I think the SLIP/PPP/TCP-IP route may be a bit of an overkill if you control both ends of the communication.

However, if you would like to leverage the Internet, then you do need support these protocols. The distributor AllAmerican sells a board called the iKit2000 that has a Triscend E5 (8051-compatible) and the Seiko S7600A. It also has some example code on create a web-based "micro server" where you can remotely monitor sensors, etc. See ...

http://www.ikit2000.com

They have tutorials in the kit on connecting via a modem, on using the CMX MicroNet TCP-IP stack, and on using the CS8900A Ethernet chip.

http://www.cmx.com/

Also see the following Triscend application note on connecting to the CS8900A.

http://www.triscend.com/products/an30.pdf