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Author Topic: Explanation of decibels in the context of ADSL  (Read 52482 times)

Pwiggler

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Re: Explanation of decibels in the context of ADSL
« Reply #15 on: October 16, 2007, 12:31:19 PM »

after all that .... thanx eric  :)
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Paul

roseway

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Re: Explanation of decibels in the context of ADSL
« Reply #16 on: October 16, 2007, 12:35:57 PM »

:)
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  Eric

farmergiles

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Re: Explanation of decibels in the context of ADSL
« Reply #17 on: December 17, 2007, 06:14:08 PM »

Quite true. I mention it only because I had a hell of a time finding out what BT referenced their measurements to - I think it took 3 or 4 months before either I or Mr S on adslguide could find anyone in BT Wholesale (as was) who knew what the baseline was. It's a milliwatt (or it was) if anyone is interested  :)

If nothing else you should all remember that 3dB more attenuation (power) halves the signal strength and each additional 3dB halves it again - logarithmic you see. Start working out what 45dB and 60dB (old BT wholesale cutoff figures) actually are in power terms and you'll perhaps see how incredible it is that some people get a connection at all.

It is also possible to discriminate (detect and use) signals below the noise floor. ie you could have a 0dB connection which worked. Not with consumer router/modems though for the filters (electronic, not passive) would cost 100 on their own  ;D


If a 100 filter  made a big difference to people like me on long lines,  many including me would buy one.    ;D
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Ezzer

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Re: Explanation of decibels in the context of ADSL
« Reply #18 on: October 29, 2009, 03:47:30 PM »

Just to expand the subject a little more the following is a ratio for round db numbers

0 db  = ratio of 1   to 1
1 db  =            1.3 to 1
2 db  =            1.6 to 1
3 db  =             2   to 1
4 db  =            2.5 to 1
5 db  =            3.2 to 1
6 db  =             4   to 1
7 db  =             5   to 1
8 db  =            6.3 to 1
9 db  =             8   to 1
10 db =           10  to 1

In the teens multiply by 10, 20's by 100, 30's 1000 etc etc

so 2db   = 1.6 to 1
     32db = 1600 to 1
     52db =  a ratio of 160,000 to 1
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roseway

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Re: Explanation of decibels in the context of ADSL
« Reply #19 on: October 29, 2009, 04:25:41 PM »

I'll agree with that :)
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  Eric

xreyuk

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Re: Explanation of decibels in the context of ADSL
« Reply #20 on: June 17, 2011, 02:46:50 PM »

I realise this is an old thread, but I have a question regarding SNR Margins.

I understand what you were saying about, SNR Margin being the the difference between what you are attaining, and what the lowest acceptable SNR is.

My question is, why does have a smaller SNR margin appear to have an increase in speeds? I am referring to using the DMT Tool to lower your SNR Margin to attain faster speeds.

Please someone correct me if I have got this completely wrong :)
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HPsauce

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Re: Explanation of decibels in the context of ADSL
« Reply #21 on: June 17, 2011, 03:07:48 PM »

You use DMT to adjust the TARGET margin. ;)
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xreyuk

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Re: Explanation of decibels in the context of ADSL
« Reply #22 on: June 17, 2011, 03:09:12 PM »

Ah okay, so why does having a smaller target margin increase speeds?
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tuftedduck

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Re: Explanation of decibels in the context of ADSL
« Reply #23 on: June 17, 2011, 04:16:31 PM »

Hello xreyuk,  :)

Have you read through this page..........http://www.kitz.co.uk/adsl/linestats.htm ..........it may help your understanding.
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HPsauce

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Re: Explanation of decibels in the context of ADSL
« Reply #24 on: June 17, 2011, 04:17:03 PM »

Have a read here: http://www.kitz.co.uk/adsl/adsl_technology.htm
You need to understand how it works, there's no quick explanation. Bit loading is at the heart of it.

It's all a trade-off between speed and stability and while reducing the target margin CAN increase connection speed it's not "free".  ;)

Edit: TD got in first.  :P
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xreyuk

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Re: Explanation of decibels in the context of ADSL
« Reply #25 on: June 17, 2011, 05:48:31 PM »

After reading that, I'm still a bit confused about how a lower target margin means higher speed :S
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HPsauce

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Re: Explanation of decibels in the context of ADSL
« Reply #26 on: June 17, 2011, 06:47:39 PM »

Well as it says (my bold):

Bit Allocation is not actually quite as straight forward as in the above example and there's more to it during the sync negotiation period which has to cover an allowance for errors as defined by the Bit Error Rate (BER) and involves a fairly complicated process called Quadrature Amplitude Modulation (QAM) which is beyond the scope of this tutorial, and this is what determines the final sync speed.  Somewhere in that process is the required overhead for Interleaving and/or more correctly Error Correction, and of course the Target SNR which sets some sort of base line, but...

The QAM rate is said to be 4,000 symbols per second, therefore each 3dB of SNR available in the sub channel over the base line will give approx 4kbps of sync speed, subject to a maximum of 60kbps (15 x 4kbps) per carrier.

 :graduate:
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jeffbb

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Re: Explanation of decibels in the context of ADSL
« Reply #27 on: June 17, 2011, 09:36:52 PM »

Hi

OK .
first  SNR   (total SNR)  this is the total signal to noise ratio the Higher the better . In other words NOISE is very low so plenty of room for signal . lets say that the average SNR over the ADSL band is 48 db then in each of the ADSL channels you can have 16 data bits ,each data bit is 3db. giving us 64Kbits per tone (approx 4Kbits for each data bit) .

 Target SNR margin  this is the minimum SNR ratio for stability  .So for our example the target of 6db has to be taken off the total SNR of 48 db which leaves 42 db for data  , in effect now only 14 data bits are available ,this translates into 56Kbits of data available tone  .# see note 

Now lets say that the line is unstable so the DLM says OK reset target to 12db this is another 2 data bits lost so only 12 data bits which gives only 48 Kbits per tone . #see note

that's how the target SNR affects the synch rate

Notes
 1 Remember my example would be a very good line  averaging 48db per tone  most are below that but the effect of increase Target SNR margin of course applies even if the TOTAl SNR was say only 20db .

 2 There is another thing to consider . If all available databits were actually allocated then there would be problems with stability . There would be no way to do bit swapping . So during the synchronisation routine some data bits are NOT used but are available for bit swapping

Regards Jeff

Regards Jeff
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razpag

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Re: Explanation of decibels in the context of ADSL
« Reply #28 on: June 17, 2011, 09:43:35 PM »

Good explanation Jeff. You've expelled a myth that I believed 15dB was the maximum SNR for a bin. Love this site for knowledge. :)
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xreyuk

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Re: Explanation of decibels in the context of ADSL
« Reply #29 on: June 20, 2011, 10:05:07 AM »

Thanks guys for trying to explain, and thanks Jeff, I think I understand. First question is, it says in the explanation on the website you can only have 15 bits per tone, not 16, which is correct? :) Also, how do you know that 3dB equates to one bit?

Also, 15*4Kbs = 60Kbps per tone. According to the website, ADSL has 223 downstream tones. However, 223*60Kbps = 13,380Mb/s which is too high for ADSL, can you explain this to me?

Second, how do you know how many bits are allocated per tone based on SNR?

I understand now how Target SNR affects speed, but how does having a lower target SNR decrease reliability?

Finally, SNR is obviously defined by the line, and what things are around it. So lets say your SNR is 50dB, and your target SNR is 3dB, how does the line synchronise with a selected SNR, because SNR is defined by the line, and things around it?

@ HP Sauce - I'm actually an apprentice telecoms engineer, and have covered QAM in a basic format, so know what it is. However, can I ask, how is QAM doing 4,000 symbols per second? What exactly does this mean? It's just ADSL I haven't done much of because I work for a major business supplier, who doesn't do residential, meaning ADSL isn't one of our bigger prodcuts. Hence why I'm trying to learn myself!

Thanks guys :)
« Last Edit: June 30, 2011, 02:33:04 PM by xreyuk »
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