This sort of situation happens all the time. I'm not sure what sort of expanation you want, but here goes. Let me draw a simple analogy with voice phones:
At home, you have a telephone. You have one telephone line, or maybe two. That is your ordinary domestic telephone. That is like a BRI interface.
At work, the company you work for has a switchboard, with lots of connections to the telephone exchange. They might be able to handle, say, 30 telephone calls all at the same time. This is like a PRI.
When someone at your work calls you at home, he has no problem to do so. It doesn't matter that he is calling from a company switchboard, and you have a normal home phone. He is using only one channel of the company switchboard.
Maybe you have two telephone lines at home. He can phone you on both of them at the same time by using two phones in the company. He uses two channels of the company switchboard.
If he takes a third phone in the company and calls you at the same time, the third call will fail because all your lines at home are occupied. He can still tell you what he wants to tell you by using the two calls that are already connected. The data still gets through.
The point is that the telephone line goes only as far as the exchange. It is then up to the telecom provider to connect call by call. You don't need to know anythikng about the type of line in use at the other end of the call. The line is not end-to-end.
Sorry if the explanation is a bit basic, but I find analogies very useful.
Kevin Dorrell
Luxembourg.
