Line codes rise to the occasion, CAP vs. DMT continues, but does the market really care?

In the evolution of the asymmetrical digital subscriber line market, industry interest has shifted in the past year from standards debate over modulation schemes to new equipment and service rollout promises.

However, that doesn't mean the line code modulation war is over. Discrete multitone (DMT), carrierless amplitude phase (CAP) modulation and quadriture amplitude modulation (QAM) line code vendors continue to aggressively promote what each considers to be the superior technology. The DMT standards group is finalizing Issue II, which clarifies Issue I and addresses compatibility, rate adaptation, transporting asynchronous transfer mode over ADSL, extending the 8 Mb/s data rate and reducing overhead for increased upstream capabilities.

While DMT is still the only standard approved by the American National Standards Institute, CAP proponents were given an ad hoc committee and are making progress toward their own standard.

The T1E1 CAP/QAM RADSL ad hoc committee is still aggressively pursuing a standard and plans to offer one by the September T1E1 general meeting, said committee chairwoman Marlis Humphrey. That means the group has three monthly meetings left to finish writing its specs.

So far the committee has made decisions on coding and encoding, a modulation scheme that will be a combination CAP/QAM and spectral placement. The group has yet to decide between two interleaving methods, as well as baud rate and step sizes for rate adaptation, scrambling, framing structure, and starting and initiation sequences, Humphrey said. The last one may be the most challenging, judging from the DMT Issue II process, she said.

What does all the continued standards jockeying mean to the industry? "One standard matters in the long term, but it's not apparent to me or to anyone else in the industry which one will win," said Bobbi Murphy, chief analyst with Dataquest, San Jose.

Two different kinds of issues stand in the way - one political and one technical, she said. The political problem is that DMT is the standard and CAP is not; however, many of the current ADSL trials use CAP. The technical problems have to do with DMT design, commercial availability and power consumption, Murphy said.

"What DMT has to prove - and pretty quickly - is prove it can compete in power, [smaller] real estate, price and commercial availability to be able to take precedence," Murphy said.

In the end, it doesn't' much matter which line code is used, said Daniel Briere, president of TeleChoice. He compared the issue with T-1 coding variety, where users chose which one they want when they get service. In fact, service providers will probably not only chose just one line code, but probably just one vendor, he said.

"Rolling out new service is enough trouble without having to worry about multiple vendors in the same line code," Briere said. "I don't think any telco will deploy multiple vendors out of the box."