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Author Topic: Bit-Loading Question  (Read 1821 times)

jack21

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Bit-Loading Question
« on: May 09, 2009, 10:25:32 AM »

Hello,

On the bit-loading chart given by my BT 2700HGV (typical below - sorry about the spacings being unaligned), I'm interested in how the bit-loading relates to actual line rate. On several occasions, I've added up (or rather my PC has!) all the bits-per-tone totals, and don't see an obvious relationship between the total and the actual line rate.......typical is a total (for downstream) of 461 while the actual line rate (the Max3 value) is 1252. From the Kitz article on bit loading, I was expecting the 2 figures to be equal. How does it work out in practice? - is there a 'factor' be be applied?

My line is 6km+, 68.2db attenuation, target SNRM 9

Thanks,
Jack

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jid

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Re: Bit-Loading Question
« Reply #1 on: May 09, 2009, 10:40:03 AM »

Firstly, what did you use to get the bit loading data, if you used RouterStats, could you post up the actual screenshot of it to give us a better idea, or if anything else, post the actual screenshot.

Also, can you post your full line stats:D

Kind Regards

Jamie
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Kind Regards
Jamie

TalkTalk Fibre 76meg | Sky+2TB | See My DSL Connection Stats | Bridgend Weather

roseway

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Re: Bit-Loading Question
« Reply #2 on: May 09, 2009, 10:48:32 AM »

I don't think it's a simple as just adding up the bits. A 4-bit number for example can have 2^4  (=16) different values. So there's some algorithm which sums these individual values and combines them into a connection speed. But I'm merely speculating, because I really don't know.
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  Eric

jack21

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Re: Bit-Loading Question
« Reply #3 on: May 09, 2009, 11:17:30 AM »

Sure, here's the screenshot and linedata (uptime of almost 48 hours); a quick script I wrote adds up all the bucket contents Hex values (as detailed in the screenshot).

Time     Line                                       Rate     Max1     Max2     Max3     Mgn1     Mgn2     Attn     Pwr     CRCs     FECs     INP     DLY         Rate     Max     Mgn     Attn     Pwr     CRCs     FECs         Mode     Vendor     State     Exit Code     Echo     VCXO     Rx Gain     INC Tones

2009/05/09 11:06:11 BST     1         1504     1536     1268     1268     9.4     7.0     68.1     17.2     976     199959     1.23     8.00         448     632     13.0     31.5     12.3     166     1151         G.DMT Annex A     TI     229/103     N/A     -16.5     0.8     34.5     0


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kitz

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Re: Bit-Loading Question
« Reply #4 on: May 09, 2009, 12:09:48 PM »

It something to do with QAM (Quadrature Amplitude Modulation) which is beyond my full comprehension..  
But each bit is worth approx 4kbps of speed, as the QAM rate for dsl is 4,000 symbols per second.

Bits per channel can constantly change due to bitswapping and may not always be the same as when sync was first attained which is when your speed is negotiated.  The total downstream range must be able to support sufficient bits though or you will loose sync.

Theres also something else thrown into the mix which can affect your sync.. and thats any line profiles for error correction & interleaving.  But I dont know exactly how much effect this has.

Ive not checked mine that often but when I have, the 4kbps per bit loaded whilst never exact, its never been too far off - but I am on a low profile with interleaving off.

If youre good with maths and electronics and understanding algorithms.. then researching QAM is the way to go.   If someone does and can come up with a nice easy explanation, then I'd love to know myself.
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jack21

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Re: Bit-Loading Question
« Reply #5 on: May 09, 2009, 12:48:15 PM »

Thanks Kitz, that roughly ties in with what I've seen, tho the factor I'm arriving at seems to be between 2.75 and 3.0 - that is actual at-the-moment line speed (the Max3) divided by the at-the-moment bit-count total. Maybe that reflects the error correction and interleaving (which is ON).

The main reason I asked is that I wanted to have an easily-recorded summary of the actual bit-loading profile without having to compare the complex shapes - just as an aid for my tuning-for-best activities. As it is, the Max3 figure has been the best, and I do have to look at the chart to see when the periodic (but significant) loss in the higher frequencies causes a loss in speed. The discrepancy between actual bit-count and speed had puzzled me.

Cheers,
Jack
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