After a long run on Profile 17a, my VDSL2 connection was dropped down a few weeks ago to Profile 8b by the Dynamic Line Management that Chorus uses. What this means is that the connection, which on 17a would sync at 65Mbps or so down, now syncs at 42Mbps on 8b which uses less spectrum.
Why this happened isn't clear though - you don't get any warning of the profile downgrade, and the connection is terminated and then restarted by the DLM. In my case it happened twice and took about twenty minutes. Prior to the profile downgrade, the connection had been rock-solid with no outages for months.
However, DLM had dropped down the connection before, after power outages in area causing the modem to reboot even though the monitoring system isn't meant to be that aggressive apparently - at the time, Chorus switched off the DLM on my line, but somehow it got turned back on again.
According to the documentation, DLM is a module that is part of Alcatel-Lucent's 5530 NA-C network analyser. It's automated and monitors the phone line to apparently maximise the data rate while "respecting quality and stability needs of a group lines, called a service class."
DLM can clearly shift user connections to lower VDSL2 profiles and also enable interleaving on the line (ugh). I'm not sure it works as intended though and my ISP, Snap says the same. They have customers asking why when their connection appears to be stable it all of a sudden shifts into a much lower gear, causing helpdesk loading.
The problem from a user point of view is that the shift to a lower VDSL2 profile results in a drastic drop in performance, which isn't what anyone would like to see happening. Adding interleaving to the line bumps up the latency too, which also means lowered performance. This is not how you expect the service to behave.
Now, although DLM is meant to be able to adjust the line speed upwards when it figures the connection is stable again, that doesn't seem to happen. According to the documentation, the two bandplan profiles used by Chorus, 8b and 17a, have sixteen service templates or profile hierarchies that guide the transition - presumably between lower and higher speeds - so it is a bit odd that the only thing DLM appears able to do is to drop your connection downwards by a huge jump, and not move it upwards.
I'd be interested to hear more about this. My VDSL2 connection has gone from "awesome" to "pretty good" which is a shame, and there doesn't appear to be any reason why. Does DLM work, or does it need more work - or be turned off completely?
Yeah, I know the answer is a fibre-optic connection. Unfortunately, it doesn't look like the UFB will come to my part of Auckland this decade so am interested in getting the best possible performance out of Ye Olde Copper line.
Before anyone suggests otherwise, yes the line is in very good nick, with a new one expertly installed to the house by a Chorus/Visionstream technician.
Other related posts:
The problem with VDSL2, part 2
The problem with VDSL2
Patchy UFB coverage for Auckland?
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