Best Practices for Mitigating the Effect of Uncancelled Crosstalk in Vectored VDSL2

Offering downstream bit rates in excess of 100 Mbps, Vectored VDSL2 uses state-of-art noise cancelation techniques to eliminate crosstalk between VDSL2 services operating from the same DSLAM in the same cable.

However, there also can be uncancelled crosstalk, i.e. crosstalk that vectoring is not able to cancel.  If not properly managed, uncancelled crosstalk can void the benefits of Vectored VDSL2.

The following illustration shows how uncancelled crosstalk occurs.

figure-1-tr-320-blog-sm

The crosstalk generated by all lines originating from DSLAM A that belong to the Vectored Group A can be canceled using vectoring techniques (although some residual crosstalk is always present due to imperfect cancellation). However, those Vectored Group A lines suffer from the uncancelled crosstalk generated by:

  • All vectored lines originating from vectored VDSL2 DSLAM B
  • The lines originating from vectored DSLAM A that terminate on one or more legacy VDSL2 CPEs
  • All non-vectored lines originating from non-vectored VDSL2 DSLAM C

The Broadband Forum has recently released two documents that present a broad industry consensus on best practices on how to mitigate the effects of uncancelled crosstalk in vectored VDSL2:

  • MR-257i2 (2014): “An Overview of G.993.5 Vectoring – Issue 2”
  • TR-320 (2014): “Techniques to Mitigate Uncancelled Crosstalk on Vectored VDSL2 Lines”

MR-257, a Broadband Forum Marketing Report, provides an overview of the benefits of Vectored VDSL2.  The new Issue 2 adds a summary on the remedies that allow mitigating or avoiding the impact of uncancelled crosstalk on vectored lines. TR-320, a Broadband Forum Technical Report, contains a detailed and quantitative analysis on industry best practices that mitigate the effects of uncancelled crosstalk.

A first key finding in the Broadband Forum Reports is that uncancelled crosstalk is likely to occur in a number of common Vectored VDSL2 deployment scenarios which include both bundled (where a single carrier operates all DSLAMs on the same cable) and unbundled (where multiple competing carriers operate different DSLAMs on the same cable) regulatory environments. These scenarios include: particular vectoring implementations (e.g. Board Level Vectoring), gradual deployment of Vectored VDSL2 (when the service on all lines in a Vectored DSLAM is not simultaneously upgraded), presence of multiple DSLAMs (vectored or not, either owned by a single Service Provider or not) connected to the same cable, customers’ choices not to upgrade to a vectored service, technological choices driven by the operator, or where remote firmware update of CPEs to vectoring-friendly CPE is not possible, or when the legacy CPE cannot be upgraded because it is not owned by the same Service Provider deploying vectoring.

The techniques analyzed in these new Broadband Forum reports are the use of Dynamic Spectrum Management (DSM), Vectoring-friendly CPEs, Binder management, Cross-DSLAM Level Vectoring (xDLV) and Cable Level Vectoring (CLV) in two important situations:

  • Coexistence of vectored and non-vectored lines in the same cable.
  • Coexistence of multiple vectored groups in the same cable.

A key finding contained in the Broadband Forum Reports is that DSL management based on DSM Levels I and II techniques is the only mitigation tool that can be applied to every scenario for mitigating uncancelled crosstalk and improving both upstream and downstream performance across either single or multiple uncoordinated DSLAMs. Specifically, TR-320 states that DSM-based management “provides a framework for preserving most of vectoring gains while limiting non-vectored lines to data rates that are often typical of legacy non-vectored VDSL2 service levels.”

TR-320 also concludes that although none of mitigation tools can completely eliminate uncancelled crosstalk, DSM and other techniques can be used to preserve vectoring gains in environments with unbundling or mixed DSL technologies.

Both Broadband Forum Reports validate the importance of using DSM Levels I and II to manage impairments other than DSL crosstalk such as noise ingress and physical plant issues.  Such impairments will impact vectored VDSL performance as they have all previous DSL technologies.

DSM-based management is one of the unique features of ASSIA’s DSL Expresse® Smart Vectoring. ASSIA Smart Vectoring is a solution that accelerates the profitable and immediate deployment of 100+ Mbps vectored DSL services. Its unique DSM-based management mitigates uncancelled crosstalk and, in nearly all cases, legacy lines can maintain their current speeds while also allowing 100+ Mbps vectored line speeds. Furthermore, DSM-based management accomplishes all its benefits without requiring costly hardware or plant rearrangement.

Other important Smart Vectoring features include support for multi-tenancy, which allows a wholesaler of Vectored DSL services to provide individualized DSM-based management to each of their retail providers, and support for multi-vendor DSLAM networks, which allows Service Providers to adopt the multivendor sourcing strategy that best fits their needs without being locked-in to solutions that rely on proprietary tools.