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uk.telecom.broadband (UK broadband) (uk.telecom.broadband) Discussion of broadband services, technology and equipment as provided in the UK. Discussions of specific services based on ADSL, cable modems or other broadband technology are also on-topic. Advertising is not allowed.

SNR and Loop ATT



 
 
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  #1  
Old August 24th 04, 10:32 AM posted to uk.telecom.broadband
Julian Knight
external usenet poster
 
Posts: 56
Default SNR and Loop ATT

From Brian Morrison on 24/Aug/2004 09:09:

On Tue, 24 Aug 2004 08:43:54 +0100, in article
Tx2
wrote:


Running a Draytek 2600 into Plusnet ADSL, i have an SNR of +/-41, with a
loop attenuation of circa 23.5



I have Loop Att 44.0dB with SNR margin of 30 - 35 dB with occasional dips
to lower values. Approx 3.8km from the exchange on 512kbps.


Good, bad, could be better, or nothing to worry about?



Very good, the modem has a sufficient SNR to be a huge amount above any
line noise so you have *absolutely* nothing to worry about.


SNR 13.0, Att. 62.0 so really close to the "limit" but I get no problems at all.

--
Julian Knight, http://www.knightnet.org.uk/
Sheffield, United Kingdom
Security, Directory, Messaging, Network & PC Consultant
Yahoo! IM=knighjm, Skype Internet Phone: callto://j.knight
  #2  
Old August 24th 04, 01:36 PM posted to uk.telecom.broadband
robert w hall
external usenet poster
 
Posts: 242
Default SNR and Loop ATT

In article ,
Julian Knight ] writes
From Brian Morrison on 24/Aug/2004 09:09:


SNR 13.0, Att. 62.0 so really close to the "limit" but I get no problems at all.

Is it 'really close' ??

- from previous correspondence on this subject people have suggested the
SNR can get down below 6db before the modem starts losing sync.
Anyone comment?
And, even though BT won't offer you 1Mbps on this attenuation, would you
expect success??
And what about SNR of 13dB and atten of 58db??
Again, anyone comment (preferably from some knowledge :-))
--
robert w hall
  #3  
Old August 24th 04, 02:33 PM posted to uk.telecom.broadband
robert w hall
external usenet poster
 
Posts: 242
Default SNR and Loop ATT

In article , Brian
Morrison writes
On Tue, 24 Aug 2004 13:36:05 +0100, in article
robert w hall
wrote:

In article ,
Julian Knight ] writes
From Brian Morrison on 24/Aug/2004 09:09:


Please trim the attribution more carefully, I wrote none of the quoted
material.


my apologies


SNR 13.0, Att. 62.0 so really close to the "limit" but I get no problems
at all.

Is it 'really close' ??


No, clearly not,


why 'clearly' - not to me or I wouldn't have made the point ('clearly')

but it would be if the SNR margin was below 6dB or so.
That would imply a high noise level rather than low signal level.


In what you (I think :-) are saying below I find I'm (I think) getting
confused by Your (I think) differentiation between SNR and noise margin.

SNR= signal to noise ratio (yes?) and is the ratio of two powers

BUT

it is usually expressed as a log. (for many reasons, not the least that
that's the way it comes in Shannon's theorem and similar, and so many
detection systems depnd directly on it)

Since its log goes through zero when the ratio is unity, in my usage the
terms SNR (expressed as a log.) and margin are synonymous.

what's yours that's different, please

When we've clarified this point perhaps we can discuss the remainder!
--
robert w hall
  #4  
Old August 24th 04, 08:27 PM posted to uk.telecom.broadband
robert w hall
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Posts: 242
Default SNR and Loop ATT

In article , Brian
Morrison writes
On Tue, 24 Aug 2004 13:36:05 +0100, in article
robert w hall
wrote:

In article ,
Julian Knight ] writes
From Brian Morrison on 24/Aug/2004 09:09:


Please trim the attribution more carefully, I wrote none of the quoted
material.


my apologies - but the correct level of quoting is of course correctly
preserved by the carat marks



SNR 13.0, Att. 62.0 so really close to the "limit" but I get no problems
at all.

Is it 'really close' ??


No, clearly


not to me or I wouldn't have asked :-)

not, but it would be if the SNR margin was below 6dB or so.
That would imply a high noise level rather than low signal level.


- from previous correspondence on this subject people have suggested the
SNR can get down below 6db before the modem starts losing sync. Anyone
comment?


It can, but usually this is SNR *margin* that is quoted,


er, is it?

the absolute SNR
required for low enough error rates depends on implementation,


the 2nd part of this remark appears true, but the first part puzzles me.
It was my understanding that
Signal to Noise Ratio (SNR), (the ratio of signal to noise powers,
usually quoted on a log scale), and
SNR margin, (as reported say from a Speedtouch 510 router or similar,
or quoted in normal usage),
are the same thing.
(ie the 'margin' is relative to unity ratio, or zero db, NOT the
threshold SNR at which the box unsyncs)

But you seem carefully to differentiate between 'absolute SNR' and 'SNR
margin'.


Please help me here

Bob



--
robert w hall
  #5  
Old August 24th 04, 11:03 PM posted to uk.telecom.broadband
Phil Thompson
external usenet poster
 
Posts: 2,720
Default SNR and Loop ATT

On Tue, 24 Aug 2004 20:27:48 +0100, robert w hall
wrote:

But you seem carefully to differentiate between 'absolute SNR' and 'SNR
margin'.


one can envisage some devices reporting the SNR and that is clearly
defined as signal to noise ratio.

however several devices report "SNR margin", now the problem is that
we don't know if SNR margin = SNR or if SNR margin = Actual SNR -
minimum SNR required

so an SNR of 12 dB on a modem needing 6 dB to achieve the specified
error rate may equal an SNR margin of 6 dB.

the proof of the pudding is in the eating - if it works at a margin of
zero (as some do) then it is a margin and not an SNR, if it stops
working below about 6 then its reporting SNR.

How does that sound ?

Phil
--
spamcop.net address commissioned 18/06/04
Come on down !
  #6  
Old August 24th 04, 11:04 PM posted to uk.telecom.broadband
robert w hall
external usenet poster
 
Posts: 242
Default SNR and Loop ATT

In article , Phil Thompson
writes
On Tue, 24 Aug 2004 20:27:48 +0100, robert w hall
wrote:

But you seem carefully to differentiate between 'absolute SNR' and 'SNR
margin'.


one can envisage some devices reporting the SNR and that is clearly
defined as signal to noise ratio.


yes agreed

however several devices report "SNR margin", now the problem is that
we don't know if SNR margin = SNR or if SNR margin = Actual SNR -
minimum SNR required


for that 'trained state of the modem' - see below)


so an SNR of 12 dB on a modem needing 6 dB to achieve the specified
error rate may equal an SNR margin of 6 dB.

the proof of the pudding is in the eating - if it works at a margin of
zero (as some do) then it is a margin and not an SNR, if it stops
working below about 6 then its reporting SNR.

don't forget when SNR gets low, the correct factor is (in theory)
log(1+SNR)

How does that sound ?

Phil


Yes, that certainly seems to be a plausible interpretation of what he's
saying.
And there are articles on the web (google on 'SNR margin' & find eg
www.aware.com/support/x200/faqs.htm) which DO differentiate 'SNR margin'
from SNR, pointing out how that SNR margin changes if you change the
number of signal levels in a channel, and so the data rate (eg a change
of 6db if going from a binary system to a 3 level system).

But I had not previously thought that this margin, which thus varies
depending on how the modem has trained itself, was what bog standard
Alcatel modems reported, rather 20.log(SNR).
(Indeed I asked a similar question about 3 weeks back, and was then
reassured that the 'Noise Margin' reported by an Alcutel 510 was indeed
just 20.log(SNR).
Ideally you'ld expect a good diagnostic to report both, or give you
enough data to deduce both.

For one thing, one is a property of your wires and the other is a
property of wires+(setup state of modem), so you'd expect some
uniformity to be needed.

Doh! I tort I unerstood this last week
--
robert w hall
  #7  
Old August 25th 04, 05:14 AM posted to uk.telecom.broadband
Reg Edwards
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Posts: 168
Default Shannon & the transmission of Information

For the technically inclined, the information capacity of a communication
channel is given by -

C = B*T*Ln2( 1 + P/N )

Where

C = Capacity in bits (binary digits) of information,
B = Bandwidth of the channel in Hertz, cycles per second.
T = Time or duration of a message in seconds,
P = Power level of the transmitted signal,
N = Noise power level in the channel,
Ln2 = Logarithms to the base of 2.

Capacity is not limited by signal power because a signal can be amplified to
any power level by an amplifier at the receiving end. The ultimate capacity
is limited only by signal to noise ratio.

Signal quality is measured by error rate, the errors being caused by
superimposed noise. The above fundamental equation applies at a negligble
error rate but not zero rate. No channel is perfect.

But note that an arbitrary low error rate can be acheived with a signal to
noise ratio much less than unity by increasing the amount of time taken to
transmit a message. In the most simple case the time is increased simply by
repeating the message.

In practice the error rate is kept under control by the coding method, ie.,
the manner in which the message itself is translated into 0's and 1's and
back again into the original message. There are a variety of electronic
methods in use.

Coding methods include error-correcting codes. Error correction requires
time. If time is an important factor, ie., as in the transmission of speech
or TV pictures, then to maintain a low error rate (low distortion) there is
no alternative but to increase channel bandwidth. Thus we have bandwidths of
60 kHz to transmit speech and music messages which themselves may have
bandwidths of only a few kHz.

The first engineer to place the transmission of Information on a sound,
mathematical basis was C.E.Shannon, appropriately at about the same time as
the first transistors were being produced.

The modern commercial economic limit on signal to noise ratio is of the
order of +3 or +6 dB. But if you have to, you can dig signals out of noise
which are 20 dB below the noise level. Not on the Internet of course.

For further study see Shannon's short classical paper on the subject. He
uses a geometrical representation. You don't have to be a professional
mathematician to appreciate its beauty. Gloss through the more difficult
non-geometric bits.

Do a Google on "Communication in the Presence of Noise", Claude E.Shannon,
and download it.
---
Reg.


  #8  
Old August 25th 04, 08:56 AM posted to uk.telecom.broadband
robert w hall
external usenet poster
 
Posts: 242
Default Shannon & the transmission of Information

In article , Reg Edwards
writes
For the technically inclined, the information capacity of a communication
channel is given by -

C = B*T*Ln2( 1 + P/N )

Where

C = Capacity in bits (binary digits) of information,
B = Bandwidth of the channel in Hertz, cycles per second.
T = Time or duration of a message in seconds,
P = Power level of the transmitted signal,
N = Noise power level in the channel,
Ln2 = Logarithms to the base of 2.



Good to be reminded of this, thanks.
We have debated the use of Shannon's Theorem in adsl in this group
before, in the context of what the limiting data rate might be, and what
the 'noise' is. (The answer I think we were told was mainly NEXT & FEXT)

But the main issue here as I see it is a book-keeping one - when the
modem manufacturers print out 'noise margin', or similar, do they really
mean that, or do they just mean SNR

I can appreciate that noise margin depends on how the modem is setup
and in particular how many bits per channel the limiting channel is
carrying - thus if a modem reports 'true' noise margin, it's a quantity
which is modem dependent, and could undergo significant changes if the
modem retrains for any reason. Whereas SNR is a function of line
properties such as crosstalk characteristics, (and so less likely to
show large shifts?)
Bob Hall
(G3XIY)
--
robert w hall
  #9  
Old August 25th 04, 10:57 AM posted to uk.telecom.broadband
Phil Thompson
external usenet poster
 
Posts: 2,720
Default SNR and Loop ATT

On Tue, 24 Aug 2004 23:04:20 +0100, robert w hall
wrote:

Ideally you'ld expect a good diagnostic to report both, or give you
enough data to deduce both.


indeed, or as a minimum to define it precisely. The SNR required for
512k is probably lower than for 1 Mbit/s so on changing service you
would see the SNR stay the same but the SNR margin drop if you were
given both.

Phil
--
spamcop.net address commissioned 18/06/04
Come on down !
  #10  
Old August 25th 04, 04:20 PM posted to uk.telecom.broadband
robert w hall
external usenet poster
 
Posts: 242
Default SNR and Loop ATT

In article , Phil Thompson
writes
On Tue, 24 Aug 2004 23:04:20 +0100, robert w hall
wrote:

Ideally you'ld expect a good diagnostic to report both, or give you
enough data to deduce both.


indeed, or as a minimum to define it precisely. The SNR required for
512k is probably lower than for 1 Mbit/s



so on changing service you
would see the SNR stay the same but the SNR margin drop if you were
given both.

Phil


yes, on a simple argument, Iguess, to double the bits/channel implies
using half voltage per level and so halving the noise _volts_ to
maintain same noise margin - which suggests 6db; but that argument
seems to cut a few corners :-). And Shannon's formula suggests it all
depends on whether S/N is much greater or less than 1.

Anyone one know the 'right' answer?

--
robert w hall
 




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