Reading a DMT graph

[Stelios]

Hi,

I heard that it is possible to locate/identify problems on a DSL line from a DMT graph.
Anybody knows any tips how to read the graph?

Is it possible to manipulate any parameters ,from modem or DSLAM side , to fix problems on the DSL line?

Thanks

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Stelios

 

[CiscoStu]

Hi Stelios,

I'm in a similar situation and trying to understand DSL frequencies; all downstream frequancies seem to flatline.

However here is a tool for converting the DMT bits into a graph, if you dont have one already:

http://cosi-nms.sourceforge.net/cgitools/dsl_show/

Cisco Stu doesn't advertise

 

[Stelios]

Hi CiscoStu,

thanks i saw that tool before.
But our provider does not use Cisco DSL routers.

I found a link to a page that explains DSL in detail. Check this out (also the other links on the left panel)
ADSL Technology and DMT

http://www.kitz.co.uk/adsl/adsl_technology.htm

I'm still searching the net on how to interpret the DMT graph.
There must be something out there.



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Stelios

 

[Stelios]

CiscoStu ,

Did you manage to get any readings from your modem.
The values of the parameters can show what is happening.

I found a thread in a forum that explains lots of things

http://mybroadband.co.za/vb/showthread.php?t=84785&page=12

i'm reposting...
"If slow service is indicated,technicians can look at the bits per tone measurement. Dips in the bit per tone graph indicate interference. Checking the frequency of the interference often makes it possible to identify its cause.The accompanying chart shows the frequencies associated with common interference sources, such as T1, E1 and HDSL. If the bits per tone are taking a hit and it is determined that noise is not the cause, then the cause could be a DC fault, such as a bridge tap or a wet section. Very low or nonexistent bits per tone in the high frequency band indicate the presence of a long loop. If there is a major dip in the bits-per-tone graph but the noise power-per-tone graph does not show any abnormalities, the most likely troubles are bridged taps or wet sections on the span. Use a loop-troubleshooting tool with a TDR to find and fix these problems. If the bits per tone are low across the whole bandwidth, the cause is most likely DC troubles on the loop, such as shorts or grounds. A major dip in bits per tone indicates AC trouble on the loop.To verify the type of AC trouble, compare the bits per tone to SNR. If the noise influence and bits-per-tone dip occur at the same frequency, the degraded performance is most likely due to a transmission influence. The particular frequency will help identify the source of influence or crosstalk. An example of possible shared frequency zones could be T1 centered at 770 kHz or HDSL at 196 kHz."

There is also a link to a PDF file that explains more

I hope that the above will help you.


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Stelios