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Broadband Related => Broadband Technology => Topic started by: roseway on September 17, 2006, 11:06:06 AM

Title: Explanation of decibels in the context of ADSL
Post by: roseway on September 17, 2006, 11:06:06 AM
I just came across this document which I put together a couple of years ago. It attempts to explain what decibels are and how they relate to ADSL connections.

-----------------------------------------------------------------------------------------------------------------
Attenuation, SNR and decibels
This is an attempt to explain what these terms actually mean, and their significance for broadband performance.

The decibel
---------------
The decibel is a logarithmic unit which specifies the ratio of two powers. It was devised by the Bell Telephone Company to simplify calculations of gain and loss in cascaded transmission systems. The original unit was the Bel, and they defined this as:
 
 Number of Bels = logarithm (base 10) of the ratio OutputPower / InputPower.
 
Because this was rather too coarse a unit for many purposes, they specified the decibel (dB) as one tenth of a Bel.
This may seem a rather cumbersome way of specifying a power gain or loss, but it has some advantages:

1. Because of the way that logarithms work, the overall gain of several cascaded stages is simply the sum of the decibel gains of each stage. This is a lot easier than multiplying and dividing the power ratios.
2. The log of a number less than 1 is negative, so all power gains have positive dB values and all losses have negative dB values. If the input and output powers are equal, then the gain is 0 dB.

Attenuation
--------------
Ordinary twisted-pair telephone lines were never designed for the high frequencies used in ADSL transmission. Not surprisingly a lot of the ADSL signal gets lost between the exchange and the user. What may surprise you is just how much gets lost...

Prior to 6 September 2004, the limiting attenuation figure for a UK ADSL installation was 60 dB (i.e. a gain of -60 dB). This is equal to 6 Bels, and therefore the logarithm of the power ratio is 6. In other words, the signal at the end of the line is just one-millionth of the signal transmitted from the exchange. And as from 6/9/2004 the limits were relaxed further, with 75 dB being the level of attenuation at which a usable ADSL service is considered extremely unlikely - that's a power ratio of about 32 million. Lower levels of attenuation are obviously preferable.

Signal-to-noise ratio
-------------------------
Just to make life even more complicated for the receiving equipment, spurious signals get induced into the lines carrying the ADSL signal. These spurious signals can come from other telephone lines running alongside, from external electrical sources, from poor quality connections, and from electronic circuits processing the signals. Together, these unwanted signals are referred to as 'noise'. Clearly, to enable the receiving equipment to separate the wanted signal from the noise, the wanted signal must be greater than the noise by a reasonable margin. The ratio between the wanted and unwanted signals is called the signal-to-noise ratio, or SNR. The greater this ratio is, the better.

The limiting value of SNR is generally taken to be 6 dB (= 0.6 Bel). This corresponds with a power ratio of 4 (log 4 = 0.6). So the wanted signal power must be at least four time the noise power. And because power is proportional to the square of the voltage, this means that the signal voltage must be at least double the noise voltage. Quite obviously, any lower values of SNR will make it very difficult to separate out a clean ADSL signal.

It should be noted that ADSL modems and routers generally report noise margin, not SNR. The manufacturer specifies a SNR at which the error count reaches some threshold beyond which the connection would not be acceptably stable; the noise margin is the amount by which the measured SNR exceeds this base level of SNR. So, for example, if the minimum acceptable SNR was set at 6 dB, and the measured SNR was 10 dB, then the modem/router would report a noise margin of 4 dB.
-------------------------------------------------------------------------------------------------------------------------

Eric
Title: Re: Explanation of decibels in the context of ADSL
Post by: Astral on September 17, 2006, 11:14:07 AM
Bravo Professor Roseway! I actually understood that, which fits in very nicely with the ethos of this site, techy stuff in plain english.
Title: Re: Explanation of decibels in the context of ADSL
Post by: mr_chris on September 17, 2006, 11:18:06 AM
Ooh, nice one Eric :) That's one of the best ways I've ever seen it described.

The one thing that makes me chuckle is when people say (and I've been guilty of it myself too!) "oh look, my SNR is only 3 or 4 dB different to yours"... not realising that a 3dB gain actually means twice the power or 1.41 times the voltage - a significant change!

One thing I would say is that you might want to cover SNR Margin, which ADSL modems report, rather than absolute SNR? Just a thought..

A truly excellent guide :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: kitz on September 17, 2006, 11:25:12 AM
excellent explanation thank you for sharing.

So good Ive stickied it :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on September 17, 2006, 04:15:36 PM
Thanks for the kind remarks, people. I've edited it to add a comment about noise margin, as suggested by Chris.

Eric
Title: Re: Explanation of decibels in the context of ADSL
Post by: guest on October 15, 2007, 07:04:52 PM
As an ex-electronics engineer who specialised in radar I'd like to point out that dB measurements are meaningless unless you reference what the measurement is based upon.

eg a dBm would usually be a reference against a milliwatt of power at the source. Which brings the question of power and voltage into play (voltage is 20 Log and power is 10 Log) but that is just going to confuse most of you - no offence intended.

I could come up with a dB measurement which would sound incredible - but what is the baseline? dB on its own means nothing - it is what it is referenced to that is important.

Edit - rule of thumb is that a loss of 3dB halves the power. So 6dB = quarter the source power, 9dB = an eighth etc etc

Edit 2 - may seem like I'm a "know it all"? I don't but I do know dB measurements as defence contractors stretch radar detection "specs" to the limit (and beyond) so I do know how to "manipulate" the figures.
Title: Re: Explanation of decibels in the context of ADSL
Post by: Astral on October 15, 2007, 07:40:33 PM
To be fair to Eric the title does state "in the context of ADSL". One can go to PhD level and beyond with one's explanations, but that would tend overload/send to sleep a high proportion of your intended audience.

As I said, above, the beauty of this site is that it (mostly) imparts technical information in plain English.
Title: Re: Explanation of decibels in the context of ADSL
Post by: guest on October 15, 2007, 08:12:10 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
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on October 15, 2007, 10:58:45 PM
Just to be clear about this, decibels are a ratio not an absolute measurement. dB measurements aren't meaningless when used in the way I defined them above, i.e. as a ratio of two different power levels. Attenuation and SNR are both ratios, not absolute measurements. So while your point is correct, it doesn't (with respect) apply to what I wrote above.
Title: Re: Explanation of decibels in the context of ADSL
Post by: guest on October 15, 2007, 11:48:17 PM
Fair enough Eric - I spose for the average user it doesn't. Like I said though it took a bit of work to get the reference point BT used - at one point they told me it was dBW. I suggested they may wish to go into the central heating business as 13k lines with 25% ADSL enabled = 3-4kW ;D I suspect that'd give Openreach even more faults than now :P

Everyone just remember that every extra 3dB = half the signal. Useful rule of thumb. Always was for me when working out how far to stand away from radomes anyway <can't find a smiley being zapped kitz?>

Twas just a suggestion - as you say dB is a ratio but ask any engineer (real engineers rather than technicians called engineers) about it and they will say the same thing. A "ratio" is meaningless unless you know what the "ratio" is measured against.

Oh dear I'm causing trouble again. I really didn't intend to annoy or offend.
Title: Re: Explanation of decibels in the context of ADSL
Post by: kitz on October 16, 2007, 12:08:28 AM
>> Oh dear I'm causing trouble again.

Yeah you are so behave or you'll get sent to the naughty corner  :lol:

/me goes to try and find a zapped smiley

Seriously though this forum is perhaps a bit more less techy than some - crikey you lost me cause Im no electronics person - and probably way more relaxed about certain things since most of the people asking for help need things pretty low key.   Ive managed to ramble on about stuff more than a few times not realising that I'd perhaps taken it a bit too deep for whom Im trying to respond to and stuff has gone way over their head :(
Title: Re: Explanation of decibels in the context of ADSL
Post by: guest on October 16, 2007, 12:26:11 AM
Yeah I know I'm the same.

The best thing really to remember (again!) is that the 3dB point on any signal is the point at which the signal has halved in power. It does bring the problem of long lines into very clear perspective.

Eg I have a 23dB figure on downstream - that means that the 1 milliwatt (one thousandth of a watt) which comes from the DSLAM is actually somewhere around 50 microwatts* (50 millionths of a watt) when it gets here. That gets me 20Mbps. I still find that amazing and I used to design radar systems  ;D

*I'm doing that in my head - seems roughly right but correct me if you think its wrong (it may be)
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on October 16, 2007, 08:01:44 AM
Quote
Twas just a suggestion - as you say dB is a ratio but ask any engineer (real engineers rather than technicians called engineers) about it and they will say the same thing. A "ratio" is meaningless unless you know what the "ratio" is measured against.

I'm sorry to labour the point, but none of this is relevant to the article at the top of this thread. A ratio is not meaningless, it's what it says - a ratio, i.e. one number divided by another. It doesn't need a reference point, because it's not an absolute value. SNR is the ratio of signal power to noise power, and attenuation is the ratio of transmitted power to received power. Both are normally expressed (quite correctly) in decibels.

The fact that many people misuse decibels is not a point which has relevance here, and I wouldn't want people reading the article to be confused by the last few messages. The article was a serious attempt to explain a technical subject in reasonably simple terms, and it doesn't misuse the term dB in any way.
Title: Re: Explanation of decibels in the context of ADSL
Post by: guest on October 16, 2007, 11:29:12 AM
Fair enough - I really didn't intend to annoy you and I obviously have. My sincere apologies.

See kitz I'm playing nice :P
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on October 16, 2007, 11:35:58 AM
No problem. :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: Pwiggler on October 16, 2007, 12:31:19 PM
after all that .... thanx eric  :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on October 16, 2007, 12:35:57 PM
:)
Title: Re: Explanation of decibels in the context of ADSL
Post by: farmergiles 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
Title: Re: Explanation of decibels in the context of ADSL
Post by: Ezzer 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
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on October 29, 2009, 04:25:41 PM
I'll agree with that :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: xreyuk 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 :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: HPsauce on June 17, 2011, 03:07:48 PM
You use DMT to adjust the TARGET margin. ;)
Title: Re: Explanation of decibels in the context of ADSL
Post by: xreyuk on June 17, 2011, 03:09:12 PM
Ah okay, so why does having a smaller target margin increase speeds?
Title: Re: Explanation of decibels in the context of ADSL
Post by: tuftedduck 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.
Title: Re: Explanation of decibels in the context of ADSL
Post by: HPsauce 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
Title: Re: Explanation of decibels in the context of ADSL
Post by: xreyuk 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
Title: Re: Explanation of decibels in the context of ADSL
Post by: HPsauce 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:
Title: Re: Explanation of decibels in the context of ADSL
Post by: jeffbb 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
Title: Re: Explanation of decibels in the context of ADSL
Post by: razpag 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. :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: xreyuk 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 :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: xreyuk on June 30, 2011, 02:24:18 PM
Any update on this guys? :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: GunJack on July 03, 2011, 07:53:16 AM
one thing to bear in mind is that, in the examples quoted, all the numbers are theoretical maximums, and therefore some allowance needs to be made for variances in everything. If every line was perfect, then you could work out a definite formula for bits/tone, sync speed for a given SNRM, etc. Unfortunately, as we all know only too well, there's no such thing as a "perfect" line :(   

..and it's not only the line that has variances. Take any amplifier circuit - whilst the theoretical frequency response curve will be absolutely perfectly symmetrical about the centre frequency, in practical terms this will pretty much never happen. So, every router, DSLAM, etc involved in the overall circuit will degrade the signal to some degree, another reason why an absolute speed/line attenuation/snrm equation can only ever be an estimate :)
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on July 03, 2011, 08:10:30 AM
In addition to that, some available bits will be deliberately be left unused, in order to leave some space for bitswapping. Without this breathing space, the connection wouldn't be able to cope with bursts of interference.
Title: Re: Explanation of decibels in the context of ADSL
Post by: Weaver on January 14, 2012, 03:12:22 AM
In ezzer's table back there, the ratios on the right are in Watts where it says 2:1 it means double the power in Watts which is approx 3dB from the table. By definition, 10dB means "ten times the power (W)".

In comms, we often don't want to talk in Watts, but rather about the voltage level of the signal on the line, in Volts (V). So watch out for this. Saying something such as merely "double", means "double what", Watts or Volts? You have to say which units you're talking in.

Now, power W = V2/R which is just Ohm's law combined with the definitions of Volts and Watts, so for example 100 x power in W = 102 equates to 10 x Voltage, because  power is proportional to voltage squared. Another example: 2 x V equates to 22 x W = 4 x W.

In ezzer's table, if you want to convert dB to V, you need to take the square root of the power ratio given. Talking in voltage terms not W, to convert voltage to dB, double voltage means 6 dB roughly, look down the rh col for the ratio you want, then double the number in the dB column. (why, the doubling of dB comes from the square in the previous para). Remember double V = 6dB approx.
Title: Re: Explanation of decibels in the context of ADSL
Post by: NewtronStar on April 27, 2014, 12:18:20 AM
Cheers I created a thread a a few weeks ago about power (dBm)

you see I got confused as because one Modem Stats says 11.1 (dBm) and another says -11.0 (dBm) i take it +11.1 is the correct one in my case.

 
Title: Re: Explanation of decibels in the context of ADSL
Post by: kitz on April 27, 2014, 12:44:04 AM
I think my brain has just exploded in another thread, so I may be missing something...  but both of your figures in the screen caps show 11.1 dB for the downstream?

-11.0 is for the U0 channel.   Its not unusual to see this on the upstream VDSL channels as they have aggressive PSD masks applied to them.  My U1 is -28dB...  but my aggregate upstream TX is still 4.4dBm.


Apologies if I mis-understood what you were getting at, my head hurst
Title: Re: Explanation of decibels in the context of ADSL
Post by: NewtronStar on April 27, 2014, 12:54:32 AM

Apologies if I mis-understood what you were getting at, my head hurst

Thats fine Kitz if U0 is meant to be -11.0, I have been reading this stuff for over a few weeks it's not registering in my brain to much maths envolved, maybe Brian Cox can explain it better with a stick and a large sandy beach  :D
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on April 27, 2014, 07:52:13 AM
Remember that 0 dBm = 1 milliwatt (one thousandth of a watt)
Then -10 dBm = 0.1 milliwatt (one 10,000th of a watt)
So -11 dBm is just a bit less than that.
Title: Re: Explanation of decibels in the context of ADSL
Post by: NewtronStar on April 27, 2014, 05:21:54 PM
Remember that 0 dBm = 1 milliwatt (one thousandth of a watt)
Then -10 dBm = 0.1 milliwatt (one 10,000th of a watt)
So -11 dBm is just a bit less than that.

Thankyou Roseway the + dBm I can get my head around now, it was the - dBm that got me all confused and your post has enlightened me some what.

0 dBm = 1 milliwatt then -1 dBm should = 0.79 milliwatts if this calculator is correct  :-\
http://www.ringbell.co.uk/info/dbm.htm (http://www.ringbell.co.uk/info/dbm.htm)

Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on April 27, 2014, 06:30:14 PM
That calculator seems to be correct.
Title: Re: Explanation of decibels in the context of ADSL
Post by: NewtronStar on April 27, 2014, 08:17:07 PM
That calculator seems to be correct.

Great Eric, now comes the hard part for me I have 11.1 dBM (12.88 milliwatts) on the DS line with 6.0 dB SNRM so because of the background noise on my line the TX power will be higher so as to overcome the noise and receive the BB signal louder than the noise on the line.

so if i've got it right then a lower dBm on broadband meens the line has less noise and so less milliwatts is used in TX as theres less background line noise ?
Title: Re: Explanation of decibels in the context of ADSL
Post by: roseway on April 27, 2014, 10:37:33 PM
That's a question for someone more knowledgeable than me. :(

Title: Re: Explanation of decibels in the context of ADSL
Post by: kitz on April 27, 2014, 11:35:42 PM
>> if this calculator is correct

Theres also one on the main site, but I rounded a bit more Output Power (http://www.kitz.co.uk/adsl/linestats_explanation.htm) :)

Quote
a lower dBm on broadband meens the line has less noise and so less milliwatts is used in TX as theres less background line noise ?

No, not necessarily :(

The over-riding fact when it comes to power is PSD masks & power cut back. These control the amount of power that is given to a line, and the shape of maximum bits that can be loaded at any particular frequency.
iirc BT use 5 different Power profiles for adsl/adsl2+  (havent ever seen anything specific about vdsl), these masks are profiled depending enitrely upon your line length. 

I'm assuming the profiles will be something like : very short, short, medium, long and very long...  and depending upon your line length I've no further info how they categorise. The profiles and psd masks will control the maximum amount of power given to each individual tone.  Regarding the masks, they principally shape on frequencies to prevent x-talk within the sub channels and types of dsl that may use tones differently ie annex a/m.   Regarding profiles, The idea is that short lines are given less power than longer lines so that short lines signals dont drown out longer lines.

For adsl2+ you will find that use of masks ensures that no-ones power goes much above 20dBm.   The starting point is 18dBm, but theres also a little bit held in reserve for bitswap or if the line is struggling.   This means that all lines (except the very shortest) should always see the downstream power in the region of 18-20dBm.  Under performing lines that are capable of much higher speeds or not reaching their full potential will also show reduced power.

With FTTC, I dont know the profiles, but each of the subchannels (U0,U1,U2,U3,D1, etc) will have their own profiles based on line length.   The reason why my U1 power is -28dB is because my line must be classed (guessing) as short therefore its throttling back to make sure U1 doesnt get too much power meaning a better signal quality for the longer lines.   

Every single tone in every sub-channel will have its own power shaping going on depending upon which profile BT class your line.  Although our router may show a total power for upstream & downstream, these are just the aggregate of all the tones in use. 

Im trying to think of an easy way to explain it, the best way I can think of is by using my own line as an example.


These are my power profiles

Code: [Select]
VDSL Band Status U0 U1 U2 U3 U4 D1 D2 D3
TX Power(dBm): -6.6 -28.0 4.0 N/A N/A 11.8 7.4 7.4


Note how my U1 band has very little power.   Now look at what effect this has on bit loading.. (cap below)..  see how it means that hardly any bits are loaded in this sub-channel.     However, my line is good..   the effect is that it forces my line to load more bits into the U2 subchannel, leaving U1 cleaner (x-talk) for longer lines that can use those frequencies.


PSD masks and profiles are important factor in adsl/vdsl. They help reduce crosstalk and gives longer lines less chance of being drowned out by short lines - which is exactly what would happen if BT didnt control the channels using masks and all lines were given the same amount of power.

----
ETA

Also uploaded SNR per tone and QLN & Hlog graphs to show that we have to be very careful in assuming that less dBm means less background noise.

If you look at the SNR per tone & QLN, then you could assume that these tones were more noisy...  but this isnt the case..  what has happened is power cut back has been applied... which means my signal over these tones isnt as strong.    In turn this causes less bit loading.
If someone didnt know what was going on, then they could assume by just looking at the SNR & QLN that I had a problem in U1... when in fact I dont.   My Hlog confirms that all is actually very good and its power cutback thats causing the lower SNR and bitloading.
Title: Re: Explanation of decibels in the context of ADSL
Post by: konrado5 on April 28, 2014, 12:02:33 AM
What is relation between overall power output value and power output on particular tones? Are unused tones included in overall power output value?

Best regards
konrado5
Title: Re: Explanation of decibels in the context of ADSL
Post by: kitz on April 28, 2014, 10:20:18 AM
What is relation between overall power output value and power output on particular tones?

Although our router may show a total power for upstream & downstream, these are just the aggregate of all the tones in use. 

Although Im unsure if its tones in use or all available tones across the sprectrum
Title: Re: Explanation of decibels in the context of ADSL
Post by: Loading on April 28, 2014, 12:02:14 PM
When the connection is syncing slower, the output power is also lower, as less tones are used.
Title: Re: Explanation of decibels in the context of ADSL
Post by: kitz on April 28, 2014, 12:22:05 PM
When the connection is syncing slower, the output power is also lower, as less tones are used.

Thank you for that clarification..   if you mean what I think you mean... then I tried to cover that in "Under performing lines that are capable of much higher speeds or not reaching their full potential will also show reduced power."

If it meant something else it (less tones used) then that got me thinking on the question " if its tones in use or all available tones across the spectrum"..  so therefore if less tones = less power and knowing that the output should in theory be the same between syncs (ignoring the reserve for bitswap), then it would imply power is calculated across all tones regardless if in use or not.   


A bit of digging in some router manufacturers documents, would seem to confirm at least some (all?) routers calculate across all the available tones in the spectrum.
Title: Re: Explanation of decibels in the context of ADSL
Post by: konrado5 on April 28, 2014, 04:33:47 PM
But I've also notices when sometimes I disable many tones in my router I have higher power output (not lower). Moreover, I have PSD masks for HAM-band and high power output: 18.6-18.8 dBm.
Title: Re: Explanation of decibels in the context of ADSL
Post by: Loading on April 28, 2014, 06:12:44 PM
Are you sure? As i end up with a lower power output 16.8, to 16.0 when tones aren't in use
Title: Re: Explanation of decibels in the context of ADSL
Post by: NewtronStar on April 30, 2014, 01:15:08 AM
Fantasic Explanation Kitz on the dBM power even I could understand it  ;)

So it looks like a fine balance to keep all customers happy with their BroadBand at the Exchange and in the FTTC cabinet.

I have learnt so much about Broadband the How's and Why's from the Kitz site than any other site in this 5 year journey to understand the BB workings.

Kitz and Members I give you my Five Star rating.