So if you're transmitting 1000 bytes of data you can't send 500, and then multi-task for awhile, and then send the rest. It has to go out as one, big chunk. Its a lot like a rollar coaster ride. And, contrary to popular belief, you cannot change the clock speed on the line. Given a certain amount of traffic, the data should flow through the backbone without delay.
Congestion is when there is more aggregate data in the network than there is bandwidth.
Well, its like an eight lane freeway that turns into a four way intersection. Most of the time you just zip in. But whenever you've got more than four cars trying to get on the freeway at once someone has to slow down. If no-one slows down you have a serious crash. Now the problem with frame relay is that there is no slowing down.