Help with DLM please (ADSL2+)

Advice welcomed. Looks like DLM has capped my uplink at 797 Kb/s. What should I ask Sky CS to do?

My migration was Friday at 5 pm.
Saturday: stayed stable and fast (for the line) at 18.48/1.268
Sunday: DLM first resync 2:05 am; synced at 18.698/1.191
Monday: DLM resync 2:04 am, at 18.013/1.188
Tuesday: DLM resync 2:11 am, at 18.015/0.797
Wednesday: no DLM resync, but I forced a resync at 9:35 am -- exactly the same speed (18.015/0.797)

I'm not using the Sky Hub. My Billion modem is set to force the downstream target margin low, but it looks like DLM has raised the upstream target from 7 to 14. I've not seen any significant errors reported up or down.

I also notice that INP has gone up to 57/2.5 -- never seen that before.

What should I ask Sky CS to do?

Stats below (counters reset ~48 hours ago):

Mode ... ADSL_2plus
Traffic Type ... ATM
Status ... Up
Link Power State ... L0
Downstream ... Upstream
Line Coding (Trellis) ... On ... On
SNR Margin (dB) ... 1.1 ... 14.0
Attenuation (dB) ... 35.0 ... 21.9
Output Power (dBm) ... 18.6 ... 12.1
Attainable Rate (Kbps) ... 11114 ... 1153
Rate (Kbps) ... 18015 ... 797
MSGc (# of bytes in overhead channel message) ... 65530 ... 15
B (# of bytes in Mux Data Frame) ... 73 ... 36
M (# of Mux Data Frames in FEC Data Frame) ... 1 ... 1
T (Mux Data Frames over sync bytes) ... 0 ... 2
R (# of check bytes in FEC Data Frame) ... 6 ... 10
S (ratio of FEC over PMD Data Frame length) ... 0.1412 ... 1.5864
L (# of bits in PMD Data Frame) ... 4532 ... 237
D (interleaver depth) ... 1 ... 16
Delay (msec) ... 0 ... 7
INP (DMT symbol) ... 57.00 ... 2.50
Super Frames ... 0 ... 0
Super Frame Errors ... 1106 ... 22
RS Words ... 38418464 ... 3416088
RS Correctable Errors ... 54910 ... 16
RS Uncorrectable Errors ... 0 ... 0
HEC Errors ... 0 ... 33
OCD Errors ... 0 ... 0
LCD Errors ... 0 ... 0
Total Cells ... 57518661 ... 2541109
Data Cells ... 3821147 ... 421900
Bit Errors ... 0 ... 0
Total ES ... 322 ... 12
Total SES ... 14 ... 0

Unfortunately on Sky ADSL2+ most lines, even good ones, have the upstream capped to about 800 kbps (mine's at 798 kbps with 16.16 dB noise margin). Ringing Sky usually results in them restarting DLM training, with the end result after a few days being the same upstream cap.

No-one's raised a formal complaint to my knowledge so you could try that if normal CS gets you nowhere.

Here is my stats from my Sky ADLS2 connection, it can vary ever so slightly depending what time of year or day I reboot it at, I had to persevere on the phone until I got someone that new what was I on about when trying to get my line back to what it should be as I knocked on a "safer" profile rebooting my router too many times one day when making adjustments to it.


Upstream Downstream
Current Rate (Kbps) 1429 21020
Max Rate (Kbps) 1425 22770
SNR Margin (dB) 6.1 3.2
Line Attenuation (dB) 10.5 14.5
Errors (Pkts) 0 0

I'm not using the Sky Hub. My Billion modem is set to force the downstream target margin low, but it looks like DLM has raised the upstream target from 7 to 14. I've not seen any significant errors reported up or down.

Forcing downstream target noise margin is going to completely mess up DLM.

Sky do not work on the basis of noise margin, they set a fixed cap on the download speed and upload speed. Hence even if you tried to lower the upstream noise margin it would not work.

Lines which are both stable on the downstream and upstream get a 1.2-1.4Mbps upload speed assigned. With a noise margin of 1db DLM will set you to one of the less stable profiles which have slower uploads. In the long term DLM will also reduce your maximum downstream speed to compensate, ie your line now syncs at 18Mbps, Sky are likely to fix the maximum sync speed to something such as 15Mbps in the longer term, which will mean regardless of your tweaking you cannot go over 15Mbps and therefore the noise margin remains higher.

I cannot imagine DLM will accept a 1db noise margin in the long term on the downstream, certainly that upload profile is demonstrating DLM is trying to do something and as I say these low upstream speeds on Sky are where DLM deems the line to be of concern, hence they cap the upload speed and usually the download speed with time.

On stable lines, Sky will set a downstream cap higher than your sync speed, and then a target noise margin of 3db, 6db or 9db... Here you can play around with the noise margin a little (up to the cap). On unstable lines, the sync speed becomes the physical restrictor.

Have you just moved to Sky?

Edited by ukhardy07 (Fri 23-Dec-16 13:43:13)

In reply to a post by ukhardy07:

Lines which are both stable on the downstream and upstream get a 1.2-1.4Mbps upload speed assigned.

This hasn't been the case for many people who have had DLM line training in the last 9 months or so. The Sky forum is full of people who are either new customers from other ISPs, or existing customers who have had over 800 kbps upload on Sky ADSL2+ before their latest DLM run.

I complained about my upstream and they started a new DLM run which completed a week ago (SR102 up and stable the whole time). On a couple of the 24 hour periods of the DLM run I was assigned 1.2 or 1.3 mbps upload speed, which the line was perfectly stable at, but it ended up at this:

Broadband Link		Downstream	Upstream
Connection Speed	15752 kbps	798 kbps
Line Attenuation	27.5 dB		15.6 dB
Noise Margin		5.9 dB		16.16 dB

Downstream is fine, but there's no justification for 16.16 dB margin on the upstream. Many people are seeing such upstream margins on Sky ADSL2+ now.

There must be a trigger for these lower uploads, certainly a rental property I help out with for family got Sky broadband in November and have an upstream over 1.2Mbps now.

Traditionally the lower upstreams occurred for two reasons:
1) Sky manually set a download speed, e.g. you call and ask them to make changes instead of DLM... Sky CS only had upload speeds of below 1Mbps to select for profiles they could manually set below 15/16Mbps.
2) DLM deemed a line to be unstable, for whatever reason in this instance DLM nearly always sets a restricted upload as well as download.

There have been recent changes to DLM so do take my above statement with a pinch of salt, as that was the way it originally worked when it was implemented.

Edited by ukhardy07 (Fri 23-Dec-16 15:12:10)

These overly restrictive upstream profiles certainly do not seem to affect all lines, which adds to the frustration. And of course we'll never hear from a Sky network engineer who could confirm any changes made to the DLM system, so we're just left guessing as to what has happened.

I had a perfectly stable 1.2/1.3 mb upstream on BT before moving to Sky, so it's certainly disappointing to have my upstream crippled in this way for no reason.

Does your line drop out ever or did you play around during DLM?

In the old days my experience was always even just a couple of disconnections meant DLM would set a slow upload. This could be the line dropping or turning the box off a few times...

Sky can reset DLM easily, well they used to be able to, so if you changed hardware or did a couple of reboots in the first few days it might be worth asking for it to be re-run.

EDIT: It is worth noting your downstream noise margin is not 3db either, so DLM has not determined your line to be one of the very stable ones.

Edited by ukhardy07 (Fri 23-Dec-16 16:31:31)

In reply to a post by ukhardy07:

Does your line drop out ever or did you play around during DLM?

No and no.

In reply to a post by ukhardy07:

Sky can reset DLM easily, well they used to be able to, so if you changed hardware or did a couple of reboots in the first few days it might be worth asking for it to be re-run.

I haven't done either. I've been with Sky 6 months, and I've requested DLM line testing a couple of times with the same resulting upstream.

In reply to a post by ukhardy07:

EDIT: It is worth noting your downstream noise margin is not 3db either, so DLM has not determined your line to be one of the very stable ones.

The competency of Sky's DLM, at least on my line, is what I'm questioning. 6 dB on the downstream I have no issue with, 16 dB on the upstream is ridiculous. I log my noise margins regularly and I'm not seeing anything to suggest any instability to justify it, and my experiences reflect those of many others who have posted on the official Sky forum over the last several months, i.e. decent downstream and crippled upstream.