It is actually to do with adsl and how it works and is actually more complicated than this.. but Ill try simplify it.
Adsl uses frequencies which are carried over your telephone line.
Think how if you were at the exchange and shouted to a house nearby and they would hear you.. but the further away then the less chance of hearing the signal.
The faster you sync, the higher frequencies are used. The higher frequencies are more subsceptible to Noise.. and depending upon the length of your line the the higher frequency signals are obviously the ones that will "fade" first.
Because your on Tiscali LLU - Im sepcifically talking about the Tiscali Profiles and NOT the BTw IP profile in the next bit.
In order to stop your line syncing too high and possible loss of syncs tiscali could say limit your line to say sync at 4096 kbps.
Theres a few other things that the DLM can do and thats turn
Error Correction and
Interleaving on. Turning both of these on helps your router recover from any errors and prevent loss of data.
Both Interleaving and Error Correction are turned on at the same time, but either of these can have different levels. The depth of each
interleaving profile is shown in the last column of that
table (eg depth 8).
The table also shows in some profiles on that table the
Target SNR, which is the "spare amount" of SNr Margin that your line must have to be able to sync at a certain speed.
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>> dslMAX: 7904mbps
Thats a guide as to the average speed you would expect a line of your length to sync at. However it cant possibly take into account local variations... and some will sync higher and some slightly less.
>> I am currently synced at 6144kbps as per my router
Yes I would expect a 30dB line to sync a bit higher than that. Are you plugged in at the master socket? Theres a few tips which may help on this page
~ Can I do anything about low SNR?