Nortel, Siemens

GlobeSpan: Next Level Communications, Samsung

Hyundai: Cisco, NEC

Intel: Alcatel, Cisco, NEC, Nortel, Pairgain, Samsung

ITeX: Alcatel, Cisco, NEC, Nortel, Pairgain, Siemens

Lucent: Alcatel, Cisco, ECI Telecommunications, Nortel, Samsung, Siemens

Motorola: Hyundai, NEC, Nortel, Samsung

Next Level Communications: ECI Telecom, Next Level Communications

Orkit: Centillium, Fujitsu

Samsung: Cisco, NEC, Nortel

Texas Instruments: Alcatel, Pairgain, Siemens