Recently upgraded, had a performance drop - any explanation?

Hi, I upgraded to FTTC on 4 March and get a download speed of roughly 50mbit. On Monday this week my speed dropped down to 1mbit and then slowly increased over some hours. I contacted my ISP and they said there were no faults found. After about two to three hours, speeds were back to normal and it's been fine since. I'm just wondering what it might have been - DLM? Here are my results since upgrade. Note that on 10th March the line was upgraded from Infinity 1 to Infinity 2.

IP_ADDRESS,TEST_DATE,TIME_ZONE,DOWNLOAD_KILOBITS,UPLOAD_KILOBITS,LATENCY_MS,SERVER_NAME,DISTANCE_MILES
"46.18.xxx.xxx","3/4/2015 1:39 PM","GMT","37268","9357","8","London","0"
"46.18.xxx.xxx","3/8/2015 11:43 AM","GMT","37463","8981","10","London","0"
"46.18.xxx.xxx","3/9/2015 8:33 PM","GMT","37070","8963","9","London","0"
"46.18.xxx.xxx","3/10/2015 7:12 PM","GMT","52341","12127","8","London","0"
"46.18.xxx.xxx","3/10/2015 8:35 PM","GMT","52391","11572","9","London","0"
"46.18.xxx.xxx","3/11/2015 7:55 PM","GMT","52930","11492","11","London","0"
"46.18.xxx.xxx","3/12/2015 10:57 PM","GMT","53151","12060","9","London","0"
"46.18.xxx.xxx","3/14/2015 3:11 PM","GMT","52860","11462","10","London","0"
"5.63.xxx.xxx","3/16/2015 3:02 PM","GMT","1044","11285","9","London","0"
"46.18.xxx.xxx","3/16/2015 3:06 PM","GMT","3982","11247","10","London","0"
"46.18.xxx.xxx","3/16/2015 3:08 PM","GMT","4377","11381","10","London","0"
"46.18.xxx.xxx","3/16/2015 3:17 PM","GMT","5391","12067","8","London","0"
"46.18.xxx.xxx","3/16/2015 3:21 PM","GMT","3296","11712","9","London","0"
"46.18.xxx.xxx","3/16/2015 3:50 PM","GMT","8688","10780","9","London","0"
"46.18.xxx.xxx","3/16/2015 4:28 PM","GMT","23532","11997","8","London","0"
"46.18.xxx.xxx","3/16/2015 4:34 PM","GMT","26650","11283","10","London","0"
"46.18.xxx.xxx","3/16/2015 4:50 PM","GMT","52389","11896","8","London","0"
"46.18.xxx.xxx","3/16/2015 5:45 PM","GMT","52775","11196","10","London","0"
"46.18.xxx.xxx","3/17/2015 8:08 PM","GMT","53216","11322","11","London","0"

Hard to tell. Speedtest results only inform you that there was a problem, but are hopeless at identifying what the problem was, or where.

DLM might have been part of the problem (or rather a reaction to the real problem), but it rarely has that degree of impact. It would be more normal to see a drop of 10-20%. Speeds of 45-48Mbps would be more likely.

DLM also rarely de-intervenes quite so quickly, nor in such steps. It usually takes days or weeks to begin to recover, with resyncs happening overnight. Where recovery happened in stages, each stage would likely be a day apart.

Could also have been a problem with the speedtest server - without a different set of tests at the same time the speedtest results don't help pinpoint where the issue might be.

Just to add: I did a test using this site's speed tester and the BTW one with similar results at the time of the slow speeds, so it's not isolated to www.speedtest.net

Could it be new lines being installed as my cabinet only went live last month?

Edited by keymoo (Wed 18-Mar-15 14:32:42)

Were you able to check your line stats at the same time?

In reply to a post by ian72:

Were you able to check your line stats at the same time?

I haven't hacked my modem, so no. Not sure I want to risk flashing the firmware so soon after getting my new fibre connection.

I'm of the opposite opinion ... I want to get the line stats copied away from day 1, long before DLM makes a decision about whether to get involved, and long before any other lines get installed.

In reply to a post by keymoo:

Could it be new lines being installed as my cabinet only went live last month?

No.

Adding a line to the cab is likely to cause crosstalk on your line - essentially adding noise.

The amount of impact is, essentially, random - it might be almost nothing, while it might be large. It depends on the physical proximity of twisted pairs within cable bundles, and over what distance, and what imperfections exist.

At the instant the line goes active, you may see more noise (so a reduced SNRM) and/or more errors. If you see a reduced SNRM, then a subsequent resync may be at a lower speed. If you get more errors, then DLM might trigger overnight. DLM itself will cost you a single large drop of 10-20% of speed, and add 10ms to ping times.

This process repeats and repeats, as each line gets added. Each impact is random. Small or large. DLM may, or may not, trigger.

But, there are a few things that tend to happen...

The normal type of speed drops accumulate over time: It is possible to see 10-20Mbps disappear over 2 years, but not in one go.

The normal type of speed drops don't magically disappear, except for a brief respite when a neighbour's modem resyncs.

Your speed will not drop by 95% then recover by 500%. It just doesn't happen like that.

When DLM triggers (using old-style FEC and interleaving), you see a single large drop in speed of 10-20%, with a simultaneous increase in latency of 10-20ms. DLM may make subsequent adjustments, but the changes are relatively small.

Again, DLM doesn't work with the speed drops you saw, nor with the kind of recovery you saw.