Kitz Forum
Broadband Related => Broadband Technology => Topic started by: Weaver on November 11, 2020, 07:33:38 AM
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referring to say https://i.postimg.cc/gj3FL1D2/0926-F881-C9-EE-4-CA8-9-CD1-2-BBDC4691-EC7.png
In the upstream band of ADSL, why does the bit-loading go down with increasing frequency? Is it a power spectrum thing for crosstalk avoidance purposes or some such? Or is it just a really bad filter approaching the inter-band gap ? - seems very unlikely.
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A small part of it will be the power spectrum limits.
However, in your case, because your line is so very long, mainly it's for the same reason as downstream bit-loading goes down with increasing frequency. The higher the frequency, the higher attenuation, and therefore lower signal and lower SNR with increasing frequency.
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If it's useful Weaver, this is what the bitloading looks like on my 900m long ADSL2+ circuit. On the upstream, I have a few steps down in bitloading when approaching the POTS side and the same on the downstream side. Presumably, as you say, this is due to restricted power for these tones to avoid interference with the voice service and the downstream tones.
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15 bits per tone is maximum. Weaver, you're only seeing 11 max. on the upstream, which is probably mostly down to your length. As ejs said, it's going to drop off at higher frequencies due to attenuation.
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This is the idea that came to me, probably stupid; if we mentally take out crossover-type filter effects and all power cut-back spectral allowances, why isn’t the bit loading monotonically decreasing regardless of upstream / downstream ?
It would be instructive to look at VDSL2 also, because of the multiple upstream bands.