VDSL2 Crosstalk Classifications
VDSL2 crosstalk on DSLAM is classified as near-end crosstalk (NEXT) and far-end crosstalk (FEXT).
Figure shows the crosstalk of the two types.
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- In NEXT, TX signals are sent from the interfering pair, coupled to the interfered pair, and then sent to the near-end RX end of the interfered pair. For example, in a bundle of lines, when signals in the upstream direction of a line interfere with signals in the downstream direction of an adjacent line, or signals in the downstream direction of a line interfere with signals in the upstream direction of an adjacent line, NEXT occurs.
- In FEXT, TX signals are sent from the interfering pair, coupled to the interfered pair, and then sent along the interfered pair to the far-end RX end of the interfered pair. For example, in a bundle of lines, when signals in the upstream direction of a line interfere with signals in the upstream direction of an adjacent line, or signals in the downstream direction of a line interfere with signals in the downstream direction of an adjacent line, FEXT occurs.
In other words, NEXT is interference between upstream signals and downstream signals of different pairs, and FEXT is interference between upstream signals or between downstream signals of different pairs.
How Can We Eliminate NEXT and FEXT
- VDSL2 uses the frequency division multiplexing (FDM) technology to transmit data. Therefore, TX signals of the interfering pair and RX signals of the interfered pair use different frequencies. Therefore, the impact of NEXT can be eliminated or mitigated using a filter.
- TX signals of the interfering pair cannot be eliminated using a filter because these signals use the same frequency band as the RX signals of the interfered pair. In addition, VDSL2 uses a high frequency band (up to 30 MHz) for short-distance transmission (usually within 1.2 km). As a result, FEXT has a more severe effect on VDSL2 than on other DSL access modes. Therefore, FEXT is the main factor of degrading VDSL2 performance. To eliminate FEXT, the ITU-T Recommendation promoted G.993.5-compliant vectoring.
NOTE:
- To eliminate or mitigate crosstalk, the DSL industry promoted a series of techniques totally called the dynamic spectrum management (DSM) technology. The DSM technology involves four stages, level 0 through level 3 stages. At level 0 through level 2 stages, the AN manages the spectra of the TX signals of single- or multi-DSL pairs, which eliminates FEXT only to a certain extend. To completely cancel FEXT on VDSL2 lines, the ITU-T Recommendation launched vectoring. Vectoring uses vectors to cancel FEXT on VDSL2 lines, thereby significantly improving the bandwidth and performance of multi-pair VDSL2 lines. Therefore, vectoring is also called level-3 DSM.
- Vectoring can significantly eliminate only FEXT.
