DSL Prime: Technology Advances

Although some improvements are overhyped, DSL technology is getting better.

by Dave Burstein of DSL Prime and Future of TV and the Web Video Summit [January 8, 2008]

Alcatel: DSM at Least a 25 percent Improvement
Third level in 2009-2010
The world's largest DSLAM maker has put advanced DSM Level 3 on the roadmap "as soon as it's ready." Paul Spruyt offered a conservative estimate of at least a 25 percent improvement, while acknowledging that the improvement might be as much as 100 percent in certain conditions. Telecom Italia and Deutsche Telekom like the results they are getting. AT&T is the (unpublicized) early leader. DSM five years ago was a dream of John Cioffi. Alcatel sells a network management package that competes directly with Cioffi's Assia service, so their endorsement is particularly meaningful. Now, "everyone" is making plans.

Dynamic Spectrum Management is a term coined by Cioffi to describe a set of techniques for improving DSL performance by understanding and working with the actual line conditions. Lower levels of DSM make simple adjustments, like using only as much power as absolutely necessary to deliver the speed the customer buys. This reduces the noise, and improves the performance of other lines.

Cioffi predicted chips would be powerful enough to "vectorize" all the lines in a binder before the end of the decade. He told me in Berlin the current 65 nanometer chip generation has enough processing power to virtually eliminate crosstalk noise. If so, performance on VDSL lines could actually double.

Vectorizing and DSM are limited in the typical deployment by lines in the binder not controlled by the DSM engine or connected to other DSLAMs. It works best in a new deployment such as the tens of thousands of new remote DSLAMs installed by AT&T and Deutsche Telekom.

Upzide: New Chipmaker Promising Full DSM
Plans to sample chips late 2008
Mikael Issakson has been designing VDSL chips for years, including one for a major chipmaker a few years ago. He and his team at Upzide have quietly been designing what will probably be the first fully vectorized chips. The theoretical work by John Cioffi is now being confirmed by major carriers and early field trials. In the right circumstances, such as a new VDSL remote DSLAM, performance improvements might be as much as 100 percent. Upzide, based in Stockholm, will need to raise additional funds to bring the chips to market.

Separately, PMC-Sierra in their booth had a reference board with working silicon for their new VDSL chip. They told me samples were now in the hands of key customers. Their design in based on Aware. PMC-Sierra is now featuring complete gateways for fiber from their subsidiary Passave. They believe they will find a market selling VDSL as a complement to their PON chips.

Confronting the Increasing Noise Problem
Help requests up 18 percent
When more lines are turned on in the same binder, noise goes up and performance goes down. One European carrier had an 18 percent increase in calls to the help line and identified noise related problems as the primary cause. Many carriers are over 50 percent penetration and the problem gets worse with every line added. Some suggestions from BBWF:

If the connection has too little margin, the line drops and the modem has to retrain. Impulse noise is unavoidable, from elevators, light dimmers, garage doors, and new customers being added in the same binder. IPTV providers suffer angry customers, who have to wait seconds or even minutes for the modem to retrain when it goes down. Customers far from the DSLAM see performance go from bad to dismal.

DSM at levels 0 and 1 can reduce the problem substantially, which is why AT&T and France Telecom have long supported the work at Stanford. Alcatel and Assia are selling competing systems to put that to work. Telecom Italia and AT&T have reported good results. In the future, DSM Level 3 may prove the answer, but that years away from production.

Alcatel's Paul Spruyt reported customers were adding "artificial noise" when setting up the modems. Rather than profiling the customer based on the actual line conditions, they add 2 or 3 "false disturbers" when the modem is configured. This increases the margin and results in fewer problems, but causes the line to run at lower speeds.

Marco Polano of Telecom Italia noted that working with line parameters can produce substantial improvement. In one test, line adjustments allowed 88 percent of subscribers to download at 5 megabits, compared to only 67 percent with ordinary settings. He also asked for tools to adjust modem parameters at different times of day. Noise problems become worse in the early evening, as people come home, use elevators and electrical equipment, and also increase their use of their broadband lines. He suggests profile switching on chip autonomously and proactively with noise. He warns, however, that many customers adjusting at the same time could create instability. If modems reset, as they often do, IPTV can drop out for ten seconds.

As take rates move up, the problem will get worse. It's not typically a crisis, but is a definite challenge that every carrier needs to address.

Copyright 2008 Dave Burstein.
The DSL Prime Newsletter is reprinted with permission.

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