Vectoring greatly improves the performance of VDSL2 by using physical layer signal processing to enable cancellation of crosstalk between all the lines that terminate on a single DSLAM. Once crosstalk is removed, the performance of VDSL2 becomes independent of the number of VDSL2 lines in a cable and approaches (ideally) that of a single VDSL2 line which is unaffected by crosstalk. This has the effect of as much as doubling VDSL2 speeds on very short lines, with diminishing speed increases on longer lines where the crosstalk is weaker. Vectored VDSL is most suitable for deployment from a node, and is the most economical in terms of required capital expenditure. The figure below shows speeds achievable by vectored VDSL2, using profile ADE17, with 20 dB of crosstalk reduction in a vectored group of 12 lines, on 0.4 mm cable, with 10% rate overhead. Speeds reach over 100 Mbps in the downstream directions for loops shorter than 550 meters (1800 feet). The sum of upstream and downstream rates exceeds 200 Mbps for loops shorter than 200 meters (700 feet).
Care needs to be exercised to ensure that all the lines emanating from the Vectored VDSL DSLAM are either in the same vector group, or are managed to avoid interference. Node-level vectoring allows large vector group sizes (e.g. 96 to 384), so that every line from a given node can be vectored. Board-level vectoring uses smaller vector group sizes (e.g. 16 to 64), and is appropriate for small nodes or for FTTB deployment. In practice, vectoring cannot cancel all crosstalk. There is always some residual crosstalk present due to imperfect cancellation, Furthermore, there can also be uncancelled crosstalk, i.e. crosstalk that vectoring is not able to cancel. The following illustration shows how uncancelled crosstalk occurs. The crosstalk generated by all lines originating from DSLAM A that belong to the Vectored Group A can be canceled using vectoring. However, 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 and that terminate on one or more legacy VDSL2 CPEs
- All non-vectored lines originating from non-vectored VDSL2 DSLAM C
The presence of this uncancelled crosstalk has initially caused a misunderstanding of the conditions under which vectoring can deliver its gains. Fortunately, the Broadband Forum has released two documents that help dispelling these misunderstandings by presenting 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”
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. Another 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.