ADSL Bitloading Query

[#1]

Below is a picture, taken from DSLstats, of the current Bitloading map on my ADSL2+ link. I am curious about something.

Looking at the bitloading in the downlink spectrum range (in red) there are several dips that, if considered as being continuous and also looked at as inverted, would look like what I imagine they are - spectral lines. That is, they are likely caused by broadcast stations in the MF band. This seems confirmed as they are always present in the same position, session by session. Using DSLstats you can check the frequency corresponding to the Tone number and use a Medium Wave tuner to identify a likely station.

My interest for now is rather in the first single tone dip (to 2 Bits) in the lower register of the red downstream spectrum. In the example shown this is at about Tone number 56, identified as a frequency of 241.5 kHz
This single dip is also always present and stays fixed through any continuous ADSL session. But unlike the 'spectral' dips, its position alters session by session - after a restart of the router or a reconnection. Sometimes its position changes by a significant amount, though it is always somewhere in the lower quarter or so of the spectrum.

Can anyone suggest what could be causing it - particularly regarding its frequency hopping nature?

 BTW I am using a Technicolor TG 582n router.

TIA

[#2]

It's the pilot tone, an integral part of ADSL.

[#3]

OK. Thanks. Does that hop about between sessions? If so why, or is it dependent on the exact characteristics of the line at the start of the session?

[#4]

I have found this sort of thing has been discussed on here previously:

ADSL2+ Tones & Bitloading

Though presently not all of the posted links show the expected images.

[#5]

I don't know exactly how the pilot tone is chosen, it's usually towards the lower end so I assume the modem is choosing a good tone to use for the pilot.

On ADSL1, the pilot tone is 64 and carries no bits, on ADSL2/2+ it moves and can carry 2 bits.

The introduction of the ITU-T G.992.3 document states:

Improved robustness against bridged taps with receiver-determined pilot tone.