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Standard User j3214
(learned) Sat 31-May-14 19:12:27
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Accurate way of estimating maximum speed with bit loading?


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Hello thinkbroadbanders!

Does anyone happen to know an accurate way of estimating the maximum speed a phoneline can receive using only the bit loading information?

Now, before I begin, I wish to make it clear that I am not entirely sure if I have the calculations 100% correct. So if I happen to be wrong, please help by correcting me.

I am going to base my information on ADSL2+ Annex A entirely as this is the mode I currently use.

To my understanding, each tone is 4.3125 kHz and can load upto 15 bits (56 kbps). On the upstream, tones 6-31 are usable (total 25). And on the downstream, tones 33-511 are usable (total 478).

I took some bit loading information earlier just after synchronising and did some calculations.

For the downstream, I had 0 tones at 15 bits, 13 tones at 14 bits, 23 tones at 13 bits, 37 tones at 12 bits, 48 tones at 11 bits, 26 tones at 10 bits, 17 tones at 9 bits, 33 tones at 8 bits, 23 tones at 7 bits, 24 tones at 6 bits, 16 tones at 5 bits, 35 tones at 4 bits, 28 tones at 3 bits, 34 tones at 2 bits, and 40 tones at 1 bit.

For the upstream, I had 0 tones at 15 bits, 6 tones at 14 bits, 10 tones at 13 bits, 4 tones at 12 bits, 2 tones at 11 bits, 1 tone at 10 bits, 1 tone at 9 bits 0 tones at 8 bits, 0 tones at 7 bits, 1 tone at 6 bits 0 tones at 5 bits 1 tone at 4 bits, and 0 tones elsewhere.

Based on the information above, I get 10628/1168 kbps (omitting everything beyond the decimal place). Compared to the actual sync, the upstream is only off by about 20 kbps. However, the downstream is off by over 800 kbps.

This information is based on a connection with an approx. attenuation of 38-40 dB on the downstream.

Is calculating the maximum speed using bit loading accurate? Is there something I am missing here? If someone could enlighten me, it would be appreciated.

If any of my above calculations are wrong, please do let me know.
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