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:
- New connections are left wide open so should run at the upper limit that
the modem and DSLAM can negotiate. - On the first day if severe instability (levels governed by which of the
three modes the line is running in) then DLM will intervene. - 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. - 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. - The upshot is that usually within 48 hours most lines have had their DLM
conditions set. - 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.
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..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.
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.
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.
Also, noise margins are reset upon re-sync or intervention!
@Andrue Rebooting the /router/ cannot affect DLM.
How can you tell if 4. your line has been banded?
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?
@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.
Andrue, I know but I have no idea why you have mentioned this when I said a noise margin reset to 6db.
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.
@ryant704: Oops, I meant to say ‘@dogbark’. Sorry about that.
If you don’t want DLM then avoid BTW/Sky.
@otester So which FTTC provider does not have a DLM that people can order?
Digital Region in South Yorkshire and who else.
@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?
fibre on demand product isnt FTTC.
I was referring to DSL products, with FTTC I think it’s almost a non-issue.
A definition of DLM would be a good idea in the article, would it not?
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.
Interleave has not be turned off, its been high since December 2012
I think I’m right in guessing that the default selected by BT and Plusnet is:
Speed – prioritise speed over stability for online gamers.