exchange negotiates deliberately unstable 3dB target SNR from today- why ?

[#1]

I've had a stable 18k down/ 1k2 up ADSL2+ line since I was switched a month back. The target SNR was 6dB and that resulted in a line that was faster than whatever BT/Plusnet could stuff down it.
Today I notice that my line speed has dropped to 1.5Mbps - something I never saw under ADSL1. This is the worst ever figure I have seen since having broadband at this address.
The connection now attempts to negotiate an SNR of 3dB. It negotiates a line rate of 20252kbps
and there are huge numbers of errors.
Something has changed and my line is now being deliberately negotiated as unstable - clearly 3dB is an inappropriate target SNR. Even 6dB was only just sensible.
Can someone at Plusnet investigate why the exchange is now doing this and reset my target SNR to 6dB ?

[#2]

The 21CN DLM uses a 3dB to 18dB range for setting target noise margin and obviously felt that 3dB was worth trying on your line. It should see that 3dB is inappropriate and left to its own devices reverse the change in 24 hours.

[#3]

I thought Plusnet were able to override this behaviour by specifying a target SNR for their customers ?

[#4]

Nope.  Plusnet can get the current target SNR reset (by raising an order to BT) to the default 6dB but the DLM can override this at any time it feels like it.

[#5]

Ugh. Why do they bother when they refuse to carry data at anywhere near that rate anyway ? My BT Speedtester results are nowhere near the sync speed. Presumably their system isn't well-enough integrated to be able to instruct the DLM logic at the exchange not to optimise past the actual capacity of the line ?
Meanwhile, their 'optimisation' strategy results in the attached line test - capped at 1.5Mbps for an unknown period - perhaps a week.

[#6]

To get an IP Profile of 1500kbps implies your router must have synced in the sync speed range 1728-2015kbps. That will have been caused by a burst of noise on the line, *not* as a result of the DLM trying a 3dB noise margin. If that was just a one off your IP Profile should increase again within a couple of days.
My results from the speedtester are 74% of sync speed (89% of IP Profile). IP Profiles which allow for protocol overheads are typically 85% of sync speed. I use a wired connection to the router. Is yours wired or wireless?

[#7]

My BT Speedtester results are always wired - as they instruct.
You say you get 89% of IP Profile - but what's your IP Profile ?
My stable IP Profile was 16Mbps before today. I've never got more than 10Mbps out of the Speedtester.
And why should 1 noise burst ruin my line for days ? It should be monitoring on a timescale of minutes/hours if it was actually designed to maximise customer throughput.

[#8]

My IP Profile is 6000kbps and speeds from the BT Performance Tester (and ThinkBroadband) are typically 5335kbps.
BT obviously think the DLM is a masterpiece. Users might not share that opinion.

[#9]

Hi disfroot,
I can see that your profile was moved to 1500 on the 12th, but uplifted to 17000 on the same day (which is what it's set to now both on our side and on BTs). Please let us know if you're not seeing those sorts of speeds though?
I hope that helps,

[#10]

Quote from: spraxyt

My IP Profile is 6000kbps and speeds from the BT Performance Tester (and ThinkBroadband) are typically 5335kbps.

I would argue that it is relatively easier for the BT 21CN network to saturate a line profiled at 6000 than one profiled at 17000. It would be very interesting to see a graph of percentage of IP Profile achieved vs. IP Profile.

Quote from: spraxyt

BT obviously think the DLM is a masterpiece. Users might not share that opinion.

No - I fail to see the 'logic' of a system that continues to set a target SNR of 3dB on an un-interleaved line after a 'noise burst' that has caused a connect at 1.5Mbps. That makes no sense to me - you can't assume the burst event is statistically independent of the persistent line condition - there's a conditional probability involved here.
It seems to me the algorithms have the effect of keeping a customer's line at a conservative margin, by the coupled factors of

• always training on the edge of minimal noise resilience


• having asymmetric time constants - an instant reduction in the IP profile downward, but a massively slugged recovery of the IP profile upward

.
Across an entire network this has the deniable effect of reducing the amount of capacity that has to be provided.

[#11]

Quote from: Matt

Hi disfroot, I can see that your profile was moved to 1500 on the 12th, but uplifted to 17000 on the same day (which is what it's set to now both on our side and on BTs). Please let us know if you're not seeing those sorts of speeds though? I hope that helps,

Yes, my IP Profile is now at 17000kbps. As always, however, I only see a fraction of that speed from the BT Speedtester ( see attached).
So, no, I'm not seeing those sorts of speeds - nowhere near - never have.
If you compare this test with the one I posted yesterday, something odd appears - the BT Speedtester reports 50% download is achievable whether the IP Profile is 1.5 or  17.
How can that be real data ?

[#12]

Hi disfroot,
have a look here for some tips on troubleshooting that.
Jojo

[#13]

I'd already pretty much done all of that, but thanks for the link.
The only thing on that list that I can't test is whether my Linksys modem is, by design, the bottleneck, as I don't presently have access to any other ADSL2+ hardware.
My internal LAN is not the problem, as I can push files between my PCs faster than ADSL2+ speeds, wireless or wired.
So what next ? The BT Speedtester says that only getting 10M from a 17M IP Profile is not a fault, after all.

[#14]

Morning! I hope the Digi Care chaps don't mind me jumping in
Your profile is currently set to: WBC 160K - 24M No delay (INP 0) 3dB Downstream, UC No delay (INP 0) 6dB Upstream (ADSL2+)
A quick translation might be in order...
WBC 160K - 24M - ADSL2+
No delay (INP 0) - No interleaving
3dB Downstream - 3dB target SNR margin on the downstream
UC - uncapped upstream
Unfortunately I can't tell you when or why it was changed from a 6dB target, but this can be hard set if required - just ask.
At the time of your last speed test you were profiled correctly on both our side and BT's side but it was run in peak time. Could you run another one out of peak hours (i.e. as early in the morning as feasibly possible) and let us know the results?
Your line's never been perfectly stable, with errors shown on both the up- and downstream which may be the cause of some loss of throughput.
To maximise your throughput I'd first recommend a change to interleaving on (INP1) to reduce erroring then go for a hard-set 6dB profile.
Please let us know if you'd like to go ahead with this.

[#15]

Quote

Unfortunately I can't tell you when or why it was changed from a 6dB target, but this can be hard set if required - just ask.

Ben,
I have a similar issue since being regraded from 20CN. The DLM has set a 3db target initailly causing it to be unstable, I manually resynched when the SNR was down to 2db ( in the evening ) and the line has held synch for about 4 days now with the SNR varing from about 3.9db in the morning to about 2.9 at night. The error counts, however are pretty high about 30000/day CRC errors and since I use voip on my line this is not ideal. I'd willingly sacrifice a little speed for reduced errors so a fixed 6db target would be really useful ( but not interleaving please as that will increase the latency and not good for voip ). I've already raised a ticket #41592331 requesting this so if you wouldn't mind having a look I would be very grateful.