I did try to unlock the modem but after i browse to the firmware file it seems to freeze up and do nothing

Yea im going to wait a few days before i restart it , it did reconnect/disconnect quickly a few times before the fault was fixed , router logs showed CCA failure > 1 second several times , not been able to find out what that means ?

In reply to a post by Bapteeed:

Ok thanks for the info , i thought the DLM worked by increasing the SNR to compensate all the noise on the line

That may not be the case with FTTC/VDSL.
I'm able to read the connection data from my Huawei modem and during the Christmas break (following a few disconnections) my connection speed dropped to low 50's (from maximum).
I assumed DLM was at work (following disconnections - most likely due to the excessive number of external Christmas lights around here.
However; my SNR was still at 6dB (well; 6.1) on downstream and well over 15dB on upstream (although upstream was connected at 20Mbps, so it's understandable the SNR was showing well above 6dB).

After a couple of manual reboots (just to check if the noise conditions had cleared) the connection speed stayed about the same (if anything it dropped by a few Mbps).

So I left well alone to see if DLM would fix it (or remove what it broke).
After 5 days and a few hours, the connection dropped and came back on again.
Remembering I'd been connected for months on end at full speed, I guessed maybe DLM had removed (or reduced) whatever "fix" it had applied.

Sure enough, my connection speed had jumped by around 25Mbps, and the SNR was still at 6dB.
Still not at max connection speed, but only a couple of Mbps away from it (so I'm none too worried at this stage).

So clearly BT/Openreach have a DLM which has methods (other than just messing up your SNR) to reduce connection speed and improve stability, which are far more successful than the rubbish DLM we had with ADSL.
It also looks like this DLM is a lot more forgiving and much happier to give you back your missing speed.

I'm leaving well alone again (currently at 4 days 16 hours since the DLM trigged resync, mentioned above) so maybe (just maybe) DLM will tweak my connection a little more and the speed will gradually creep back up by a few Mbps.

In reply to a post by Bapteeed:

Do you think its worth calling them or should i wait and hope DLM fixes it ?

I'd be tempted to sit and wait out DLM.
Leave well alone.
No resyncs, no rebooting the HomeHub, no clicking on 'disconnect' on the HH.
Just leave it alone and wait out the 5+ days.

Maybe give it six (it seems things tend to happen at midnight or midday - so wait out one midnight and one midday after the HH has showed a solid 5 day connection) before getting back onto BT - mainly as they'll only have you disconnecting & plugging in/out, which may well restart the DLM timers.

Thanks for the info Ade sounds exactly the same as my problem i guess its different levels of interleaving that gets applied rather than the SNR being changed

It has stayed connected since the engineer left so ill take your advice and leave it , hope it fixes the ping and upload speed soon though

I spent a lot of yesterday doing revisions to my website, particularly re FTTC DLM and IP Profiles. The stuff I put up soon after FTTC was introduced was way short of what we know now.

Have a look at the Openreach DLM page.

Edited by RobertoS (Sat 12-Jan-13 00:42:55)

Well that [banding] would certainly fit what I've seen over the last week or so.
Maybe the next 24 hours will show whether my connection had either one level of a variable banding, or also had a level of interleaving set up.
Basically I started off (having had 80Mbps almost since Infinity2 was released) at full speed and nothing less than that, to suddenly having around 28Mbps lopped off my connection speed sometime during mid-December.
Then five (and a bit) days later, with a solid connection, around 25Mbps of that was restored (giving me a sync of 77539kbps with an attainable rate of 77984kbps).

I wonder if the only minor difference between connection speed and attainable rate is a symptom of banding?
Since day one my attainable rate has been something like 5Mbps above my connection speed (indicating my line had a reasonable amount of headroom, over and above full speed) but since banding appeared to have been applied the attainable rate has been within maybe 400kbps of my connection speed.
So I'm guessing (and it is a guess) the current level of banding on my line has a max sync of 77984kbps (a fair assumption - why else would my attainable rate be an exact multiple of 32 (in fact 32 * 2437) , whereas my connection speed is 32 * 2423.09375).

I've now had another 4 days 21 hours solid connection - hopefully DLM hasn't finished trying to put my connection back to where it started and there's still a small amount of banding to be removed (or maybe not... in 4 days 21 hours I've had a total of 1381 FEC errors, but zero CRC errors - who knows what Openreach deem to be an acceptable error rate, but 11.8 FEC errors an hour doesn't seem that high to me - given my old ADSL2+ connection would have been in the millions in five days).

Unfortunately I didn't bother to check to see if my line had FEC errors before (which would have shown if I've had interleaving since day 1, or whether interleaving has recently been applied).

I will report back as to whether my speed increases (as the level of banding is reduced), but I wonder if interleaving is automatically switched off (once applied) or whether it's like it used to be on ADSL (i.e. once it's on, you're stuck with it for all eternity).

That bit about the division by 32 is interesting.

Interleaving - I meant to have in there that the consensus so far is that drops sync by about 10% compard to Fast Path, but Max Attainable isn't affected. Your figures don't tie in with that, which is either "worth knowing" for us all, or "odd" for you.

that sounds bizarre.

if you still only had a 6db margin then the line has no extra buffer so in theory should be no more stable.

my theories are either it was a dlm applied to manage crosstalk by reducing the signal power hence lower sync with same snrm. It then maybe decided later the older setting was better so reverted the change? or simply you had temporary crosstalk or fault and it cleared up. Although the latter wouldnt trigger an automatic resync.

is odd what you described.

In reply to a post by RobertoS:

I spent a lot of yesterday doing revisions to my website, particularly re FTTC DLM and IP Profiles. The stuff I put up soon after FTTC was introduced was way short of what we know now.

Have a look at the Openreach DLM page.

Your link above needs revision because it misleads people into thinking that Openreach are in charge of DLM, which is not the case as BT Wholesale is.

It's all a little odd...

1. The division by 32 bit (from attainable rate) and ending up with a whole number could just be the way the Openreach modem calculates the rate. Alternatively it could be registering the banding cap applied by DLM. Or it could just be a coincidence.

2. The 10% speed drop (caused by interleaving) could be showing on my connection. Although not obvious, my current sync is around 10% below my old attainable rate (which was around 83-85Mbps - ish), so maybe interleaving is causing my slightly lower than maximum sync? But wouldn't the max attainable rate be the max figure ignoring any loss due to interleaving?

3. Chrissy mentioned maybe a DLM applied power drop (which also occurred to me, when I first noticed a low 50s sync, whilst the SNR was still at 6dB). I'd have thought a sync cap, due to banding alone, would be reflected by a massive rise in SNR (giving the false impression the loss of speed was due to an SNR increase) and I'd seriously doubt a line, which has an upstream sync of 20Mbps (with an upstream SNR of 15dB) couldn't manage more than 50ish downstream, at maximum power, at 6dB SNR.

It does look like DLM has applied at least two "fixes";
1. Switched on interleaving (but it may have been on anyway, as Speedtest.net pings are still around 5-10ms, to London & south Midlands servers, which is where they've always been).
2. Either applied banding (to cap downstream sync) or reduced transmission power (to limit downstream sync by way of the standard 6dB margin).

At least we don't seem to have the excessive (and frankly annoying - because it's next to impossible to have undone) ADSL DLM overkill first step of banging the margin straight up to 15dB (OK; not strictly "straight up to 15dB", more like up froim 6 to 15dB in a matter of a few resyncs).

Space to be watched; I think

Edited by adebov (Sat 12-Jan-13 11:04:05)

In reply to a post by robertcrowther:

Your link above needs revision because it misleads people into thinking that Openreach are in charge of DLM, which is not the case as BT Wholesale is.

They can't be as the FTTC DLM works for Sky and TalkTalk users who don't have anything to do with Wholesale.