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    FTTC DLM - What it is & How it works

    This is a discussion on FTTC DLM - What it is & How it works within the Sky Broadband (Fibre) Help forums, part of the Sky Broadband help and support category; The following has been posted in the Plusnet Support Library* and provides some useful info about FTTC DLM . What ...

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      FTTC DLM - What it is & How it works

      The following has been posted in the Plusnet Support Library* and provides some useful info about FTTC DLM.


      What is DLM?

      DLM stands for Dynamic Line Management. It’s an automated system used to ensure that the user received 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:

      FTTC DLM - What it is & How it works-bands1.jpg

      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).
      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:

      FTTC DLM - What it is & How it works-caution-counter.jpg

      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.


      *The original post can be found here.
      Meteosat likes this.

    2. The Following 6 Users Say Thank You to Saturday For This Useful Post:

      Angel_Rex (27-05-14),asamkv (29-05-14),James_Mitchell (29-05-14),joe pineapples (27-06-17),Meteosat (28-05-14),prestonradar (03-12-14)


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      Re: FTTC DLM - What it is & How it works

      If anyone has any specific questions surrounding this, feel free to drop me a PM.

      Chris

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      Re: FTTC DLM - What it is & How it works

      Thanks Chris - both for the article and the offer, though unless there's a need for a private conversation via PM, we always encourage people to post questions/answers to the forum as then others can benefit.

      I really like the way that Plusnet offers quality technical support via their official forum. In contrast, Sky's general forum can be woeful at times though maybe things will be better for those on the pro BB products with their new dedicated (but private) forum. In the meantime, I think Skyuser does a pretty good job helping their customers but we're always glad for any contributions of knowledge and advice.

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      Re: FTTC DLM - What it is & How it works

      "If DLM ever needs to act on your line, you’ll never be set to that open profile again".

      I have had quality issues with the telephone, the crackling was so bad sometimes I had to terminate the call. During this time my connection speed was variable (middle 30s).
      All fixed now, the engineer connected me to another pair due to an earth leak. My connection speed is inching backup towards the open speed, but my low pings are never going to recover ?
      Is there any chance of getting the DLM reset as the repair was approx 6 weeks ago ?

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      Re: FTTC DLM - What it is & How it works

      @Saturday - That's no problem at all, I'm happy to respond to any questions on here for the benefit of other viewers Yep, I do like this Forum, I've had a read through it before so it's nice to finally be here!

      @146Kg - I'm glad that things have picked up. First of all, I'm not 100% sure of Sky's internal processes, so forgive me. If a network change was made to your service, then a DLM Reset should have been initiated. It looks to me as though you're going through the Caution Counter and your profile is inch by inch improving. Your line probably has Interleaving on which is unlikely to be removed until you start hitting some of the higher profiles (40-80M) for example.

      My advice would be to have a chat with your broadband provider (assuming it's Sky) and explain that you've had an engineer out and to see what your options are to get a DLM reset initiated.

      I hope that helps!

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      Thumbs up Re: FTTC DLM - What it is & How it works

      Quote Originally Posted by cpettitt View Post
      If anyone has any specific questions surrounding this, feel free to drop me a PM.

      Chris
      As per other note, rather than sending PM hopefully my question and (hopefully answer) may be of use to others.

      I've just switched to Fibre so am probably luckily still on an Open Profile. I have good wiring in that when I was on DSL I'd never suffer random drops due to REIN or other factors, and indeed my DSL modem would hold a line down to a 2db SNRM, often less.

      Anyway, the default SNRM for Sky Fibre (presumably others) looks to be 6db and is giving me around 47Mb sync. That's fine, I only had 13Mb@6db on Sky ADSL (was 19Mb@3db on Be, I know the difference in the DLSAM chipset influenced that).

      Since installation yesterday, I've rebooted about 3-4 times during configuration, but all spaced out deliberately over several hours so as to not make DLM think there is a problem.

      My question relates to trigger thresholds.

      The OP mentions:

      "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."
      Do you have any further details on what these trigger thresholds are?

      e.g. How many CRC errors per X time duration puts you into to the 'red' zone, how many reboots/retrains per hour/day might trigger DLM to take action. etc? I'm asking for clarfication as I'm guessing you used the numbers above as examples, but wanted to find out if there was any more detail available.

      Thankfully my home network is pretty much set up now so things shouldn't change, but of course I may need to reboot at times when tweaking Port Forwarding etc. I'm an active gamer so keeping interleaving off is a priority for me, but ultimately I know this will be out of my hands. I do however have a phone line issue (when Sky activated the Fibre, the landline stopped working) so I'm concerned that any work they do to fix my line might introduce noise/drops which may trigger DLM.

      Aside from the landline issue, on my router I have the ability to tweak my VDSL target SNRM from 5db to 30db, so I'm keen to understand if it's worth me using that feature to help reduce CRC errors if I'm getting too many, to preempt DLM and stabilise my line so I can stick with an open (or at least Fastpath) profile, even if I lose a bit of top speed with a higher SNRM.

      e.g. At the moment I've had 2267 D/S CRC Errors down across a period of 15h30m - an average of 2.43 CRC errors per minute at 6db. This rate may rise over the evening period of course.

      Is this high? Is this low? If it's high, I can adjust my SNRM to a higher value to help reduce these, but I'll lose some sync. Conversely, if this is low and I have 'headroom' I could drop target SNRM to 5db and get a bit more speed upto a 50Mb sync (but more CRC errors).

      Any more details on these trigger thresholds would be very much appreciated if available.

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      Re: FTTC DLM - What it is & How it works

      Sorry been away on holidays!

      Do you have any further details on what these trigger thresholds are?
      I'll be honest. Yes I do know what the thresholds are, however, I'm not able to provide those details as the information isn't yet in the public domain (BT Openreach would release these). They are similar to ADSL I'll tel you that much


      Is this high? Is this low? If it's high, I can adjust my SNRM to a higher value to help reduce these, but I'll lose some sync. Conversely, if this is low and I have 'headroom' I could drop target SNRM to 5db and get a bit more speed upto a 50Mb sync (but more CRC errors).
      That's a reasonably low number of errors to have on the line. That said FTTC DLM can be really picky and if it see's constant errors above a certain threshold, let's say 5,000 just as an example, it'll probably cap you and apply Interleaving. If you're an active gamer, the best thing for you, and to keep Interleaving off, would be to actually have a lower error count, meaning a slightly lower sync rate potentially. That will mean that the quality of your connection will be at it's best.

      Apologies that I'm not able to provide you details you're after. If/when BT OR make such details public, I'd be happy to share.

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      Re: FTTC DLM - What it is & How it works

      Great article, I am particular curious about two parts

      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 had some wiring issues (read builders) which caused me to lose sync on two consecutive days at around 4am (approx 4Mb each time). In line with the above. Also having seen the following I was expecting to have to wait 9 days or so for the sync to improve. I assumed a caution counter of 8

      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.

      Now here is the strange thing only 2 days or so after fixing the wiring and rebooting the router my sync has jumped back up (not the full amount just 4Mb) and this re-sync happened at 8am approx

      So my questions
      Is there a caution counter of 1 or 2 ?
      Do DLM re-syncs UP happen outside 3am-5am ?

      Or have just had another fault re-sync and just got lucky on the new sync rate ?



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      Re: FTTC DLM - What it is & How it works

      Changes that are made between 3am-5am aren't necessarily those which take effect instantly. It's the decision that's made at that time and the drop in connection for the DLM change to take effect is somewhat staggered. This is done purely so that BT don't have a huge amount of people dropping and reconnecting and hitting their radius servers at once.

      In the example you've given, it's difficult to tell whether or not your sync rate actually dropped temporarily or if DLM took swift action to force the sync rate down using banding. A couple of drops don't generally cause too many issues. I'd say that 4 or 5 disconnections within a 2 hour window would be a good threshold to look at.

      Is there a caution counter of 1 or 2 ?
      There isn't. I think the slight drop you had was potentially a slight blip. were you able to look at your actual sync rate at the time? That will be the only way to tell, other than having your provider run a Knowledge Based Diagnostics test on your line.

      Do DLM re-syncs UP happen outside 3am-5am ?
      Yes they do. Only the decisions of what changes will take effect on your line happen between 3am-5am. That said, there are a lot of people's lines that you'll see that do drop within that time since it's the quietest time on the network.

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      Re: FTTC DLM - What it is & How it works

      Thanks - reading my own post again its a little unclear. When I say I had two consecutive days of re-synch at 4am (they where drops to lower sync) as the result I suspect of many disconnects during the daytime because of the builders (they kept turning the power on and off several times in the hour !!!!

      So I expected DLM to lower my sync but was surprised it has started to rise again after only 2 days

      Anyway problems are solved now and I'm back to 58Mb from a low of 54Mb but still short of my peak of 65Mb.
      So I guess its sit tight and see what happens
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