This article is from the Programming VCOMM FAQ, by firstname.lastname@example.org (Taed Nelson) with numerous contributions by others.
[Contributed by Taed Nelson (email@example.com).]
Microsoft Dial-Up Networking uses asynchonous PPP. Most, if not all, ISDN
systems use synchronous PPP. Therefore, to communicate over ISDN, your port
driver or the hardware must convert between async and sync PPP. This isn't
actually too hard, and is described in RFC 1662.