When your router syncs with the exchange, it negotiates the speed dependent upon the
target SNR margin as set by BT. Depending on the time of day and various interference present / not present on the line, the speed will differ.
Once you are connected, the SNR margin can rise above or fall below the initial target, without causing a resync. The only thing that will cause a resync is if the SNR margin gets too low (i.e. there's too much noise on the line) to allow the modem to hang on. It can't 'hear' the ADSL conversation properly and so gives up and tries again.
This "target" SNR is initially 6dB. If you have problems, and your line resyncs more than an expected number of times (I don't know the exact figures, I wish I did!), then BT's DLM (Dynamic Line Management) will automatically raise the target SNR up, because it knows your line is disconnecting. It assumes the reason is because your SNR margin is falling too low, therefore you need more headroom. So it will raise it to 9dB. Then if that doesn't work, it will try 12dB, then 15dB, which it looks like you're on.
Now, when kitz said about rebooting when the SNR is at its highest... I'll give an example that should make it easier to follow:
Say you connect at 2Mbps and your target margin is 15dB. At some time during the day, the SNR may rise to 16 or 17dB. If you were to reboot at this point, the target SNR is still 15dB, but because the line is clearer, you may sync at say 2.6Mbps or something.
Now, as regards time of day. For a lot of people, SNR is better during the day than at night, but your mileage may vary. So best thing to do is if you keep an eye on your current SNR margin, then reboot the router when it's at its highest value that you observe, then you should get the most out of your line.
Good luck, and I hope this has explained things ... and not made it ten times more confusing for you!!
Any points you want clarifying please just say
