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Reminder about the reality of DLM on Openreach FTTC
Monday 15 July 2013 15:32:10 by Andrew Ferguson

With BT Wholesale ADSL and ADSL2+ services having ran for some years most people have grown to understand how the system manages their connection, but with the FTTC services there has been a lot less information made public, in addition to a lack of control for the broadband provider.

The Openreach Fibre to the Cabinet products use a DLM system that has three main modes, the only control a retail provider has is to pick which of these three profiles your new FTTC connection will run on, or swap you between the various modes.

  • Standard – best overall balance between speed and stability for general internet users.
  • Stable – prioritise stability over speed for IPTV videoconferencing, home workers and businesses transferring data and IPTV.
  • Speed – prioritise speed over stability for online gamers.

In all three modes the system will tune the connection, setting fast or interleaved mode for error correction and once in interleave mode vary the depth of interleaving, the deeper the interleave the more tolerant the line is to errors at the expense of increasing latency and some throughput speed.

Where the most myths occur is the speed with which the DLM system will react to issues and the pattern is something like the following:

  1. New connections are left wide open so should run at the upper limit that the modem and DSLAM can negotiate.
  2. On the first day if severe instability (levels governed by which of the three modes the line is running in) then DLM will intervene.
  3. Normally the DLM waits for a day after provisioning before making any DLM choices, and then only if the modem was trained for at least 15 minutes the previous day so that some data is actually available.
  4. Aside from adjusting interleaving depths, the DLM can set a maximum and minimum sync rate (where the minimum is usually half the maximum). This is often referred to as banding and the purpose of the minimum is that to stop a line syncing at a stupidly low speed due to a noise burst e.g. thunderstorm i.e. the modem will keep negotiating until the minimum sync can be obtained.
  5. The upshot is that usually within 48 hours most lines have had their DLM conditions set.
  6. VERY IMPORTANT: While the DLM is set at an early stage, changes in line conditions will continue to affect the line and potentially cause DLM changes in the future for as long as the FTTC service is active.


Posted by AndrueC over 4 years ago
..and it can be several weeks before the DLM relents and reduces interleaving. This typically happens in the early hours of the morning without user interaction though in some instances rebooting the /router/ (not the modem) may be required.
Posted by driz over 4 years ago
It took 3 weeks for interleaving to be removed from my line after some noise on NYE.

I suspect some local noise caused by someone with a fancy lighting system or sound system in a nearby flat caused the problem, which showed as packetloss on a ping graph from about 830pm until 1230am.

Interleaving was applied about 3am, and lasted for 3 weeks.

Utterly, utterly, utterly, pointless.
Posted by WWWombat over 4 years ago
Very worthwhile pointing this out.

I'd add the fact that FTTC seems less vulnerable to DLM being triggered by power-cycle of the modem. But, because of the slow speed to recover, it is still worth being cautious.

I'd also add that when DLM has intervened, the visible results are (usually) a reduction in sync speed of 10-20% and an increase in latency of 8ms.

Next installment: a reminder about the reality of FTTC speeds, and the impact of increased take-up on crosstalk.
Posted by ryant704 over 4 years ago
Also, noise margins are reset upon re-sync or intervention!
Posted by dogbark over 4 years ago
@Andrue Rebooting the /router/ cannot affect DLM.
Posted by dogbark over 4 years ago
How can you tell if 4. your line has been banded?
Posted by mervl over 4 years ago
I think the biggest problem with the DLM is that you never hear about the lines that its intervention keeps stable. I'm not ashamed to say in my case for months on end, and hasn't changed anything further beyond the initial interleaving for a couple of years now. Am I the only one?
Posted by AndrueC over 4 years ago
@rynt704: Yes but the IP profile is linked to your PPP session and if the resync is quick enough you might retain the same session throughout. This is what's called a profile mismatch. The modem might connect at a higher speed but your PPP session would be stuck at the lower rate.

By rebooting the router you will hopefully get a new session with an updated profile.
Posted by ryant704 over 4 years ago
Andrue, I know but I have no idea why you have mentioned this when I said a noise margin reset to 6db.
Posted by chrysalis over 4 years ago
dogbark the fritz 3370 reports banding info. another way is if the modem syncs at a low speed with a high snrm, that would indicate banding.
Posted by AndrueC over 4 years ago
@ryant704: Oops, I meant to say '@dogbark'. Sorry about that.
Posted by otester over 4 years ago
If you don't want DLM then avoid BTW/Sky.
Posted by andrew (Favicon staff member) over 4 years ago
@otester So which FTTC provider does not have a DLM that people can order?

Digital Region in South Yorkshire and who else.
Posted by JNeuhoff over 4 years ago
@otester So which FTTC provider does not have a DLM that people can order?

Any provider which will support the future fibre-on-demand (though the latter has yet to be made available). Surely genuine fibre does not require cumbersome DLM?
Posted by chrysalis over 4 years ago
fibre on demand product isnt FTTC.
Posted by otester over 4 years ago
I was referring to DSL products, with FTTC I think it's almost a non-issue.
Posted by nstrudwick over 4 years ago
A definition of DLM would be a good idea in the article, would it not?
Posted by MikeCR over 4 years ago
Notwithstanding anything else which has been said, so far as I am concerned, in a rural area (Moray), all I want is "Stability". A slow, but reliable, broadband connection would be very welcome.
Posted by bartman007 over 4 years ago
Interleave has not be turned off, its been high since December 2012
Posted by dogbark over 3 years ago
I think I'm right in guessing that the default selected by BT and Plusnet is:

Speed – prioritise speed over stability for online gamers.
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