1) The role of the model is that in fact it is the modem that converts signals used in the computer to signals that can be transmitted over the ISDN, and vice versa.

2) Yes, ADSL requires a special modem. ADSL needs also a splitter that is installed outside the user's home or office and its goal is to separate voice transmissions from data over the copper line.
It is just like DSL, analog signals converted into digital ones!

If I am wrong somebody correct me pls