From what I've been able to gather up to now...
* The DSLAM is setting the downstream SNRM at 12db regardless of the the speed it connects to at sync.
* The reason for the 12db setting appears to allow for a margin to compensate for a non-continuous source of interference that when present increases the noise level by approx 8db almost instantly. (Non-continuous in the sense of not being present 24/7 rather than having regular 'pulse' interference.)
* The times this non-continuous interference source is present is during weekdays between roughly 7.15am to 8.15am and from some time starting between 4pm to 5pm until around 12am to 1am. There was one occasion this week when this interference was not present for several hours in the early evening, presumably because whatever is causing the interference was turned off at this time.
* If the modem syncs when this source of interference is not present, downstream sync is at approx. 8Mbps. When the interference is present downstream sync drops to around 5.1 to 5.4Mbps. In the case of the former when the non-continuous interference becomes present the SNRM drops to 4db. The the latter case when the non-continuous interference stops the SNRM increases to 19-20db.
* Other than the +/- 8db SNR jump, the downstream SNR remains very consistent with no real changes over a 24 hour period.
* The upstream SNRM is also fairly consistent - it is slightly affected by the non-continuous interference with a roughly 1db increase in noise. It is syncing continuously and reliably at 1Mbps.
* From observing my QLN graphs both with this non-continuous interference being present and not being present, and comparing them to other ADSL users on MDWS, two things stick out for me - first, my noise floor is fairly flat & consistent with no major "spikes" with it being roughly -110db up to around tone 250, then falling smoothly down to about -123db at tone 400 before rising again to about -113db at tone 500. Second, despite the noise floor being fairly smooth, it appears that my line is rather noisy in the sense that other MDWS ADSL stats indicate a general noise floor (excluding noise spikes) quieter than this, in many cases down to -140db. It appears that this noise is wideband - however what is noticeable is that this noise is at its highest in the downstream below 1.1MHz - while the ADSL service I have is an ADSL2+ LLU product, my exchange is otherwise a 20CN with a standard ADSL1 service through BT Wholesale. Possibly the case that I have significant cross-talk problems?
* Looking at the Hlog graphs, I may have came to the conclusion that the MW broadcast station on 549kHz is not significantly causing interference to tone 127 and that the reason for this tone having no bitload is a co-incidence. A MW broadcast normally has a sideband on each side of the carrier between 4.5kHz to 7kHz wide and would surely interfere with adjacent tones if the radio signal caused interference, but it's not the case here. I don't really know the reason why some bit tones missing while adjacent tones are perfectly fine, I can only think that something on the line is causing very narrow notches on specific frequencies. Again I don't see these narrow notches on other ADSL Hlog graphs on MDWS, or at least not to the same extent.
* My downstream attenuation is 28db. According to the maximum speed calculator on site here at
www.kitz.co.uk/adsl/max_speed_calc.php it gives the following results...
* Downstream attenuation: 28db
* Approx line length: 2km
* dslMAX (20CN): 8128kbps - IP Profile: 7150 kbps
* BT 21CN WBC & adsl2+: 16512kbps - IP Profile: 14564 kbps
* adsl2+ (LLU): 16512 kbps - Throughput: 14613kbps
...and also another calculator for estimating ADSL line speed...
http://coolwebhome.co.uk/calc/index.php?param=RG4gODE5MCBEbiAyOCBEbiAxMi4xIERuIDI=#resIt appears to me that my phone line is seriously hobbled for ADSL speeds where even at the best sync speeds it's achieving only about half the downstream throughput it is potentially capable of.
* I've attached a screen grab of the Broadband Exchange & Line Checker - because I'm on an LLU it doesn't give back more accurate stats like estimated FTTC speeds but when my phone number was with Plusnet (and hence on BT's network) it was returning almost the same results for LOS & road distance from my exchange (Fintona, NIFIN).
Now I'm wondering what to do next as I've two separate issues to overcome, namely combating the non-continuous interference source, and IMO the generally noisy line levels. The former would seem to be slightly easier to deal with than the latter but would almost certainly need Openreach involved.
