TechZine – Alcatel-Lucent

Technology

How to Manage and Optimize Vectoring Networks

Highlights

  • Upgrade forecasts show attainable bit rates with VDSL2 — and with vectoring added.
  • Proactive troubleshooting addresses common line issues before vectoring is deployed.
  • Effective monitoring examines the bigger picture, over time, not just a single line.

As vectoring becomes broadly deployed, service providers must increase their focus on network analysis and optimization systems.

To take full advantage of vectoring gains, service providers need to update their line optimization and testing capabilities. The most effective vectoring support will be integrated with the service provider’s other tools, such as line diagnosis, single ended line test (SELT) and dynamic line management (DLM). This combination of capabilities can produce the best bit rates, while also helping to reduce OPEX, improve service rollout decisions, enhance service quality and accelerate problem resolution.

VDSL2 vectoring requires broader view

Today’s multimedia world is presenting greater technical challenges for wireline networks by requiring faster bit rates, along with higher line quality. Very high speed digital subscriber line 2 (VDSL2) vectoring is a key technology for enabling the necessary speeds. It can deliver downstream rates of 100 Mb/s at distances of up to 400 meters, and 40 Mb/s can be supported with loops as long as 1000 meters.

But to provide these higher rates, VDSL2 vectoring must continuously cancel the ever-changing “crosstalk” within a cable bundle (binder). A defect in any line can impact other lines in the bundle — and potentially reduce their vectoring gains. To keep this technology operating at its best, service providers need to update their line optimization and testing capabilities to provide vectoring support.

These updates should begin with proactive network diagnostic tools that can address potential problems before vectoring is deployed. Flexible monitoring and troubleshooting capabilities are then needed to maintain optimal speeds and reliability. For the help desk, a typical real-time view into a single line must be provided. But vectoring also requires an ability to look at the broader performance picture. For example, in vectoring-enabled networks, problems on one line may be caused by a physical issue on another line in the same cable or binder. Therefore, diagnostic tools must enable views across vectoring groups to binder views and even aggregated views per cabinet or digital subscriber line access multiplexer (DSLAM). Each view should also depict specific graphs and parameters to enable rapid fault tracking.

Real-world bit rate increases

To provide higher bit rates, vectoring dynamically cancels interference between multiple copper lines in a bundle. This process does not raise the theoretical maximum transport speed of a line. Instead, it reduces the gap between the theoretical maximum rate and the speeds that service providers can deliver in typical field conditions.

In most deployments, phone lines carrying VDSL2 signals are part of a cable bundle, positioned near many other lines. This close proximity results in interference (crosstalk), one of the most significant factors limiting the bit rate of a line. The higher the number of nearby lines, the more crosstalk is generated. But in field trials with leading service providers, vectoring noise cancellation improved previous downstream bit rates by 90% to 150%.

Figure 1. VDSL2 vectoring trial results from 2010 to 2012

Effective vectoring support must recognize that vectoring always involves more than one line — and that each line can affect all the others in a bundle. Those inherent properties of vectoring make monitoring and troubleshooting more important than ever. To maintain reliable performance, service providers will require a number of capabilities when introducing vectoring into their networks.

Initial assessment tools

While service providers are beginning their deployment decisions, these tools provide insights into how well vectoring can work on existing infrastructure — and where improvements may be needed.

Upgrade predictions — Service providers can start with a forecast of the attainable bit rate when upgrading a line to VDSL2 technology. This information increases the success rate for the upgrade, identifies where remote cabinets may be needed and makes it possible to accurately calculate the associated cost of offering an upgrade.

Upgrade predictions with vectoring support then extends these forecasts, by offering a separate prediction of the attainable bit rate with VDSL2 vectoring implemented. This information provides a valuable context for assessing whether vectoring can perform at its best on a particular line.

Proactive diagnostics — Service providers also need analytical tools that can assess the line health of the full network. These tools should enable quick identification of typical physical problems, such as bad contacts, bridge taps and bad wiring, which can generate excessive and varying crosstalk. Then service providers can address the problems upfront, before they cause problems on vectored lines.

Ongoing monitoring and problem resolution

Updated network monitoring tools should offer clear displays of network information that can help optimize vectoring performance and streamline troubleshooting. For example, this data needs to include vectoring parameters, G.INP parameters, line profiles, disorderly leaving events (DLEs), vectoring gain and indications of service stability.

Crosstalk sources — The next key step is to identify varying crosstalk sources that can impact other lines in the bundle, which have no real physical problem. A clear display of the disturbers allows problem lines to be rapidly detected and diagnosed separately, as illustrated in Figure 2. When the help desk has an immediate overview of the highest disturbers, they can pinpoint problems on other lines and save substantial time in making a diagnosis.

Figure 2. A view of all vectoring disturbers on a specific line.; colors represent degree of crosstalk. (Source: Motive Network Analyzer)

Integration with DLM —DLM is an automated method for optimizing the configuration of a DSLline. It continually monitors key line parameters — including bit rate, power spectrum and noise protection — and automatically reconfigures the line profile to maximize performance, while ensuring stability.

Together, DLM and vectoring can deliver higher bit rates than either can provide alone. That’s because DLM enables transmission at lower power levels, which reduces crosstalk and contributes to vectoring gains. But DLM also supports vectoring performance in other ways.

First, it plays an expanded role in monitoring line health because crosstalk sometimes masks other forms of noise on the line. When crosstalk is eliminated the line may become more sensitive to the remaining forms of noise, such as radio frequency interference (RFI), impulse noise and alien crosstalk variations.

In that case, DLM helps guarantee line stability by providing sufficient noise margins and by making sure that G.INP and seamless rate adaptation (SRA) are doing their jobs properly. In addition, DLM provides longer monitoring periods than SRA and G.INP, which can be important for addressing RFI and bad loop conditions that often involve highly variable crosstalk. If any problem emerges on a line, DLM starts a diagnosis automatically.

DLM can also allow service providers to operate vectored lines in combination with legacy lines, when not all lines in a bundle can be upgraded at the same time. By reducing power on the legacy lines with DLM, alien crosstalk into the vectored group is reduced to a minimum, while the stability of legacy lines is preserved.

Tracking disorderly leaving events — DLEs are another case where problems on one line can affect other lines in a bundle. For example, bad contacts in a line, missing splitters or customer premises equipment (CPE) disconnections can all generate a DLE with the potential to affect neighboring lines. As a result, a customer with a good line might find that his video is not stable.

In this case, monitoring tools with vectoring support can provide the fastest problem resolution. They would start by launching a diagnosis of the problem — and quickly recognize that vectoring is enabled on the line. This would trigger a check for major disturbers, leading to identification of the source of the problem.

Integration with SELT — Vectoring support also needs to take SELT measurement procedure into consideration, or else this line testing tool can seriously disrupt vectoring. The Motive Network Analyzer – Copper, for example, automatically makes sure that the highest SELT frequency remains under the vectoring spectrum, which prevents any disruptions.

Maintaining vectoring gains

Troubleshooting in a vectored DSL environment has become more important than ever before because line defects in a single line can impact other lines in the bundle. Leaders in vectoring support are now offering optimization and testing capabilities designed to keep VDSL2 vectoring performing at its best. These capabilities can maintain optimal vectoring gains and keep end users satisfied, while minimizing the time and expense required for problem resolution and customer support.

To contact the author or request additional information, please send an e-mail to techzine.editor@alcatel-lucent.com.