Showing results for 
Search instead for 
Did you mean: 

FTTC DLM - What it is & How it works

FTTC DLM - What it is & How it works

FTTC DLM - What it is & How it works

 What is DLM?
DLM stands for Dynamic Line Management. It’s an automated system used to ensure that the user receives a good, high quality, usable connection. Whilst it’s important that you get the best possible speeds, it’s just as important to ensure that it remains stable and error free, as much as possible. Depending on the technology that DLM is being used on (ADSL/FTTC) will depend on how quickly DLM reacts to the changes of line conditions. DLM ultimately works to ensure that you’re receiving the best possible experience from your connection.
 How does DLM affect me?
In a lot of cases it won’t. On 45.4% of lines across the BT Network, DLM hasn’t needed to act on it. If DLM does need to act on your line, you may find that Interleaving has likely been applied as a first port of call. Interleaving is used to correct errors on your line. Errors can be caused by both internal and external influences, such as REIN (Repetitive Electrical Impulse Noise) or something as simple as faulty equipment or internal wiring.
If DLM finds that your line is erroring enough to cause the quality of your service to become poor, it will act and either apply Interleaving or reduce the ‘speed band’ that you’re set within. You probably won’t notice the difference in service having Interleaving switched on. If DLM believes that the line will deliver much better overall performance at a slightly reduced rate, it can and will make the changes where necessary.
When your line is provisioned, it has a set of thresholds assigned to it. If the thresholds are crossed, DLM will make the changes as explained above.
So, for example, if your line retrains (errors so much that it causes your line to drop) more than 50 times within a 24 hour period, DLM is likely to take drastic action to try and stabilise the line.
Another example, if your line has an error every 10 seconds, your line quality is likely to turn ‘Red’, meaning that negative changes will be made by DLM to stop your line from erroring to ensure your line remains stable and increases the performance of your connection.
 What are bands?
Just like ADSL, FTTC has a number of bands that your connection will be categorised into. Please see below for a list of some of the different bands:
 Depending on your sync rate (connection speed rate between your BT Openreach Modem and FTTC Cabinet) you will be put on one of the above bands. When your line has been first provisioned, you may notice that your band is set to the ‘open profile’ which is likely to be either; 0.128M-40M or 0.128M-80M on the downstream and either 0.128M-20M or 0.128M-10M (depending on whether you’re on an up to 38Mbps or 76Mbps product). your line will be left on an open profile for 2 days initially to allow the line to adjust following the installation.
If DLM ever needs to act on your line, you’ll never be set to that open profile again, the highest profile you can be assigned would be 40M-80M (sync rate dependant). There are three levels of Interleaving – off, low and high, If Interleaving is applied on low your latency will be increased by around 8ms and 16ms on high.
 When does DLM make changes to my line?
Unlike ADSL, FTTC DLM does not work on the fly. DLM monitors each line throughout the day in 15 minutes snapshots. At around midnight each night, all of this data is uploaded to a datacentre where it is analysed. Based on the data received, DLM will make an intelligent decision on what changes should be made your line, whether that be Interleaving or Banding Profile changes being made, or even both.
Any changes made by DLM are executed between 3am-5am, this is to lower the impact this has on the user. Changes made by DLM will force a resynchronisation (quick disconnection between your BT Openreach Modem and Cabinet).
 I’ve had a fault and my speeds haven’t returned to normal, why?
If you’ve had a fault that’s affected the speed of your service, this is something we can look into for you. Unfortunately we are not able to do remote resets on FTTC circuits at present, so this isn’t something that can be requested nor can we send engineers out solely to complete a DLM Reset.
If you’ve had a fault with your own equipment and/or internal wiring, you will need to allow DLM to pick your speed back up. The quickest way you can do this, is by ensuring that your modem is left connected at all times. In time, DLM will put you back onto the relevant profile, it may take some time, so please be patient.
If you’ve had a fault with the service and an engineer has visited and made changes to the BT Network in order to resolve the issue, the engineer will perform a DLM Reset. If no changes have been made, a DLM Reset will not be submitted. A DLM reset takes around 5 minutes to complete and completely resets your line; you will be placed back onto an open profile.
We are working very closely with our suppliers for tools to be made available for us to do further remote diagnostics and changes to your line. We fully appreciate that waiting for your speeds to increase can take quite a lot of time and is frustrating!
 Why does it take so long for DLM to increase my speeds?
DLM uses a system called ‘Caution Counters’, this dictates, based on the quality and performance of your line what band/profile your line should be on as well as how long it will take before your line needs to be stable (Green quality) before increasing the speeds.
For example:
 In the above example, you can see the line quality is green and no changes were made to the profile. All of a sudden, your line has a bad day and errors significantly and drops out due to this, DLM reacts and changes the profile/applies Interleaving. The counter then starts to count up to 9, the line remained green; the top profile would have been re-applied. In this case, the line started to error again, DLM reduced the profile again to try and control these errors.
You can see that there is a ‘barrier’ which shows at 0, 8, and 24 in this example. If your line had a bad day, you’d be put on barrier: 8, your line will then have to wait for 9 days (barrier=8 +1) before any positive profiles changes would be made. If your line continues to error/perform poorly, the barrier is increased, following the same rule as before.
It’s important to bear in mind that it’s very unlikely that your profile will be set at its lowest without there being some kind of fault with the service, which an engineer would likely have visited and repaired and performed a DLM Reset.
 Running the BT Speedtester
If you're experiencing problems with the speed of the FTTC service, it's really important that you run a Speed Test at once the initial Speed Test has been run, click on 'Further Diagnostics' and enter your telephone number and run the more in depth test. Once the tests have completed, the results are uploaded to our suppliers servers which we can access to view your results.
You may notice that sometimes the Speed Test fails and doesn't complete fully, don't worry, the details from your original Speed Test should still be uploaded. If you've been asked to complete a TAP3 Test and you're having problems, please let us know so we can advise our suppliers. If your FTTC Circuit has only just been provisioned, please allow 14 days to pass before attempting to run the BT Speedtester as it can take some time for your circuit information to show on all the relevant back end systems.

Community Veteran
This is really useful, thanks Chris.
Community Veteran
Thanks for this Chris! I often wondered how DLM worked on FTTC and now I know.
Not applicable
I'm glad you've both found it useful Smiley
Rising Star

no mention of the stability profile?

no mention of the snr banding and the new 3db band?

no mention of levels of dropouts?


out of interest where did this info come from, seems that there is lots of 'hearsay' articles on the internet, but nothing actually directly from openreach...

Seasoned Pro

I want PN to ask openreach to be including Caution Counters for all FTTC GEA TEST report.


Can this be updated?

Aspiring Hero

This includes how DLM works on


Graphics don't appear  in the above DLM post e.g.  "long it will take before your line needs to be stable (Green quality) before increasing the speeds.
For example:"   


But if you copy and paste the post, say to Onenote you see them.

Good explanation, we need more like this.  Yes there are 21 pages of stuff in the "Library" so you may  find what you want there. 

More info on the diagnostics would be really helpful as with a recent fault I only found out about local noise when the engineer posted info in the support ticket.   Upload went from 8 to 1 Mbps but no one was going to contact me about the possibility of local noise.