DLM

[Bill]

Found a reference on the tbb Plusnet forum to a document released at the end of their ADSL2+ trial in 2009, full reference is here:

http://community.plus.net/forum/index.php/topic,80204.0.html

Not directly applicable to IDNet of course, but there's an interesting bit about the DLM:

Overlooking the whole speed vs stability piece is BT's DLM (Dynamic Line Management) system. DLM has been designed to analyse the stability of every line on a constant basis and, if necessary, make changes to line characteristics each day. DLM will look at how many disconnections a line has had (it should be able to tell the difference between a loss of sync and someone switching the router off), look at the amount of errors on line, see what changes have previously been made, see how much downtime the line has had and make decisions based on those as to what to do. The options DLM has are:
1. Change the interleaving status - interleaving is a form of error correction, it will add latency but mean that if packets are corrupted across the DSL part of the connection they can be repaired rather than retransmitted. There are several levels of interleaving that can repair more packets at a time but at the cost of higher latency. Interleaving can be disabled or enabled permanently on any line by the ISP.

2. Change the target SNR - the SNR is the signal to noise ratio, in other words how much signal is heard over background "noise" on the line. The higher this number the better in terms of quality. Over the course of time (a hour or day for example) the SNR can and will fluctuate because of background interference such as RF interference. This can be caused by all sorts of things from street lights and other lights to microwave ovens. On some lines the level of noise can outweigh the signal, when that happens the connection drops. This is more often seen an evening, the line syncs nice and happily in a morning but by the evening the background noise from the street lights etc. has increased so much that the line drops. To rectify this DLM will increase the target SNR so that the line syncs at a lower rate and the background noise doesn't get to such a high enough level that it causes dropouts. By default the target SNR is set to 6dB. DLM can increase this in 3dB steps up to 21dB.

3. Apply a banded profile - banded profiles are new for 21CN, what they do is set a speed cap on a line. So if for example applying interleaving and changing the target SNR have made no difference DLM may set a banded profile to set a line from sync'ing between certain limits. The banded profiles are:

WBC banded line profile (rate kbps)
7168 - 14336
4864 - 9728
3328 - 6656
2272 - 4544
1472 - 3072
1152 - 2272
576 - 1152
288 - 576
160 - 288

Below is the graph of the current 21CN speed distribution of our customers based on the BRAS profile (I've excluded the FTTC customers).

If a line is seen to be really unstable then it can sometimes apply both interleaving and change the target SNR at the same time, sometimes apply a big change in the target SNR (say 6dB to 15dB) or go straight to a banded profile.

DLM is also designed to work in the opposite direction, so if a line is seen to be stable then it should slowly back out changes it has made, so remove a banded profile or reduce the target SNR for example. Each time it will monitor the stability and back the change out should instability be seen.

We have raised concerns over how the DLM is currently operating. A lot of our customers prefer the option of having a faster speed but don't mind a little bit of instability. One or two dropouts a day may not be noticeable but the extra 2Mbps of speed it can have certainly is. Each 21CN line has a choice of three stability options, standard, stable and super stable. We have put most of our lines on to standard but have tested out both stable and super stable to see the difference. Over time the lines on stable and super stable tend to sync at a much lower rate than standard and are thus more stable but it would be nice to either have a fourth option of "best speed" or being able to adjust what DLM can do on a line which is currently being discussed across the industry, we can enable or disable interleaving so an opt in out of banded profiles or being able to set the line profile ourselves would be a nice to have.

I assume IDNet specify the "standard" stability option?

[Rik]

Probably, Bill, but I don't know for sure.

[Steve]

I do recall Rik being involved in a trial last year which gave IDNet the ability to adjust adsl2+ line parameters,as far as I am aware BT have not taken this any further yet.

[Bill]

I found the interesting bit that it continuously adjusts the parameters, ie if a line improves after a bad patch it will back off the limitations it imposed. Confirms the impression I've had on FTTC.

Don't agree with their fourth option though... there ought to be one, but it should be called "Disabled"

[Steve]

It is interesting but all we see at present is BT throttling connections back automatically leaving the customer to cajole and beg for manual reset.

[pctech]

For which BT can probably apply a small charge at its discretion of course.