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Here today ... Does DSL have a future?

Without question, digital subscriber line, or DSL, is the broadband technology of choice for the independent telecommunications sector. The 2003 NTCA Broadband Availability Survey revealed that 92% of respondents deploy DSL as a broadband delivery technology. Unlicensed wireless was a distant second, with 17% of respondents identifying it as a broadband delivery technology. These numbers suggest an acceptance of DSL as the "workhorse" of broadband in rural America.

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With so much investment in and activity surrounding DSL, it is worth taking pause and asking, what is DSL's future? As with most debates, there are arguments on both sides of this question.

Some argue that DSL represents the best future for independent incumbent local exchange carriers (ILECs), because it extends the life of the copper infrastructure that is so prevalent today. Others argue that DSL, at best, is an interim technology that is only laying the groundwork for a true fiber-to-the-home/premise (FTTH/FTTP) network architecture--"priming the pump," as they say.

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This by no means is a new argument. It has been debated over the past decade as both DSL and FTTP were being developed and deployed. Recent activity, including the Bell operating companies' request for proposal on FTTP and the expansion of bandwidth-intensive applications like video/entertainment via DSL, are re-igniting this debate.

The fact of the matter is that bandwidth demands are increasing, not decreasing. While it is safe today to offer downstream broadband speeds of 1.5 Mbps or less, and upstream speeds of 512k or less, will it be enough three years from now? As emerging applications like video-on-demand (VOD), high definition television (HDTV), voice over IP (VoIP) and virtual private networks (VPN), which still are in their infancy, begin to mature, 1.5 Mbps down/512k up may not suffice. Whatever the requirements for future bandwidth may be, will DSL be up to the challenge?

Making Strides

Putting aside for a moment the argument of whether DSL is up to the challenge, it is worth exploring some technology developments. The fact of the matter is, there is much development occurring with DSL. In fact, Tom Starr, chairman of the DSL Forum, says of DSL development, "Contrary to popular belief, we haven't reached the end of the road in performance for DSL." Some near-term development, where products should be available over the next six to 12 months, includes ADSL2+ and "enhanced" SHDSL.

ADSL2+ attempts to improve both bandwidth and reach characteristics. ADSL2+ effectively doubles the amount of bandwidth available over traditional ADSL. As loop length increases, bandwidth gains decrease. For a loop length of 8 kilofeet, ADSL2+ provides approximately 9 Mbps downstream. At 12 kilofeet, ADSL2+ provides approximately 4 Mbps downstream.

Of course upstream bandwidth availability is much lower in these examples because of its asymmetric nature. There also are developments in symmetric DSL, where upstream and downstream bandwidth speeds are equal. Enhanced SHDSL increases symmetrical bandwidth speed from 2.3 Mbps to 5.7 Mbps at 7 kilofeet or below.

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DSL Down the Line

There also are longer-term developments in DSL taking place, which aim to move DSL into the 10s of Mbps territory for loop lengths of up to 12 kilofeet. These advancements primarily are utilizing "pair bonding" approaches, where multiple cable pairs are bonded together to increase bandwidth performance and reach.

One such development is known as multi-megabit multi-pair DSL or M2DSL. By bonding four pairs together, M2DSL promises to deliver 10 Mbps to a loop length of up to 12 kilofeet. Conceivably, M2DSL could provide HDTV-type bandwidth in loops of less than 8 kilofeet.

Along similar lines, the next generation of VDSL--VDSL2--is under development. VDSL2 shares many characteristics with M2DSL. In fact, there is movement by the standards bodies to merge these two developing standards together. Both are in the early stages, with no near term product availability on the horizon.

These newer DSL solutions also take advantage of Ethernet protocols for more efficient and inexpensive bandwidth delivery, thus leading to the term Ethernet over copper (EoC). Because these technologies utilize pair bonding, the availability of spare pairs plays a role in their deployment.

Noise Abatement

One of the biggest challenges in increasing bandwidth performance for DSL is mitigating its noise interference. As DSL performance increases, it becomes more "noisy" and may interfere with other circuits/pairs within the same cable bundle.

Tens, hundreds, and in some cases, even thousands of cable pairs may be grouped together in bundles throughout a network's architecture. Cable pairs carrying these high-bandwidth DSL circuits can be noisy. Consequently, solutions to mitigate these noise challenges are being addressed as high-bandwidth DSL technologies are being developed.

The multiple in multiple out (MIMO) approach is similar to some approaches used in wireless technology, where noise also is a challenge. Effectively, MIMO attempts to detect noise within a cable bundle, and mitigate that noise for the DSL signal in use. In other words, MIMO works to recognize noise interference in its environment and minimize its effect on performance.

Another approach, dynamic spectrum management, is set up to, in effect, be a "good neighbor" within a cable bundle by adjusting its own signal power (a generator of noise) based on the activity within that cable bundle and by the demands for bandwidth by the end user.

In effect, it dynamically adjusts itself according to exterior factors. As is the case with the longer-term DSL developments, these noise-mitigating technologies also are in the early stage of development.

Is DSL Enough?

Conceivably, all of these developments are leading to much higher bandwidth availability over longer loop lengths for DSL. But will it be enough? The FTTH community doesn't think so. "We don't view DSL as a competing technology, we view it as an enabling technology, 'wetting the appetites of consumers' for eventual FTTH deployments," said Dan Tataka, executive director of the FTTH Council. Tataka argues that DSL is an interim technology that eventually will be replaced when it reaches its bandwidth limitation.

The questions is how long is "when?" Five years? Ten years? Does anyone really know? Perhaps Starr of the DSL Forum puts this argument in perspective best by saying, "Name me a technology that is not an interim technology." In other words, doesn't all technology eventually get replaced with more innovative and efficient technology?

DSL is no different, and the time will come when even DSL will have reached its limit. But until then, DSL will continue to be the broadband "workhorse" for independent ILECs, with its future relatively secure.

A. Bernardin Arnason is vice president of business and technology at NTCA. He can be reached at barnason@ntca.org.
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Copyright 2004 Gale, Cengage Learning. All rights reserved.

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Title Annotation:digital subscriber line
Author:Arnason, A. Bernardin
Publication:Rural Telecommunications
Article Type:Cover Story
Geographic Code:1USA
Date:Jan 1, 2004
Words:1105
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