BT DLM Issue?

Up until a couple of days ago I had a stable ADSL2+ connection at c.17Mbps connection speed. My router then went into a continuous re-sync cycle (i.e.; loss of internet, DSL training, DSL established). It would hold the connection for a minute or so and then start the whole process again. To my surprise, rather than fall, my downstream connection rose to 18.3Mbps but the time between re-synchs reduced. Prior to the problem, I had a stable downstream LA of 18 and SNR of 3. My ISP has run line tests but they have not detected a fault, There is no noise on the line.

Through my router settings, I have tweaked my line for stability over performance. My target SNR has risen to 6 and I have a relatively stable connection at 16.4Mbps: that said, I am still getting downstream errors. Yesterday's router log shows 69 resynchronisations - most of these between 7 and 10 am.

When I reduce the target SNR back to 3 instability returns and the connection rate rises to an unstable 18.3Mbps - a figure never seen before on my line. The downstream CRC error rate also increases to thousands per hour.

Could this be a DLM fault. If so, what is the best way of getting it resolved at no financial risk to me?

Have you swapped out the router with another to see if its the router causing the issue?

if you want stability than a margin of 9 or 12 would probably deliver it.

If you're tweaking the SNR you are wrestling with DLM in a way it doesn't expect, so the outcome is somewhat unpredictable.

In reply to a post by yarwell:

if you want stability than a margin of 9 or 12 would probably deliver it.

If you're tweaking the SNR you are wrestling with DLM in a way it doesn't expect, so the outcome is somewhat unpredictable.

My router log now shows an attainable speed of 18.5Mbps which is about 1Mbps higher than my line has ever attained before. By moving the slider towards stability - as recommended by AVM - I have stopped the resynchs and reduced the errors. My router had re-synched over 60 times before I tweaked the router settings. Why hasn't the DLM kicked in? My current log shows a downstream SNR of 4; a connection rate of 17.2Mbps and an acceptable error rate. Sadly, I do not have a spare router and with FTTC only a month away, I am inclined to try and soldier on.

I understand that the DLM kicks in for a set of loss of sync events. If the router says to the DSLAM, I want to resync and they shut down communications rather than the link failing through sync loss then it is no classed as a loss of sync event.

My old router had a managed shutdown/disconnect function and I did try a whole series of those but could not affect the sync speed. A block of pull plug out disconnects would bring the DLM into play and a loss of speed would occur.

In reply to a post by MHC:

I understand that the DLM kicks in for a set of loss of sync events. If the router says to the DSLAM, I want to resync and they shut down communications rather than the link failing through sync loss then it is no classed as a loss of sync event.

My old router had a managed shutdown/disconnect function and I did try a whole series of those but could not affect the sync speed. A block of pull plug out disconnects would bring the DLM into play and a loss of speed would occur.

After chatting to a Dutch expert on the Fritz!Box, he believes that my router is acting normally as you suggest. After months of stability it is trying to increase the downstream connection rate from 17.5 ish to 18.5 but it is failing to hold the higher connection speed due to errors from ADSL crosstalk. By using the built in router firmware slider to improve stability, I have increased the target SNR and reduced the errors and downstream speed back to 17.2. The connection is now holding steady at 17.15 against an attainable downstream speed of 18.5. The Fritz!Box is designed for maximum performance on long and poor lines. I was also told that the Fritz!Box suffers from VDSL crosstalk on ADSL lines (as do most routers) and this is why most European providers do not endorse the FTTC solution for VDSL. My box has remained stable for the past 6 hours so the German solution of Intended SNR; Impulse Noise Protection and Identification of RFI sliders in the firmware do seem to work.

Are those features unique to the Fritz!Box ?

In reply to a post by PiKe:

Are those features unique to the Fritz!Box ?

I do not know. I have just looked again at my router stats and I now have a steady connection at 17.2Mbps with a max obtainable throughput of 18.4Mbps. Sadly, the manual is a bit lacking in detail and more comprehensive guides are in German