How to design an ISDN exchange! :-)

[kneival]

I am looking into a final year project for university and was thinking of designing an ISDN exchange
At uni we have 2 ISDN cards that cannot be used due to high costs of using an external exchange.
Do you think its possible to design and build a very simple exchange, the computers are in the same room + it doesnt even need to go full speed.
ANY information would be most appreciated, links, books, circuit diagrams ( ha ha )

Thanks in advance for any help on this matter.