My broadband feed comes overhead from a pole more or less
opposite my house. I don't know about the overhead section of
the line, but the dropwire is Drop Wire no.8 - not a twisted
pair, just like glorified bell wire. This comes into the house
to a GPO lightning protector (essentially 2 fuses), and then
twisted pair to an early NTE5 equipped with an Openreach Mk3
VDSL filter plate.
The combined modem/router is a Zyxel VMG1312. This graph
shows the bit loading (which is obtained with the command
xdslctl info --Bits). It only goes up to bin 1863 because
there's nothing in use on my line above that bin. Upstream bands are
in blue, downstream in green. And there's a very obvious big hole.
From bin 205 to bin 306 there are 0 bits assigned (with the
exception of bins 221 and 227-231 where there are 1 or 2 bits
assigned). Then there are odd single bins with 0 bits assigned,
which are 309, 338, 359, and 443. These bins translate to these
The big hole is from 884kHz (bin 205) to 1320kHz (bin 306).
That is the section of Medium Wave with the stations
from Radio 5 Live to Premier Christian Radio.
Bin 309 is 1333kHz - ?
Bin 338 is 1458kHz - that's Lyca, from Brookmans Park
Bin 359 is 1548kHz - that's Gold, from Saffron Green
Bin 443 is 1910kHz - hmmm, something in Top Band, can't see what
The only really strong transmitter that doesn't wipe out
(at least) one bin is London Turkish on 1584kHz (which is bin 367)
- and when I look at the bin data the adjacent bins are allocated
8 bits while bin 367 is allocated 5 bits, so it's had some effect.
I suspect something is getting overloaded by the big Medium
I believe my early NTE5 has a gas-filled spark gap protector -
rather than the 26A surge arrestor which is rumoured to affect
DSL performance. And I don't think there's anything in the
VDSL filter plate to overload. So is it the VDSL modem getting
Quiet Line Noise
In looking further into the problem, I found that the VDSL modem
as part of the sync process has a look at the RF noise on the line,
before the VDSL signals are present. It sort of turns the VDSL modem
into a spectrum analyser for a brief test. The data is stored for
the most recent VDSL sync, and can be obtained by the command
xdslctl info --QLN.
So I've looked at my QLN data (actually on a different VDSL box,
a Billion 8800NL, but it has the same problem as the VMG1312), and
I think it shows a disaster area!
Here's the graph, out to bin 1883:
And, zooming in to cover bins 0 to 400 (up to ~1.72 MHz):
The hole in the bit loading graph from bins 205 to 306
corresponds to the big signals with the wide skirts. They're
pretty much destroying VDSL in that frequency range. The strong
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