Help with the mysteries of SNR

[holmbase]

I use a DG834Gv4 with Andrews & Arnold on the Wolverton exchange. After many problems, I had established a stable connection at 3,776kbps with attentuation of 50db, a default SNR of 12db (I assume) and an interleave depth of 8.

After monitoring my SNR, it varied between 11db and 13.5db. So (I thought) if the SNR is now stable, let's reset to 6db and see what happens.

A&A obliged and I connected at 5,600kbps. Joy......not. Now SNR is unstable varying between 0 and 6db. I have lost connection several times in the space of 8 hours and my connection rate has now dropped from 5,600kbps to 4,768kbps, still at 6db SNR with the interleave depth at 16.

So my question.

Why is my SNR and connection so consistent at the default 12db, but very unstable at 6db? I understand the pinciple of SNR but what causes more variability (and hence instability) at the lower level? Can anyone enlighten me?

Also, why is my sync rate dropping without the SNR increasing? What other factor affects the sync rate for a fixed SNR?

Thanks

John

[duckson]

Are you monitoring it with Routerstats? I would do so as it will work off the bat with the router you have.

See mine with my own noise margin problems:- http://www.duckworth.myzen.co.uk/router/Netgear/

If you right click on the graph you can select overlay sync speed, keeps sync speed and noise margin on the same graph to compare.

One reason why a lowered sync rate hasnt increased your margin is maybe a source of noise interference is stopping it doing so.
I stand to be corrected but each 3db is a doubling of the noise margin so what might only affect it slightly at 12db would show up more readily at a lower end of the scale ie 6db. Maybe.  

[jeffbb]

Hi
quote : Why is my SNR and connection so consistent at the default 12db, but very unstable at 6db? I understand the pinciple of SNR but what causes more variability (and hence instability) at the lower level? Can anyone enlighten me?


Remember that the its a logarithmic scale .when your SNR margin is about 6db(a linear power value of X)  say a 6db noise spike(value of X) is happening  then you will end up with 0db margin so lose connection .

but 12db margin is 4 times more margin(4X) than 6db so the 6db noise  spike(X value) in effect reduces the actual power value by X to 3X . If you margin was 15db(8X) then it would be reduced to 8-1 = 7 times the noise level.(haven't worked out the actual db value )
Regards Jeff



 

[holmbase]

Duckson
I am using routertstats although not the latest version as it isn't running on my Win98SE pc. I looked at your graphs and at 12db/3,776kbps my router is more stable than many of your picture. At 6db, my router looks very much like most of yours with noise being quite jittery.

Jeffbb
I didn't know/remember that SNR is logarithmic which would explain a lot. Very frustrating though.

Thanks

[Ezzer]

Jeff got the no.s spot on, copied this off another post

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

[b4dger]

After monitoring my SNR, it varied between 11db and 13.5db. So (I thought) if the SNR is now stable, let's reset to 6db and see what happens.

Just because you appeared stable at 12db it doesn't follow that you'll be stable at the default 6db Target I'm afraid.
The reason it's stable at 12db is due to the decreased line sync - with a higher sync things (as you've found out) may become unstable.

I would try things in smaller steps - try a Target of 9db to see if that gives you a line that remains stable but will also sync higher than when you were on a 12db Target.

[holmbase]

b4dger

It went to 9db at about 4.7mbps and seemed reasonably okay as it happens ie. it dropped about once in two days and resync'd at the same rate. After two days I thought I'd achieved something. 

Errors didn't seem too bad (at least not by the standards I sometimes see on these forums) but for some reason, DLM didn't like it and bumped me back to where I was before I started ie. 3.7mbps sync rate and a default noise margin of either 12 or 15db (I can't quite tell which as the margin flips between 12 and 14.5db now).

Am now planning to move from AAISP to BE or O2 LLU as I understand that these don't use DLM and therefore allow you better sync rates under the same line conditions. It's also cheaper so I might get a win win.

John