What is the difference between a null modem link and IPX-based links?
There is a fundamental difference. IPX is a packet protocol, which means information is transported in data units (packets). Null modem uses handshaking and then bi-directional send/receive streams to talk.
However, there is a serial link IPX driver available (contact Microcom) that allows you to use IPX over a modem dial-in link. This is commonly used to remotely log in to a NetWare server.