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Author Topic: 21CN and DLM  (Read 6338 times)

Azzaka

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Re: 21CN and DLM
« Reply #15 on: June 03, 2010, 01:54:58 PM »

Hi 7LM,

Some modems can adjust the noise levels without resync'ing. The target Margin can be adjusted by the ALMS to assist in speed and line stability. On 21CN the ALMS will use a Target SNR of 3DB as a base for speed and then work upto 15db. Say you have a target SNR of of 6db and stable, the ALMS will then drop the target noise margin down to 3db allowing the modem to drop its noise margin. This does not require a reboot of the modem nor a reset.

With this in mind, the ALMS now has a lot more choices for stabilty and speed. Drop the SNR for speed, but then enable INP (Impuse Noise Protection) for stabilty.) Remove INP and up the SNR to 9db. Speed drops slightly but the latency is now lowered.

It all works together to try and maintain a balance. SRA will allow the same but in a different way, again adding more tools the ALMS' arsenal.

Leo
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sevenlayermuddle

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Re: 21CN and DLM
« Reply #16 on: June 03, 2010, 03:14:42 PM »

, the ALMS will then drop the target noise margin down to 3db allowing the modem to drop its noise margin

Hmmm, I was about to question the concept of 'allowing a modem to drop margin', since modem's generally hang on to a connection regardless of variations in noise, as long as they can maintain sync with the DSLAM.

But, thinking further, are we saying that ALMS tells the modem to only allow the margin to drop so far, but to voluntarily reconnect if the margin falls below some ALMS-controlled  threshold?   From a quick look at G.992 (/G.997) I see that 'Minimum Noise Margin' is a defined parameter, and does indeed seem to support such behaviour.

If that's the case, then it would be interesting to see what happened if a user tried to 'tweak' the target.    ALMS may decree (say) that the target is to be 12db, and the minimum is to be 6db.  If a user then tweaks the 12dB margin down to circa 6dB, then every reconnect could be quickly followed by a 'low margin' event, triggering another reconnect, triggering another 'low margin' event, and so on...  I do hope they've thought this through.  


- 7LM
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Azzaka

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Re: 21CN and DLM
« Reply #17 on: June 03, 2010, 06:30:33 PM »

...  If a user then tweaks the 12dB margin down to circa 6dB, then every reconnect could be quickly followed by a 'low margin' event, triggering another reconnect, triggering another 'low margin' event, and so on...  I do hope they've thought this through.  

- 7LM

They have and this is why we ask our customers not to tweak the SNR as this can cause more problems than its worth.

Azz
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sevenlayermuddle

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Re: 21CN and DLM
« Reply #18 on: June 03, 2010, 08:15:56 PM »

we ask our customers not to tweak the SNR as this can cause more problems than its worth.

A 'No Tweaking' rule would get my support, even with 'old' DLM (in most cases) though I'm aware that opinions vary  :)
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Azzaka

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Re: 21CN and DLM
« Reply #19 on: June 04, 2010, 09:58:30 AM »

Oh yes they do. In some cases it can help but in others in can cause issues.
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