What is the difference between line speed and throughput speed?
A good way of describing this is with an analogy to a motorway.
70mph is the maximum speed limit on the motorway and is equivalent to the
line speed. But this doesn't mean you can do 70mph all the time - the motorway may be congested for various reasons, your car may be playing up, the weather may be playing tricks. It all slows you down.
Think of the speed that you're actually able to do on the motorway as your
Line speed is the maximum speed that your line is capable of supporting. Your line speed will vary depending on a number of factors such as line quality, how far away your exchange or cabinet is, and whether the ADSL micro-filters are installed correctly.
Throughput speed or download speed, is the actual speed that you'll get over your broadband. There are so many factors that can affect this that they've got their own FAQ:
My broadband is slow. What can I do?