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Author Topic: Continually reducing speed  (Read 422 times)


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Continually reducing speed
« on: September 21, 2020, 08:56:56 PM »

I have an ADSL line with BT. It's ADSL in an FTTC area, since they offered it free of charge.  My main connection is a Virgin Media Business Cable service, which is usually very reliable.  Since BT offered ADSL free of charge on an this BT line I accepted, as a backup.

The issue is distance from the Exchange which results in around 5.9 - 6.5 Mb/s with upstream of 832 kb/s.  This is with an SNR margin of around 9 dB.  Over time the downstream gradually drops to around 1 - 1.5 Mb/s with the SNR margin rising to 15 dB or more.   If I force a resync the original speed is restored.

I had thought that BT systems worked in both directions -  to not only reduce but to increase sync speed as well?   Clearly that seem not the case, yet the line is capable as a re-sync causes a higher sync.

Since it's a free backup, you get what you pay for, but it does seem to work just in one direction.

Earlier this year the BT Wholesale checker had an 'open order' on the line, which BT were able to tell me related to a wider re-grade of all ADSL lines that could switch to VDSL/FTTC lines at the same cost.  Naturally the pandemic delayed those plans, but the new checker no longer shows open orders so wonder if these changes are still happening?


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Re: Continually reducing speed
« Reply #1 on: September 21, 2020, 10:50:58 PM »

It is difficult to say what could be happening.

The one oddity is the (apparent) 9 dB SNRM target. I would normally expect that to be 6 dB. I wonder if there has been some situation, in the past, that has caused the DLM process to use the higher margin target?

I wonder if the CPE that you are using has a problem with bit-swapping or if the bit-swapping is turned off in its configuration? That might account for the slow decline . . . which could then be reversed by a CPE re-boot or power cycle.
:cat:  100% Linux and, previously, Unix. Co-founder of the ELRepo Project.

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