The problem with VDSL2

, posted: 26-Aug-2013 14:09

Update The below isn't correct, and my connection is now back to 67Mbps down. See the new blog post.

In May I blogged about my VDSL2 connection dropping down in speed, or rather switching profiles from 17a to 8b, for no apparent reason.

At the time, it looked like Alcatel-Lucent's Dynamic Line Management or DLM which Chorus uses on VDSL2 to monitor and adjust line conditions was behind the profile switch which shaved off well over twenty megabit/s download speed (uploads remained the same at 10Mbps).

Chorus kindly had a look at the line with an analyser and also sent out a Visionstream techie to physically check it for faults (thanks everyone, appreciate it).

Nothing's amiss with the line however, which shows good electrical characteristics.

Instead, the profile and speed drop is due to further DSL connections on the same cable as my one, resulting in poorer noise margin. This is the chart Chorus provided that shows how the noise margin on my connection has changed over time:

DSL noise margin

Not quite sure how to read it, to be honest, as it appears to show the noise margin on the downstream frequencies has improved while the upstream has deteriorated but anyway.

Apparently my line operates at 63-67Mbps at the cabinet. Signal loss between the cabinet and my place means roughly 22Mbps less download speed according to Chorus.

VDSL2 uses plenty more spectrum an ADSL2+, in higher frequencies which in turn are more sensitive to cross-talk or noise. In other words, as long as there are only a few DSL connections on the same cable sheath as your one, it works great. Add more DSL connections, and VDSL2, while still substantially faster than ADSL2+, will shift down to more sedate speeds.

This is a shame, and from this you can infer that VDSL2 isn't ever going to be an alternative to fibre-optic as I've heard some people claim. It's too sensitive to interference, and losing a third of speed at random will annoy customers especially since there's no way to restore it.

Vectoring, a form of noise cancelling, could in theory sort out the crosstalk problem. It's unlikely to be deployed here though as vectoring requires all VDSL2 lines in a cable from the cabinet to be controlled by the same system which means the technology  can't be used on unbundled copper lines (not very easily at least). Plus, it's an extra cost for the provider and Chorus, which is already spending heaps on UFB fibre.

So yeah: fibre would be nice.

Other related posts:
The problem with VDSL2, part 2
The mysterious Dynamic Line Management on VDSL2
Patchy UFB coverage for Auckland?

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