lol... you've just witnessed what I call one of my 'off the top of head blurbs' which I will on occasion do on the forum... you will notice that I say 'guess' or suspect. I usually put a warning up that what Ive written is 'afaik'.. simply because this would be an area that requires much more research and some areas are stretching my limits simply because theres little info about to be absolutely certain.
Its often something that Ive come across amongst some real techo stuff and its stored somewhere in the back of my head and doesnt come out again until asked on occasion such as this.
Stuff on the main site is much more carefully presented.. but on occasion I do like to discuss and the warning is there simply because if anyone else has anything or more knowledge to add then its appreciated.
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Something I did forget to mention in the above post - but it is something that I have mentioned previously which youve probably seen... is that routers/modems also have their own masks which are applied depending on the type of adsl (eg adsl1 or adsl2+ or annex_M) to also ensure that the Signal doesnt get too strong within each tone.
You may see these referred to as PSD (Power Spectral Density) Masks, and the router will apply different PSD masks depending upon which type of adsl you configure - eg g.dmt, adsl 2, adsl2+, gdmt.lite, annex_M etc. These masks tell the router which bins it can use and which bins should transmit a signal no more than 'x'dB.
Not all routers contain all masks - hence why some routers may not be adsl2+ compatible... or in the case of the 2wire I recently bought it (disappointingly) doesnt contain an Annex_M mask... therefore it steadfastly refuses to transmit data at the required frequencies and over-rides anything set on the dslam
Ive just done a quick google and theres a little bit more info on PSD masks from Texas Instruments who are one of the major router and dslam chipset manufacturers
here.
The opening paragraph says it all really.
"
The purpose of the limitation of the PSD mask is to reduce cross-talk noise and to limit power consumption."
By all means continue to read it as it does contain some useful info, but Im afraid my eyes often glaze over when I start to see algorithms/complicated maths and/or lots of figures.. and thats when my brain refuses to store any more and I start just using 'x'