LLU/ adsl2+ is one of the many things on my todo list.
SRA is just basically rate adaption "on the fly".
With max you sync at a particular speed and it stays like that. During poor conditions you will still retain the same sync speed until the signal gets so weak that the connection drops out. Then the router has to resync to get establish the new connection speed.
With adsl 2 aiui, SRA is meant to be able to sense these changes as they are about to occur, and adjust things without sync actually being lost.
