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Author Topic: Line attenuation suddenly lower  (Read 11841 times)

orainsear

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Re: Line attenuation suddenly lower
« Reply #15 on: January 10, 2009, 12:42:31 PM »

You could be seeing the attenuation change due to the effect of the weather.  The electrical conduction of copper improves as temperature lowers, hence you may see a subequent drop in the attenation.  I've seen the attenuation of my own line drop by around 8% with a 20 C change in the temperature.
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roseway

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Re: Line attenuation suddenly lower
« Reply #16 on: January 10, 2009, 01:10:29 PM »

I'm a bit dubious about that theory. The attenuation of a line to ADSL signals is predominantly due to its high frequency AC losses, not the resistance of the copper. That's why the downstream attenuation is higher than the upstream - it uses higher frequencies.
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  Eric

orainsear

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Re: Line attenuation suddenly lower
« Reply #17 on: January 10, 2009, 02:31:56 PM »

It's a natural physical phenomenon that designers have to make allowances for.  Telecoms Tech Reference, DSL Tech, DSL Advances

Edit: fixed hyperlinks
« Last Edit: January 10, 2009, 02:35:56 PM by orainsear »
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Re: Line attenuation suddenly lower
« Reply #18 on: January 10, 2009, 03:23:37 PM »

Well my attenuation has returned to "normal" i.e. 37.5dBm and it's still zero outside or thereabouts so there's always one example that bucks the theory.  :-\

Since moving the router from the unheated room to a floor location in a bedroom I've had no reboots in 24 hours.

Current stats are :

Uptime: 1 day, 0:04:38
 
DSL Type: G.992.5 annex A
 
Bandwidth (Up/Down) [kbps/kbps]: 1,325 / 8,187
 
Output Power (Up/Down) [dBm]: 12.0 / 19.0
 
Line Attenuation (Up/Down) [dB]: 19.0 / 37.5
 
SN Margin (Up/Down) [dB]: 7.0 / 6.5
 
Vendor ID (Local/Remote): TMMB / BDCM
 
Loss of Framing (Local/Remote): 0 / 0
 
Loss of Signal (Local/Remote): 0 / 0
 
Loss of Power (Local/Remote): 0 / 0
 
Loss of Link (Remote): 0
 
Error Seconds (Local/Remote): 0 / 0

What is strange is that even though the router is now using a hard-wired extension box (ring wire removed) I am seeing no Loss of Signal or Error Seconds which I was seeing using the main BT linebox even when the room temperature was 18c+.

There are FEC, CRC and HEC errors but nothing to concern me for a 24 hour period.
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orainsear

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Re: Line attenuation suddenly lower
« Reply #19 on: January 10, 2009, 04:05:04 PM »

Well my attenuation has returned to "normal" i.e. 37.5dBm and it's still zero outside or thereabouts so there's always one example that bucks the theory.  :-\

 ;D Doesn't surprise me at all.  For all the theory and laws of physics a large part of understanding ADSL seems to be what I would describe as an art, and every line seems to have distinct characteristics.

Out of interest how long have you been using the TG585v7?  As has already been mentioned, quite a few people have had problems with this device. 



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Re: Line attenuation suddenly lower
« Reply #20 on: January 10, 2009, 04:58:50 PM »

ADSL seems to be what I would describe as an art

Or rather a "black art"  ???

Since moving over from Orange to O2 on 16/9/08 I've used the O2 router with no real complaints until the problem in this thread which would seem to have been of my own making by expecting the router to live in an unheated room during the coldest early winter since who knows when  ::).....so it doesn't like getting hot and it doesn't like getting cold  :baby:

I still have my trusty Belkin 7633 (used since May 2005) which I set up and tested on the first day with O2 then put it away in its box.

I think routers are like any other modern technical device.......you get perfectly good ones and you get absolute lemons.....it's the luck of the draw.

The only thing I don't like is that the full-blown version of Routerstats won't work with it.....I try each new version but still no luck so I stick with Lite.
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roseway

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Re: Line attenuation suddenly lower
« Reply #21 on: January 10, 2009, 11:19:55 PM »

It's not so much an art as a very complicated science. Electronics engineers will understand all the factors, but there are so many variables that every line is different. In principle it would be possible to make lots of individual measurements on a line and so determine how to get the best result for the user, but the cost of doing this would be totally prohibitive. So we have to resort to practical experience and a fair bit of 'suck it and see'.

Of course an extra complication in the mix is that there's no universal agreement on what is meant by 'best'. For some people it will mean fastest, for others it will mean most stable, and for others again it will mean lowest latency. Or some compromise between the three. So in a practical sense, yes, it's a black art. :)
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  Eric

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Re: Line attenuation suddenly lower
« Reply #22 on: January 12, 2009, 04:06:49 PM »

Still no reboots since plugged into the bedroom extension, current stats are :

Uptime: 3 days, 0:55:11
 
DSL Type: G.992.5 annex A
 
Bandwidth (Up/Down) [kbps/kbps]: 1,325 / 8,187
 
Output Power (Up/Down) [dBm]: 12.0 / 19.0
 
Line Attenuation (Up/Down) [dB]: 19.0 / 37.5
 
SN Margin (Up/Down) [dB]: 7.0 / 8.0
 
Vendor ID (Local/Remote): TMMB / BDCM
 
Loss of Framing (Local/Remote): 0 / 0
 
Loss of Signal (Local/Remote): 0 / 0
 
Loss of Power (Local/Remote): 0 / 0
 
Loss of Link (Remote): 0
 
Error Seconds (Local/Remote): 1 / 0
 
FEC Errors (Up/Down): 458 / 5,189,926
 
CRC Errors (Up/Down): 458 / 382
 
HEC Errors (Up/Down): 4,037 / 260

So  it would seem that the cause was probably the router being in an unheated room and possibly being affected by condensation.
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jeffbb

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Re: Line attenuation suddenly lower
« Reply #23 on: January 12, 2009, 05:55:01 PM »

Hi
Hard to believe as after awhile the average router would be warm enough to get rid of any condensation . But hey don't matter its working  ok now :clap:

regards Jeff
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zen user

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Re: Line attenuation suddenly lower
« Reply #24 on: January 12, 2009, 07:36:26 PM »

Hard to believe as after a while the average router would be warm enough to get rid of any condensation.

Well that was my thinking but as that's the only change I've made, I can't put it down to anything else. I know the BT linebox is OK as I ran a check using the test socket and got the same results as when using the linebox normally. So even though I'm now using an extension, albeit hardwired from the main linebox, this is giving better stability.

Not knowing anything about dew point and relative humidity to be able to explain the results, I guess I'll just have to be thankful that the reboots have stopped  :D
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Re: Line attenuation suddenly lower
« Reply #25 on: January 13, 2009, 05:54:43 PM »

Just one more item of data before I finish with this thread.

Would anyone like to comment on the DMT graph shaping and the gaps......are these results anything to do with the reboots ?
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kitz

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Re: Line attenuation suddenly lower
« Reply #26 on: January 13, 2009, 07:48:03 PM »

Hard to say - the graph displays your bitloading only... unfortunately when Thompson b@stardised their firmware it stopped DMT displaying the SNR in some versions.
The gaps indicate that your router has deemed the SNR at that particular frequency too low to be able to transmit any data at some point. Normally you would expect the SNR and bit allocation to more or less mirror (with a caveat that it doesnt always - depends on how long the router has been up for).

Based on the assumption that your SNR and bit loading is mirrored, the the bit swapping process should have sufficient head room to keep your router alive.  The gaps do indicate though the frequencies where your connection suffers interference the worst from.
One or 2 gaps are sometimes unavoidable on some lines and the tail off at the end doesnt look too unusual so Im ignoring those. 
Tones around 200 are possibly the ones that are loosing you the most from your sync speed.. but its still a smallish amount. - off the top of my head based on your graph and very approx guess  - circa 200-250 kbps across all the gaps.

The gaps also show where to keep an eye on and where problems may most likely be occuring but for it to be more useful youd really need to see active monitoring of the SNR

eg Ive just fired DMT up now to quickly grab a screenie to show you how and where my own SNR(M) is currently falling.  DMT marks it with the little orange arrows.

If youre interested in understanding DMT and bit loading then theres more info (and graphs) on this page
http://www.kitz.co.uk/adsl/adsl_technology.htm
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How to get your router line stats :: ADSL Exchange Checker

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Re: Line attenuation suddenly lower
« Reply #27 on: January 13, 2009, 08:11:27 PM »

Thanks for that analysis Kitz.

I'd already read through that link and it's why I decided to post the graph and ask the question. It was the high end gaps that I was unsure of, as regards their importance. Also fortunately the number of gaps do not seem to increase at night.

I've also seen references to overhead phone lines (my BT service is overhead from a pole across the road) picking up interference. There are no local radio transmitters but there is a major hospital <1Km and a significant MoD establishment <4Km.
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kitz

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Re: Line attenuation suddenly lower
« Reply #28 on: January 13, 2009, 10:16:32 PM »

>> It was the high end gaps that I was unsure of, as regards their importance.

Something you are most likely to see as the line gets longer.  The frequencies are higher and more susceptible.. and because of the way DMT works and because the SNR is naturally lower, these bins are most likely the ones that your router marks as unusable and bitswap to other subchannels.
Its not at all unusual on a line that is syncing at its maximum capabilities,  to see the spike-gap-spike-gap effect on a line thats been up for a while at the tail end frequencies.
A  total resync may cause some of these bins to be used again.. or it may not..depends on the SNR at time of initialisation... and the fairly complicated calculation that decides how many bits to allocate depending on the BER (now Im beginning to get out of my depth)..  but at those frequencies on the tail end you're only looking at a couple of bits per bin (which bit swap will have already allocated else where), so youre not likely to gain much at all, if anything.


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