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Author Topic: Question about bits/tone  (Read 2475 times)

broadstairs

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Question about bits/tone
« on: November 08, 2008, 10:37:48 AM »

I have been watching my stats for some days now and have had quite a stable connection, however one thing does puzzle me a bit. Looking at the bits/tone graph from both DMT and Routerstats I can see that my connection rarely use many tones above the 0-255 region, right after a resync I usually see some in use above 255 being used but not many. Sometimes they stay being used and other times it drops them. Now if I understand correctly these change according to the noise levels associated with the various tones however I am puzzled that on a LLU connection which is supposed to be ADSL2+ (but limited to 8meg currently by Talktalk) I would have expected to see more tones being used in the upper band at least initially after a resync.

My current connection shows:-

Rate (Kbps) :     6435      443
SNR Margin (dB) :    10.1     26.1
Attenuation (dB) :    40.5     21.3
Output Power (dBm) :    0.0     12.9  Note downstream output occasionally shows on DMT as 18.9db

As I say I'm not unhappy with my connection, I'm just trying to get a sound understanding of what I'm seeing here and if possible why.
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ISP:TalkTalk Connection:FTTC Cab:ECI Router:Netgear D6220

roseway

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Re: Question about bits/tone
« Reply #1 on: November 08, 2008, 10:56:53 AM »

I've no personal experience of LLU so I'm guessing here, but I would imagine that this is a consequence of the way in which the TT 8 Mbps profile is set up. I suppose that the higher tones would only be used if needed to support a higher connection speed.
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  Eric

broadstairs

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Re: Question about bits/tone
« Reply #2 on: November 08, 2008, 12:18:41 PM »

I've no personal experience of LLU so I'm guessing here, but I would imagine that this is a consequence of the way in which the TT 8 Mbps profile is set up. I suppose that the higher tones would only be used if needed to support a higher connection speed.


Yes that could be however I have seen reference to folks being placed on a 24Mbps profile if they have a very short  line but they still get 8Mbps max AFAIK, so I wonder if there is something else which limits the speed.
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ISP:TalkTalk Connection:FTTC Cab:ECI Router:Netgear D6220

kitz

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Re: Question about bits/tone
« Reply #3 on: November 08, 2008, 12:32:14 PM »

It may to do with the way they configure their network.
Although the MSANs that TalkTalk use are actually adsl2+ compatible, as are the routers they ship out.. TalkTalk dont offer a true adsl2+ service and cap the speeds to 8Mb.

adsl2+ is a technology that allows higher frequencies to be used.. but the provider can still configure the DSLAMs/MSANs to restrict certain frequencies OR speeds.

However it may also be that just plain and simple that at the time of sync there are insufficient bits in that particular tone for it to be usable.  Each carrier bin needs the equivalent minimum of 6dB SNR to be usable.

>> Sometimes they stay being used and other times it drops them.

That indicates bit swapping process, and that theres been a decline in your SNR ('real SNR' and not your overall SNR Margin) for those particular frequencies.  As the SNR for those channels drop below 6dB, your router will attempt to bit swap those bits to use in other channels if it can. If it cant it will loose sync.

I could be wrong, its hard to say without seeing what your bitloading is like,   but in view of the above I'd say its more like you dont have sufficient SNR for the particular channels to be used.
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