21CN on a long line mystery ~ questions???

Hi All

I would welcome any insight on this, what to me is a bit of a mystery???

Though I was not 100% content with my 3204kbps sync @ 6db Margin connection on 21CN (for the record this has been dropping progressively from a figure closer to >3500 mark, I was at an IP of 3.1 for a good while, since the 10 day training period completed [I will try to find my records of periodic changes but they were not frequent i.e. I could see a solid sync for 14 days of say 7 days???]) but at approx 2am this morning (23rd April) the PPP session reset and the sync plummeted to 2519 with a Margin of (now) 9.8db

Not only that looking at the Connection Type it has changed from ADSL2 to ADSL2+, now bearing in mind under ADSL2 the Attentuation was showing as 54.5 but now under ADSL2+ it is 57.5

To me this makes no sense as everything I have read and been advised here says that long lines like mine cannot get the best of ADSL2+ hence the MSAN allocates ADSL2....................so how does that tally with with I have seen above.

For the record I have asked PN and the initial feedback is not very helpfull but as the 2519 sync is below the FTR of 2739 I have asked what is/are the next steps that should or will be taken???

I did a BT speedtest and it reports my measured speed as 1.98Mbps and the IP as 2.22Mbps......................appalling, eh!!!

TIA for your insight

Can't you set the router to force ADSL2?

In reply to a post by XRaySpeX:

Can't you set the router to force ADSL2?

Hmmm! not sure that the Netgear DG834 gives that sort of control??? Will have to check if that is possible..........................or logical to do until I can get to grips with why this has happened in the first place???

Edit, just checked and I can. The current setting is ADSL2+ ??? The options are Auto (Multi Mode), ADSL (G.DMT), ADSL2 and ADSL2+ though I thought the exchange equipment "served up" so to speak what it could offer the could handle???

Not sure what to do now as surely I would have thought the Router defaulted to that, or did it because frankly I cannot recall if I made any changes to it. I think I put the Netgear back into to use at the start of April (was running the Thomson TG585v7

So, as there is some question in regard to the FTR have been breached can or should just go ahead and change the "type" to ADSL2 and see where that gets me???

Edited by deleted (Mon 23-Apr-12 22:11:17)

In reply to a post by Routefinder:

The current setting is ADSL2+ ??? The options are Auto (Multi Mode), ADSL (G.DMT), ADSL2 and ADSL2+ though I thought the exchange equipment "served up" so to speak what it could offer the could handle???

Well as it asked for ADSL2+ that is what DLM serves up unless it's impossible. I think if you had set it to Auto (the default), then DLM might have gone for the "best" it could handle but even that seems random from what other users have reported. So it's worth a try with setting ADSL2.

Let's deal with the easy bit first.

A rise of about 3dB in attenuation on switching from ADSL or ADSL2 to ADSL2+ is very common. In some circumstances it doesn't happen, but I would describe it as "expected".

The re-sync at 2am was probably caused by the MSAN deciding, for some unknown reason at the moment, to move you from ADSL2 to ADSL2+. As 2am is a "noisy" time for broadband, you got a low sync as a result.

The 9.8dB you are seeing is probably simply most of that noise being absent, so the noise margin is bound to rise from what was probably 6dB at 2am.

Rather than mess about at this stage, I suggest a simple disconnection and reconnection well inside daylight hours tomorrow, and see what the result is. Possibly a sync increase of between 1.2 and 1.7Mbps.

Edited by RobertoS (Mon 23-Apr-12 22:52:05)

Thanks both for the added insight

As I say if for reason that escapes me now I set the Router to ADSL2+ and all the while (good many days) the MSAN was trying to 'negotiate' with the Router (could that explain the progrssive drop in sync I was seeing???) and was at ADSL2 for ages it then took a powder and went for broke and set ADSL2+ and that is what has so adversely affected the connection surely rather than 'just' a simple discon/recon the Router needs to be told to go ADSL2.................................doesn't it???

As for making the changes well into daylight hours, well if I do it tomorrow Tuesday that will be approx 9am the latest I can do it!

How stable was your line before the DLM switched the modulation over to ADSL2+ from ADSL2? If it was quite stable, that could be why - It's trying to see how well it performs on ADSL2+.

The most probable reasons why you synced at a low speed are (As RobertoS has highlighted) because the Noise Margins are lower at night time (More interference from street lighting.etc)
and because of the rise in attenuation (Although the higher frequencies that ADSL2+ utilizes might rule the attenuation part out).


The DLM is constantly monitoring your line and if it sees the need to change something it will. By default, all modulations up to the one your DSLAM/MSAN supports can usually be used by a router.
So if you have ADSL, ADSL2 and ADSL2+ all selected on auto and you are on the 21CN network, the DLM will use the best one it sees fit (Up to ADSL2+) depending on the Noise Margin available and the length of the line. Obviously if the Linecard you are on doesn't support the modulation, your router simply won't sync.

9am is a good time to resync in the morning but it varies from line to line. Different people have different things on at different times of the day which can affect interference. If you are not planning on using ADSL2+ then I would untick it from the modulations which the router should sync with.

By the way, how did your line perform on the old 20CN Up to 8Mbps network (If you had broadband on it then)?

With a Downstream Attenuation of 54.5dB, standard ADSL might bring your speeds up a bit.

Edited by chris6273 (Tue 24-Apr-12 00:51:52)

You should avoid changing several things at once when trying to diagnose this sort of thing.

Any change you make to the router settings will cause a re-sync. If the problem is solved, for a period at least, we won't know whether it was just because of the 2am connection or because of your new settings.

Which is why I said to try a simple reconnect first.

The important thing to avoid is more than 5-6 reconnections in an hour, as the MSAN will take that as a sign of instability and start reducing the possible speed even further! (The critical number is supposed to be 10, but I never dared test that).

2-3 times with a few minutes between each will be fine. So do my suggested reconnection, and if it doesn't work then try a setting change .

It seems you have to go out at 9am. 8:30 will be fine. Just avoid dawn, dusk or dark.

Thanks Chris & Roberto for the feedback.

Firstly, to reply about the ADSL Max (20CN I had before) this before the change I had measured speed mostly ranging over recent times from 2.4Mbps to 2.9Mbps

Now here is more wonders of the 21CN and the MSAN behaviour:-

At just about midnight I see that the PPP session reset and I now have the following stats:-

Channel: FAST, Upstream rate = 443 Kbps, Downstream rate = 3179 Kbps
Link Power State: L0
Mode: ADSL2
Channel: Fast
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 6.4 17.9
Attn(dB): 54.0 28.7
Pwr(dBm): 0.0 13.1
Max(Kbps): 4196 828
Rate (Kbps): 3179 443

Now I have done nothing to the Router i.e. it is still set at ADSL2+ but note the increased sync speed, reduced Margin & Attenuation and dah da! back on ADSL2

So still a complete mystery as to why the MSAN decided to go "ADSL2+"........................do you think I should set the Router to ADSL2 to mitigate for the mismatch being a possible/likely cause for what I have experienced???

TIA

Looks like MSAN is doing what it says on the tin, here is the current speed:-


Just off to see what BT says my IP is

Result is IP of 2.8Mbps whic tallies with the 3179 sync i.e. 88.2% of that figure.

Hmmm! so could the cause of the long interval drops from "post the Training Period" be down to the Router set to ADSL2+ somehow conflicting with the MSAN 'settling' on ADSL2 as was and is now again???

Oh, at least 21CN is doing what was promised i.e. reflecting the changes rapidly compared to groaningly slow 20CN DSLAM changes taking 10's of hours to increase and "instantly" dropping in comparison!

Edited by deleted (Tue 24-Apr-12 01:57:31)

There isn't a mismatch as such.

Every line has individual characteristics So I have a slight change fom ny previous recommendation, based on what we see now.

I would like you to do the morning reconnection as I said before, and post the stats. Or possibly save them somewhere and post together with the ones from what I think you should then do.

Which is: Utick ADSL2+. Tick ADSL and ADSL2. Reconnect and take the stats. Post both sets.

I'm hoping to see getting on for 4Mbps downstream sync.