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Author Topic: Not too bad  (Read 8966 times)

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Re: Not too bad
« Reply #15 on: January 20, 2012, 11:03:42 AM »

So the conclusion that I draw when comparing those 3 sets of data is that there is no reliable relationship between attenuation and road distance to the exchange.

Correct. Other than a very broad "rule of thumb", to be taken with a big "pinch of salt".  :-X

So why even begin to associate straight line distance and road distance with attenuation......it may as well be the number of lamp posts between you and the exchange.

Logically the further you are from the exchange, the more lamp posts, cats eyes in the road (apologies bc), telephone poles etc etc. there will be.
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Bald_Eagle1

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Re: Not too bad
« Reply #16 on: January 20, 2012, 11:21:57 AM »

So the conclusion that I draw when comparing those 3 sets of data is that there is no reliable relationship between attenuation and road distance to the exchange.

Correct. Other than a very broad "rule of thumb", to be taken with a big "pinch of salt".  :-X

So why even begin to associate straight line distance and road distance with attenuation......it may as well be the number of lamp posts between you and the exchange.

Logically the further you are from the exchange, the more lamp posts, cats eyes in the road (apologies bc), telephone poles etc etc. there will be.


Just a little food for thought..............

Logic, BT and ISPs don't always fit nicely in the same sentence.

I think attenuation AND physical distance from the exchange/cabinet need to be considered together.

e.g. Using the Kitz ADSL calculator, a DS attenuation of 30dB suggests an Approx Line Length of 2.2 km.

If a given user knows (for a fact) that their own line length is say, only 1000m, but they are seeing 30dB DS attenuation, it could indicate a severe, repairable fault (High Resitance etc.).

It may just be that their connection includes a lot of aluminium cable, or the copper has corroded down in thickness over time (in which cases there would be very little, if anything, the user could do about it).
 
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roseway

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Re: Not too bad
« Reply #17 on: January 20, 2012, 11:35:05 AM »

The figure of 14 dB per kilometre has always been reckoned to be a reasonably typical figure for the UK (and indeed for many other countries too), but as you say, there are many ways in which the actual value can be quite a bit higher. There's a lot more to it than the electrical resistance of the cable of course - we are dealing with AM radio frequencies being carried over cables designed for low frequency audio. So the largest element in the attenuation is not the resistance, it's the reactive effects (capacitance and inductance), and these will vary depending on the type of cable, its proximity to other cables and metal objects, and so on.
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Bald_Eagle1

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Re: Not too bad
« Reply #18 on: January 20, 2012, 11:56:31 AM »

If I'm not mistaken the Kitz calculator uses 13.81 dB per km (I like exactness  ;)) at an assumed SNRM of 6dB, also assuming 0.5mm copper (AWG24 or AWG 26 - can't remember which) for ADSL frequencies.

It's probably a lot different again for the higher VDSL2 frequencies.

Before razpag departed, he mentioned that quite often BT's twisted copper pairs have corroded to less than 0.4mm in thickness, thus throwing yet another spanner in the works to consider.

As b*cat mentioned:-
Quote
Correct. Other than a very broad "rule of thumb", to be taken with a big "pinch of salt".
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Oranged

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Re: Not too bad
« Reply #19 on: January 20, 2012, 03:45:28 PM »

If I'm not mistaken the Kitz calculator uses 13.81 dB per km (I like exactness  ;)) at an assumed SNRM of 6dB, also assuming 0.5mm copper (AWG24 or AWG 26 - can't remember which) for ADSL frequencies.

Lot of assuming going on for what is a calculation using technical data to 2 decimal places  ;)

Don't get me wrong I've always "assumed" that distance and attenuation could be loosely related but since I posted the query comparing UncleUB's, burakkucat's and my stats, I'm not so sure it's realistic.
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Bald_Eagle1

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Re: Not too bad
« Reply #20 on: January 20, 2012, 08:50:36 PM »


Lot of assuming going on for what is a calculation using technical data to 2 decimal places  ;)


Hey, don't shoot the messenger  ;D

From http://whirlpool.net.au/wiki/DSLAM_speeds

Estimating distance and speed

The standard signal attenuation spread for a given speed is somewhere in the region of 15-20dB for ADSL2/2+ speeds and 25-30dB for ADSL1 speeds. That is, we see users at 15Mbit with signal attenuation between 0-48dB at the extremes and the statistically significant around 12-32dB. For a given signal attenuation level we see a statistically significant spread of 10Mbit. And by statistically significant, an even spread over the range for a sample set of ~50,000 connections.

A way to "guesstimate" the line attenuation is the following: (originally posted by Just 'Adam')

Less than 1km should be 23-24Mbit
1.0km = 13.81dB = 23Mbit
1.5km = 20.7dB = 21Mbit
2.0km = 27.6dB = 18Mbit
2.5km = 34.5dB = 13Mbit
3.0km = 41.4dB = 8Mbit
3.5km = 48.3dB = 6Mbit
4.0km = 56dB = 4Mbit
4.5km = 62.1dB = 3Mbit
5.0km = 69dB = 2Mbit

A Microsoft Excel document containing a calculator to obtain these figures can be downloaded http://users.on.net/richard.moulynox/whirlpool/adsl2calculatorv1.xls. Current Version 1.4.

Until last June, when I had an ADSLMax connection, I always felt privileged to get 1Mb (on a good day).

Apparently the distance from the exchange is reported as
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5283m (approximately)  :lol:


The attachment shows my best ever DS rate, but just look at the US rate!


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Oranged

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Re: Not too bad
« Reply #21 on: January 21, 2012, 12:26:59 AM »


Hey, don't shoot the messenger  ;D

No I wasn't....it's just my slant on "21 century technology".....you would have thought by now all the relevant data and stats associated with ADSL and vDSL could be relied upon wouldn't you ?
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asbokid

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Re: Not too bad
« Reply #22 on: January 21, 2012, 01:47:52 AM »

RichardM kindly replied to queries about his spreadsheet formula for calculating likely connection speeds.

Today, we can probably build on this formula using the attenuation data [Hlog(f)] that is available for each frequency across the entire VDSL2 tone map.

The attenuation (aka insertion loss) data could be used in conjunction with other data, e.g. the constants defined in the technical specifications for BT cable reference models.

A function could be developed to use this data for deriving estimates of unknown characteristics of the loop, e.g. the loop length, its theoretical Shannon capacity and the electromechanical properties of the loop cabling itself.


[1] http://www.ja.net/documents/development/llu/llu-technical-document.pdf
[2] http://www.sinet.bt.com/349v2p3.pdf
[3] http://www.sinet.bt.com/351v4p5.pdf
[4] http://ftp.tiaonline.org/TR-30..ETSI%20Documents/970p02r3%20ETSI%20Cable%20Reference%20Models.pdf
[5] http://www.niccstandards.org.uk/files/current/nd1601_2005_05.pdf?type=pdf
[6] http://stakeholders.ofcom.org.uk/binaries/research/technology-research/asses.pdf
« Last Edit: January 21, 2012, 02:09:45 AM by asbokid »
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burakkucat

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Re: Not too bad
« Reply #23 on: January 21, 2012, 03:46:56 AM »

You have provided us with links to six reference documents, for our edification. Thank you.

It would be nice to know at which point in your post that each of those documents is contextually relevant.

I am currently obtaining a "Problem loading page" with BT's SIN site. Hopefully that is just a temporary glitch.

Perhaps you could persuade Mr Eagle to develop something along the lines of your musing . . .  :-\
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Bald_Eagle1

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Re: Not too bad
« Reply #24 on: January 21, 2012, 08:14:33 AM »

Hi b*cat,

I am currently obtaining a "Problem loading page" with BT's SIN site. Hopefully that is just a temporary glitch.

It was working earlier this week, so as you say, hopefully that is just a temporary glitch.

Quote
Perhaps you could persuade Mr Eagle to develop something along the lines of your musing . . .  :-\

As we are now moving into the realms of VDSL2 & FTTC etc. I have started another thread in the FTTC section.
FTTC - Line Length and Attenuation Calculations

I did have a half-hearted stab at this some time ago, but didn't fully understand matters at the time (still donít) :-
http://forum.kitz.co.uk/index.php/topic,9726.msg198765.html#msg198765
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Bald_Eagle1

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Re: Not too bad
« Reply #25 on: January 21, 2012, 08:31:43 AM »

Hi asbokid,


A function could be developed to use this data for deriving estimates of unknown characteristics of the loop, e.g. the loop length, its theoretical Shannon capacity and the electromechanical properties of the loop cabling itself.


Thanks for the links. Interesting stuff.

To avoid clogging up this thread with VDSL2/FTTC specific matters, I have started another thread in the FTTC section:-
FTTC - Line Length and Attenuation Calculations


Paul.
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asbokid

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Re: Not too bad
« Reply #26 on: January 21, 2012, 06:15:36 PM »

I will try and thresh out a coherent post around the documents.

BT SIN351 can also be downloaded from here: https://issues.asterisk.org/jira/browse/ASTERISK-11859   (The page is amusing. Apparently BT's overnight testing of its Metallic Path Facilities caused Asterisk PBX servers to crash!)

EDIT:  BT SINet is up and running again.
« Last Edit: January 22, 2012, 08:49:01 PM by asbokid »
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