ISDN internet access

I cannot get broadband and have signed up for ISDN. My sons computer is networked with mine and he can access the internet using internet sharing. When we change to ISDN, what do I need so that he can still get on line, and can he do this without my computer being turned on. Do I need a router and is a ISDN router different from an ordinary broadband router. Do the two computers still have to be networked, when using a router to connect to internet. Thank you