VDSL2 vectoring helps fixed-line communications service providers offer fast broadband over existing copper wires—100Mbps and beyond—but there’s much confusion when vectoring is introduced in unbundled or multi-operator environments. Does vectoring help in multi-operator environments?
The according to a recent Alcatel-Lucent (News - Alert) TechZine article “VDSL2 Vectoring in a Multi-operator Envrionment- Separating Fact from Fiction,” by Marketing Director for Wireline Fixed Access, Dr. Stefaan Vanhastel and Paul Spruyt and the company’s xDSL Strategist for Fixed Access,” is that it depends. And, there is a lot to take into consideration.
Vanhastel and Spruyt note that, “Vectoring enables higher bit rates over copper by canceling crosstalk between all VDSL2 lines in a telephony cable… This is achieved through the continuous monitoring of crosstalk coupling within a cable, and the real-time generation of ‘anti-noise’ that cancels out this crosstalk between all pairs. As a result, much higher bit rates can be achieved.”
But if control of all lines is not with a single cable operator, VDSL2 vectoring might not help. It all depends on the type of unbundling. There are two types of unbundling, ADSLx and VDSL2 sub-loop unbundling (SLU).
This is widely used today and typically is a local loop unbundling (LLU) from a cable operator (CO). Vanhastel and Spruyt explained that in fiber to the node (FTTN) deployments, ADSLx lines from CO from multiple operators will co-exist with VDSL2 lines from the cabinet in the distribution cable beyond the cabinet. This they say will not pose a problem since it has a small effect on vectoring gains. The reason is that most gain is realized above the ADSLx spectrum.”
When trying to achieve fast broadband speeds VDSL2 unbundling can cause a significant decline in vectoring gains making deployment much trikier. The reasons cited by the authors are that crosstalk between VDSL2 lines from different providers are terminated on independent DSLAMs and they cannot be cancelled. Vanhastel and Spruyt explain that, “As the VDSL2 lines use the same frequency spectrum there is no way to avoid crosstalk through PSD shaping without significant bit rate loss.”
There is a solution to this challenge. Bitstream (News - Alert) access, that allows VDSL2 vectoring lines to remain under the control of a single access provider while granting access to non-incumbent providers, is a desirable fix for several reasons. An alernative solution to the challenges such deployments can create is to ignore the problem and take the speed gains that happen to accrue. The problem as the authors state is that these could be so slight, and because extra bit rates are not guaranteed, customers are not likely to appreciate the improvement and will be unlikely to pay a premium for them.
Alien crosstalk cancellation is cited as another way to achieve fast broadband speeds with VDSL2 vectoring. Sometimes called “SuperMIMO” (multiple input multiple output), this approach only works if multiple pairs are available per subscriber. Vanhastel and Spruyt noted that, “These limitations render SuperMIMO useless in practical deployments with SLU.” And, another alternative is using dynamic spectral management (DSM (News - Alert)) to mitigate crosstalk between VDSL2 lines from different operators. This alternative also has its strengths and weaknesses. While DSM can reduce interference between VDSL2 lines in the same cable and thus in an unbundled environment cannot offer bit rates comparable to vectoring.
What to do?
So what advice do the authors suggest to service providers? Based on their analysis Bitstream access, particularly the latest VDSL2 vector technology from Alcatel-Lucent, is the superior option for VDSL2 SLU if available. In fact, the authors not that, “Today’s ‘next-generation bitstream’ or virtual unbundling enables everyone to enjoy full vectoring bit-rate gains, along with the flexibility and control needed to differentiate service offerings…None of the alternative solutions, such as alien crosstalk cancelation and dynamic spectral management, can deliver bit rates similar to vectoring.”
All of the above my sound like a lot of technical talk about something that many readers may not be familiar with. The fact of the matter is that of the roughly 600 million broadband connections worldwide, more than half are DSL and another significant slug are part of FTTx (hybrid solutions involving some combination of Fiber and copper). In other words, because of VDSL2 capabilities, service providers around the world can provide customers with quality, high-dpeed broadband using much of their existing plant, while gracefully migrating to all fiber solutions when and where they are needed.
With things like LTE (News - Alert) getting traction as people rely more and more on wireless for their primary broadband experiences, being able to use the existing plant to provide access and compete without a heavy investment in fork-lift projects is crucial. That said, knowing how best to deploy VDSL2 and under what circumstances is key.
Edited by Peter Bernstein