The mtnl broadband speed is set from prabhadevi exchange. I personally have the contact number of the lady in charge of speed change cause I have called them many times , they take about 30 min to change. Its cause when MTNL changes ur plan they dont upgrade ur speed immediately . U need to call them up to change it.
Yes, you call up Prabhadevi, but DSLAMs are managed remotely. Can you imagine what would happen if MTNL had to send a technician out to the local cabinet/exchange/DSLAM site each time someone wanted a plan change?
They send a command from their computer at Prabhadevi to the DSLAM which handles your line which tells the DSLAM to limit your speed to whatever - your modem can sync with the DSLAM at the maximum achievable rate but the DSLAM will only pass on data at a certain speed.
We are implementing a similar structure to our network whereby we can manage everything from a single location.