DLM, dB and other line stats

Is DLM a continuous process, is it always checking my line and updating or something which only kicks in after x number of disconnects/errors within y minutes?

My ongoing effort is to get my 40/10 connection as 'best as' in order to make it worthwhile upgrading to 80/20. I've done all I can at my end including stripping out some internal wiring, I'm hoping this to be reflected by an increase in attainable speed (currently ~48/13).

I followed the advice given by my ISP (Zen) and looked at my line rates. They haven't been updated for 11 months. So either any change I've made internally which could make a difference will be picked up now or I have to ask Zen for the DLM to be reset?

Meanwhile I have a SNR (is this the SMRM, or are they two different things?) of 6dB. Do I want this to be high or low? What can I do to make it better if 6dB isn't as good as it should be?

The attenuation is 20, I guess that's a fixed product of the line length (~550m) and can't be changed.

Continuous!

In reply to a post by Woolwich:

Is DLM a continuous process

Continuous.

My ongoing effort is to get my 40/10 connection as 'best as' in order to make it worthwhile upgrading to 80/20.

That may not be possible, the DLM is optimising for the speed cap you are assigned, so if you hit 40/10 sync then the DLM won't bother doing any more optimisation, as the goal has been achieved with a stable line at the sync cap.

Meanwhile I have a SNR (is this the SMRM, or are they two different things?) of 6dB. Do I want this to be high or low? What can I do to make it better if 6dB isn't as good as it should be?

This will be Margin. If you are getting 40 download, with 6dB, then the DLM won't need to try you on a 3dB profile.

It is called margin because you have 6dB of signal above the "noise floor", so you have plenty of signal in hand for any noise events that might occur (e.g. ancient car or motorbike going past) that could impact the DSL signal over the copper wires. As speeds go up, you may have to sacrifice margin (and stability) for higher speeds. This is only possible if you pay for the higher product to get the cap increased.

The attenuation is 20, I guess that's a fixed product of the line length (~550m) and can't be changed.

That is my understanding. 20 is pretty good to be honest. I was also 550m and my attenuation was 28. Openreach in 2013 suspected it was higher due to the internal joints running through the block of flats before it made it outside.

Edited by jchamier (Thu 11-Feb-21 14:02:43)

In reply to a post by jchamier:

the DLM is optimising for the speed cap you are assigned, so if you hit 40/10 sync then the DLM won't bother doing any more optimisation, as the goal has been achieved with a stable line at the sync cap.

Thanks, that was all helpful.

It sounds as if the only way I'll know if I'll see a worthwhile increase in speed is to cough up. But knowing my luck so far I'll be taking a bit of a risk.

So where does the 'attainable' figure my router reports come from?

Current theory as I understand it: I upgrade and then DLM starts testing. With luck it will increase me further than the current reported attainable.

With luck...

Edited by Woolwich (Thu 11-Feb-21 15:42:07)

In reply to a post by Woolwich:

So where does the 'attainable' figure my router reports come from?

From a maths algorithm, which is not reliable on any future speeds you may attain.

Current theory as I understand it: I upgrade and then DLM starts testing. With luck it will increase me further than the current reported attainable.

An upgrade (or migration to an new ISP) will cause a reset of DLM, which may turn off some things (e.g. GINP) currently on, and so may get slower speeds for a bit. Then in a few days (2 or 3) these should return, and the DLM keep testing and where it can reduce the SNR margin, maybe down to 3dB if the line is stable. At that point you will be getting the highest sync possible.

Should the line be unstable, DLM will intervene and increase the margin, and maybe interleave, until the line is stable.

Predicting is hard!

Interesting. I had the same gamble when I switched from Virgin to Zen in December.

The Zen estimate was 49M so I gambled on the higher speed. My line length estimate was about 520m from both the Openreach guy who hooked the line up and then also the Fritzbox estimate. I've only managed to sync at 46M so far giving throughput of 43M (upload is sync is 10M). So not really worth the extra but better than always wondering though! Maybe I'll go for the 40M if/when I re-contract.

Every sync I've done (Fritzbox 7530, Huawei HG612, Zyxel VMG1312-B10A) has come up at 46M from the very first day and my SNR has always been 6-7dB with line attenuation of 18.5dB. DLM has never diddled with anything. The line came up as fastpath with g.inp on day 1. No aspect of any of my stats have changed since the very first day. Errors are low but DLM hasn't nibbled at the SNR at all. This is probably a good thing as what is the point of a few extra Mbps which will be unnoticeable versus reduced stability which is definitely noticeable.

I settled on the Huawei feeding the Fritzbox in the end, The Fritz and the Zyxel drop the line once or twice a week but the Huawei sticks to it like glue. (I have OpenWRT on the Fritz now also which doesn't support its built-in modem anyway.)

Another oddity is the Zen portal. It recorded my sync speeds for about a week or two then gave up. It has two other sections (forget what they are) but they are always empty anyway, When I deliberately take the modem offline (to fiddle) I always wait at least 30 minutes before resync, so DLM sees it as an 'unforced retrain'.

G.INP is now retained on an ISP migration and a DLM reset.

What modem are you using and could you post some stats?

If you have an attainable of 48, capped at 40 sync and an SNRM of 6dB then that sounds interleaved.

With G.INP enabled I would expect the SNRM to be above 6dB if there's 8Mb spare attainable.

Something doesn't sound right from that description.

In reply to a post by j0hn83:

G.INP is now retained on an ISP migration and a DLM reset.

That is good!

In reply to a post by j0hn83:

What modem are you using and could you post some stats?

FritzBox 7490. Latest firmware.
***
Receive Direction Send Direction
Max. DSLAM throughput kbit/s 40000 10000
Min. DSLAM throughput kbit/s 128 128
Attainable throughput kbit/s 50216 14010
Current throughput kbit/s 39998 9997
Min. effective data rate kbit/s 39973
Seamless rate adaptation off off

Latency fast fast
Impulse Noise Protection (INP) 38 0
G.INP on off

Signal-to-noise ratio dB 6 8
Bitswap on on
Line attenuation dB 20 26
approximate line length m 537

Profile 17a
G.Vector off off

Carrier record A43 A43
***

I've now placed a regrade order with Zen. Worst than can happen is a get a couple of Meg more upload which would be okay. Its a £90 bet.

The attainable has stuck around where it is since I 'lost' the internal wiring. It was hovering around the 47-48 range with upload at 13 - 13.5.

Meanwhile OR have been spotted running FTTP two streets away. But they're not coming to my bit of the estate.