VDSL2 - Can DLM instruct the DSLAM to change TX power?

Having recently moved from VM DOCSIS 3.0 to Sky VDSL2 I'm curious about the details of how my new broadband service works.

I switched to Sky Fibre Max just over four weeks ago on an EO line of about 150m length. The fibre cabinet is placed in the exchange compound.

Initially sync rates were 79987 / 19999 with a d/s loss of 9.9dB and SNR margin of 18.7dB.

Initially the connection was very stable with very little variation in loss or SNR and no line resync.

Just over two weeks after installation the modem resynced overnight and came back up at 79999 /19999. At the same time the SNR margin dropped to 17.5dB and then about a week later to it's current 16.2dB.

The line still seems very stable. Upstream noise margin has not changed from the initial 23dB.

Whilst it could be that this change in SNR margin is caused by additional connections to the cabinet causing crosstalk, it seems strange that the change occurred overnight.

As an alternative explanation I was wondering if DLM may see my line as particularly stable on the original high TX power.
Could it then instruct the DSLAM to reduce this power in stages to help reduce the crosstalk from my line on to others in the same bundle?

Thanks for any insight.

Watch for changes in the maximum attainable speeds, they'll probably correspond with the noise margin changes.

In reply to a post by IanBB:

Watch for changes in the maximum attainable speeds, they'll probably correspond with the noise margin changes.

I wish I could but the stats provided by the Sky Q Hub are very limited.

I'd certainly expect the max attainable to reduce if the DSLAM TX power was reduced. Do modems like the DrayTek Vigor report the TX power of the DSLAM?

In theory it is possible to tell the DSLAM to reduce power, but I would wager money it doesn't. The most likely explanation is that it's simply increased cross-talk causing the reduction in SNR margin. However, given that you have such a high SNR margin I really can't see that it's likely to impact on your current sync levels.

From my understanding DLM makes positive changes between 4am-6am, negative changes can happen straight away and at any time.
initial SNR of 18.7db is probably down to the time of day and interference in your area.
16.2db is after a re-sync at 4am ish, and less interference.

Hi

Yes power can change. I saw that on my new FTTC connection, started off at 11db SNR and 13.3db transmit power to me, and an overnight re-sync on the 1st night saw the transmit power drop by 1db and SNR down to 10.6db.

Reducing power is a crosstalk preventative measure and also helps prevent echos back up the line, and if you have plenty of margin, it makes sense to reduce the power.

Regards

Phil

Ummm . Not quite that simple re SNRM.

On a line that easily achieves the 79999kbps sync the SNRM which is what is actually the figure reported will be higher when there is low interference such as at 4am. Then lower as daylight approaches.

On a line that does not achieve the product cap then it will always be approximately 6dB, soon on good lines 3, 4 and 5 also becoming available to DLM.

I find on FTTC mine varies very little, two or three decimal points. On ADSL2+ it was about 3.5dB

+1

Can you clarify please?

Generally "power" is referred to with such changes - strictly that would infer a change of voltage; but that I recollect, there are strict limitations to the voltage applied to the PSTN copper network, probably 24 Volts for general signals and 60 Volts for Bell.

Is it not a case that it is the Rate as "(Mega) Bits per second" that is altered?

My post was responding to what was said about SNR(M) in the post I replied to, and was entirely about that. Nothing to do with power levels.

Edited by RobertoS (Wed 23-Nov-16 15:01:29)