Questions and answers on DLM, sync speed and real ...

Question

I was confused about some DML details and posted on a BE forum (https://avatar.bethere.co.uk/forum/viewtopic.php?t=43542) and got what was to me an incredibly helpful response from Tom (drsox).  I thought it would be of interest to some readers here so copy it below.  Once again, many thanks Tom.
1) FTTC DLM applies only between the cabinet and the modem? 
>> Yes, as there isn't any broadband / data signal going anywhere further down the line to the exchange if you are on FTTC! 
2) What are the different parameters that apply on this hop? I know there is a current sync speed, a current snr, a target snr and an ip profile. Anything else? 
>> The IP profile isn't really related to DLM, it just "follows" your sync rate. In the past the IP profile would take up to 3 days to "follow upwards", leaving you with a high sync and awful speeds! This has been changed now on 21cn and "RE-ppp"ing (rebooting the router) would force the IP Profile to update instantly. 
DLM changes the SNR target. So where BE have hard coded / set SNR targets such as 3db, 6db, 9db, 12db etc.. They only change upon user request. BT use their DLM system where line instability or line error counts are collected and analysed, the SNR target and interleaving settings are then modified in increments to try and stabilise the line. 
This image shows quite well how, over a 6 day period, BTs DLM noticed that my line fault was fixed. I didn't get the full speed back for a week. 
3) Is the ip profile the same as the BRAS profile? 
>>Yes 
4) The DLM keeps statistics of errors etc to dynamically change the ip profile? 
>> Changes the SNR target and interleaving for the line which will reduce the speed, and therefor the IP profile. 
5) There is a general theory that the DLM is incapable of telling the difference between an error and a modem power cycle, so that power cycling can cause your profile to lower. Is that correct? 
>> Yes, if you resync / power cycle your router multiple times in a day it is likely to try and "combat the line errors" and reduce your sync rate by increasing the SNR and / or interleaving on the line. 
6) Can other connections and disconnections have the same effect? For example, if I reboot the router (where this is separate from the modem)? Or use the router interface to disconnect/reconnect the broadband? 
>> PPP drops (router reboots while leaving the BT Openreach VDSL modem on and connected) won't affect DLM. The IP profile will match your sync rate upon reconnection of the PPP session. 
7) If you have a low ip profile, does this mean that the sync is limited to the ip profile? 
>> Other way around, if you have a low sync rate then the IP profile will match the sync rate. If you get an awful sync rate and the IP profile follows downwards, then you resync higher but the router "keeps the PPP session open" (which is possible if you resync quick enough) then your IP profile will stay low, even though the sync is high. Until you reboot your router or disconnect and reconnect the PPP session. 
8) Can the sync rate change dynamically without a reconnection? If the ip profile gradually rises, will the cabinet force a resync to make sure the customer is actually getting the benefit of the higher ip profile, or must the user intervene with a modem resync? 
>> See above - Sync can rise without affecting the IP Profile and the cabinet does not force a RE-PPP. 
8) As I understand it, if the ip profile gets stupidly low for some reason, it can only be reset by BT wholesale? This will only be done after a visit to your home and a telephone call from the engineer to the BT wholesale central admin? 
>> Mixed reports plus I don't think people understand the difference between sync speeds, line faults, the delay in DLM increasing sync after a fault.. I expect half the reports on the BT forum are not related to a stuck BRAS / IP profile. It is possible for the ISP or a BT engineer to reset the IP profile and possibly DLM training. 
9) (8) This applies to all ISPs using BT Wholesale: some may be more willing to force this process to happen on behalf of their customers, but none can cirumvent it? 
>> GEA providers (I believe) still have DLM but don't have BRAS / IP profiles unless they implement their own. 
10) Who sets the snr? As I understand it, the customer has no direct control. Is this different for different ISPs? 
>> Mixed information again, I believe that BT Wholesale give providers DLM override controls and SNR settings but none? (or few) ISPs use this system. 
11) Real download/upload speeds are physically limited by the sync rate. They are also naturally limited by local limitations (such as wireless connections), server limitations, and congestion at various points along the route? Any more? The congestion along the route will vary bewteen ISPs as they use different routes with different capabilitues. 
>> It is possible for BTs regional or local cabinet network to become congested. Supposedly the minimum speed you should achieve over the BT part of the network is 15mbps. ISPs such as Plusnet, BT etc.. then layer their own "discriminatory" traffic management on top of that (ie, slow down torrents etc.). 
12) Real speeds are 'unnaturally' limited by throttling. This will also vary from ISP to ISP. Sky claims to have none, and BT none except for p2p. PlusNet and John Lewis have significant (but well explained)throttling. All the more expensive operators have none. 
>> What you said  
13) The ip profile affects the sync rate, and therefore actual speeds. Does the ip profile effect actual speeds in any other way? 
>> Not quite right. The IP profile directly affects the speeds you can achieve but the sync rate affects the IP profile. The sync rate is determined by the stability and quality of your line and what DLM thinks it can cope with. 
14) Many people (including AndrueC above) report real speeds from speedtests that are consistent and not close to the ip profile. These can't be explained by congestion (that would make them inconsistent). They can't be explained by server limitations, as they are consistent accross servers and the same servers give better speeds to other people. They can't be explained by local limitations, too many people claim sudden drops where the local conditions have not changed. They should not be able to be explained by throttling, as they as seen for many ISPs who do not throttle (I don't hold with the ISP lying theory here). What other factors are there that can explain these cases? 
>> AndrueC and some other Sky customers have strange symptoms where speeds are around 20mbps rather than the full sync speed. The only logical explanation is either local cabinet congestion, contention between BT and the provider's network (not enough investment by the provider) or artificial traffic management on the providers side. It is also possible that the router that the customer has attached to their line is unsuitable for the higher speeds. For example an old ethernet router may only be able to cope with 20 or 30mbps! A friend of mine has a TP-Link ethernet router running Tomato on a BT 80mbps service but the TP-Link's CPU limitation causes his service to only perform at around 68mbps; he has a replacement router on order. (Obviously wifi will significantly reduce performance). 
15) If you get a stupidly low ip profile, how long can you except it to take before it climbs to something more reasonable? (eg in my particular case, my profile is currently around 15Mb, and from BT service line tests should be around 58Mb). And the classic question, is it best to power cycle the modem (1) never, (2) occasionally (how often) or (3) never if at all possible? Mine did jump from 3.5Mb to 15Mb on one power cycle. Was that just coincidence? 
>> My FTTC line took about 6 to 7 days to go from 40mbps to 80mbps after a line fault. It is possible that your 3.5mbps to 15mbps increase was the "re-ppp" affecting the BRAS profile and that your sync had slowly been increasing over time. 

Answer 1

what a waste of money sending an engineer to "fix a fault" which does not exist.  Precisely.
In my original BE post to which Tom so helpfully responded, I began:  It seems to me that DLM is an excellent concept with a highly flawed implementation, both technically and administratively.   I think that sending out an engineer to fix an obviously flawed profile is the main example of an adminastrative flaw.  I understand (I can't remember source, maybe Tom again) that they are sometimes relaxing the requirement for a visit before reset.
Maybe the DLM system is too keen on stability vs speed.  This will keep complaints down from many people: most users won't notice speed too much as long as it is reasonable, but will be upset if their Skype calls and browsing are being interrupted too often.  
However, it does lead to complaints from people who notice the drops after an incidence (as in your thread that has drawn lots of interest), or who only get 50 instead of 60.  The main technical flaw is that DLM can so easily be confused by drops from loss of power, too much modem recycling, etc, and then takes so long to recover.