Yes Power Spectral/Spectrum Density ... without going into detail about what it is and how it is calculated, you get a PSD figure for each frequency that is in use (for ADSL/VDSL/or anything else). These are plotted on a graph.
There is an Overlay MASK that sits on the graph and is the boundary figures for that particular technology and the transmissions cannot sit outside. the mask is defined to minimise transmission of interference or cross talk to other lines or services and may, for example have notches where other key transmissions opereate.
Use of a PSD mask with the chip set in the DSLAM brings in Dynamic Spectrum Management and allows continual adjustment of power levels at different frequencies to maximise throughput and minimise interference.
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