VDSL2 is the latest international DSL standard (only recently completed) and contains the latest technical developments in various aspects of the physical and link layers design. While there are numerous improvements in performance and reliability, the greatest benefit that VDSL2 promises is unparalleled flexibility to address any loop condition or service requirement from a single modem and in a better way than older technologies.

For example, in the past there have been two technologies based on DMT (Discrete Multitone) for addressing long loops and short loops (ADSL and VDSL respectively). VDSL2 is also a DMT-based technology, which unifies those two older technologies and provides a single platform to address loops of any length. Further, VDSL2 supports trading off bandwidth resources from the upstream to the downstream direction, enabling different services with symmetric or asymmetric traffic profiles.

Given the extensive flexibility and configurability of VDSL2 systems, it is interesting to consider whether these systems will also support new business services. Traditionally, business and residential services have followed different paths, are deployed out of separate platforms, and utilize different DSL technologies.

Can VDSL2 achieve the goal of a truly "universal DSL" that can address both residential and business services? Are the VDSL2 benefits significant enough to warrant a change in the way these services are deployed today? Will the industry follow these technical possibilities with the development of the associated platforms, deployment processes, backend software, etc. that is necessary to transform the VDSL2 core technologies into fully functional service offerings? These are some of the questions this article investigates.

VDSL2 versus Legacy Symmetric Technologies

Business services over copper loops go back longer in time (compared with residential DSL) and build upon a longer tradition of legacy. Many architectural elements of symmetric transceivers date back to ISDN and have been unchanged since their introduction in the mid-1980s. These include a rather primitive (by today's standards) pulse amplitude modulation, and an echo cancelled transmission with an inflexible upstream/downstream symmetry ratio. 

These modems have served the symmetric access market well over the past 20 years, supporting a variety of services such as DDS, T1, and E1. HDSL technology in the early 1990s increased the bandwidth of the ISDN basic design and HDSL2/SHDSL added improved coding. Despite these improvements, the basic architectural design of symmetric modems shows signs of aging and cannot compete with more advanced DMT-based systems.

DMT is an advanced modulation technique that was introduced in the early 1990s with the ADSL standard. It has since been the workhorse of residential DSL and is employed in the vast majority of DSL modems deployed today. It forms the foundation of a number of recent standards, such as ADSL2+, VDSL and VDSL2.

DMT breaks the available transmission bandwidth into a large number of narrow subchannels and adjusts the transmission power and carrying capacity of each channel to optimize for different channel attenuation and noise characteristics across the different subchannels. This provides for more efficient modulation, especially when combined with the powerful concatenated Reed-Solomon/Trellis codes available in VDSL2. Further, it provides a tool for easily adjusting the transmission power over different frequencies shaping the system's spectral profile. This is an important attribute in addressing difficult interference scenarios that may appear in practice.

Another major difference between DMT-based systems (such as VDSL2) and legacy symmetric systems is the way they multiplex upstream/downstream traffic. Legacy systems allow transmission across the whole band in both directions and utilize echo cancellation at the receiver. This method has since been found to pollute the copper binder with detrimental near-end crosstalk and reduce the performance of the overall network. 

Modern DMT-based systems use frequency division (different bands for upstream and downstream) and avoid the crosstalk problem. The benefits of frequency division manifest themselves in the excellent performance of DMT modems in the presence of same DMT disturbers. In contrast, legacy symmetric modems show their worst performance in the presence of same symmetric modems and sacrifice more than half the available capacity to the effects of crosstalk. DMT systems more than double the available rate under similar self-crosstalk conditions. 

There is no doubt that DMT based VDSL2 is a more flexible, modern, and well-designed system, with better performance than legacy systems. The question is whether these advantages are significant for business service offerings. The answer to this question is twofold: On the technical side, DMT enables new technologies that can further improve performance and keep up with competing solutions. On the business side, unification of service platforms can reduce costs and withstand future pricing pressures on these services. 

DMT Enables New Technologies (DSM and MIMO)

As broadband penetration increases and data rates go up, the complexity of the network increases. Along with greater diversity of services (symmetric versus asymmetric) comes a greater diversity of loop topologies, with mixtures of long and short loops and even binders sharing long (CO-fed) loops with short (RT-fed) loops. Further complicating the picture, new higher rate services may be deployed on a single loop for short range and on multiple bonded loops for longer range. 

It is important that the technologies deployed in the loop have the tools to deal with these complex interactions across links, services, and spectral profiles. The alternative would be a network that is incapable of scaling its bandwidth, crippled by unmanaged crosstalk. 

DMT systems provide the tools to:

• Monitor the effects of interference on each link and analyze its frequency distribution
  

• Shape the transmitted spectrum to avoid problem areas or limit disturbance to other services

These tools enable the generation of a set of technologies that come under the name Dynamic Spectrum Management (DSM). They promise to diagnose crosstalk conflicts and take preventive action to heal the trouble spots in the network in an automated way. These technologies are implemented as monitoring and diagnostic software running on servers and interrogating the DSLAMS for crucial health indicators for each circuit. 

Early implementations of these techniques are based on homegrown software developed by big carriers. Specialized vendors are appearing, however, that will perfect such solutions and market them to a broader base. The NAI-NIPP North American standardization group (under ATIS) is currently working on a Technical Report document that standardizes software interfaces to DSM software.

More impressive results are obtained by a new technology called MIMO (for Multiple-Input-Multiple Output), which originated in wireless communications. MIMO architectures originally appeared in wireless systems in the late 1990s in support of multiantenna transmission systems (multiple transmit-to-multiple receive antenna systems). They jointly process signals from all antennas to eliminate cross-channel or cross-antenna interference, enabling the practical use of such multiantenna systems. These techniques have recently been adapted to wireline systems, where multiple signals from several pairs are jointly processed to mitigate crosstalk interference. 

MIMO signal processing explores the correlation of the interference that a given pair is experiencing with the interference that can be measured in other pairs. This predictability of interference from pair to pair enables cancellation of a given pair's interference by judiciously processing the measured interference on other pairs. MIMO signal processing is effective even for crosstalk generated from legacy disturbers that are not under the direct control of the MIMO transceiver. 

By treating the groups of bundled copper pairs ("binder groups," typically consisting of 25 to 100 pairs per group) as a multi-input/multi-output channel and creating a transceiver which treats multiple pairs together, it is possible to separate crosstalk from signal. This MIMO transceiver must perform joint signal processing across all the pairs it controls and create a mathematical picture of the crosstalk environment those pairs inhabit. 

The mathematical picture must constantly be recalculated and updated. Each successive bit decision made by the modem must use this information to separate signal from interference and make the best possible decision using the information from multiple transmission pairs.

MIMO processing can be thought of as a set of adaptive filters which connect each transmission channel in a MIMO system to every other transmission channel in that system. In modern systems, such filters are implemented as discrete time digital filters. If the filters are very complex in terms of the amount of filtering they need to perform, it is almost always more efficient to first transform the signal being filtered into a different domain, sometimes called the frequency domain. 

DMT transceivers already perform these transformations as a normal part of the modulation/demodulation process. This means no extra complexity is required in the transceiver to perform the MIMO calculations by the most efficient means available. Hence, one can say that MIMO and DMT are architecturally synergistic. This powerful processing is ideally suited to an extremely efficient modem.

MIMO transceivers are a natural fit for supporting services over multiple bonded copper loops. Indeed, this application is the first one where such systems have already been available. MIMO technology is expected to migrate to the DSLAM, where it can improve the performance of single pair services by mitigating crosstalk from other single pair services.

Conclusions

There is no doubt that VDSL2 modems can support business services better than existing legacy symmetric technologies. Their core DMT technology offers better performance, advanced coding protection, and better reliability. More importantly, this provides a strong position for taking advantage of the next wave of DSL technologies (DSM and MIMO) that promise to further improve service speed and reliability. 

How soon VDSL2 will actually be introduced in support of new business services depends on two key factors: the intensity of competition in business access services in the near future, and the speed with which vendors will respond with new hardware and software solutions utilizing the latest DMT technology.

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