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Author Topic: Explanation of decibels in the context of ADSL  (Read 48828 times)

xreyuk

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Re: Explanation of decibels in the context of ADSL
« Reply #30 on: June 30, 2011, 02:24:18 PM »

Any update on this guys? :)
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GunJack

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Re: Explanation of decibels in the context of ADSL
« Reply #31 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 :)
« Last Edit: July 03, 2011, 07:57:21 AM by GunJack »
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roseway

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Re: Explanation of decibels in the context of ADSL
« Reply #32 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.
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  Eric

Weaver

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Re: Explanation of decibels in the context of ADSL
« Reply #33 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.
« Last Edit: January 14, 2012, 04:15:38 AM by Weaver »
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NewtronStar

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Re: Explanation of decibels in the context of ADSL
« Reply #34 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.

 
« Last Edit: April 27, 2014, 12:22:21 AM by NewtronStar »
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kitz

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Re: Explanation of decibels in the context of ADSL
« Reply #35 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
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NewtronStar

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Re: Explanation of decibels in the context of ADSL
« Reply #36 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
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roseway

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Re: Explanation of decibels in the context of ADSL
« Reply #37 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.
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  Eric

NewtronStar

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Re: Explanation of decibels in the context of ADSL
« Reply #38 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

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roseway

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Re: Explanation of decibels in the context of ADSL
« Reply #39 on: April 27, 2014, 06:30:14 PM »

That calculator seems to be correct.
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  Eric

NewtronStar

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Re: Explanation of decibels in the context of ADSL
« Reply #40 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 ?
« Last Edit: April 27, 2014, 08:29:00 PM by NewtronStar »
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roseway

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Re: Explanation of decibels in the context of ADSL
« Reply #41 on: April 27, 2014, 10:37:33 PM »

That's a question for someone more knowledgeable than me. :(

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  Eric

kitz

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Re: Explanation of decibels in the context of ADSL
« Reply #42 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 :)

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.
« Last Edit: April 28, 2014, 10:13:14 AM by kitz »
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konrado5

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Re: Explanation of decibels in the context of ADSL
« Reply #43 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
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kitz

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Re: Explanation of decibels in the context of ADSL
« Reply #44 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
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