For the dial-up connection to work between the remote client and the RAS server, a protocol must be used that takes the data packets exchanged between the client and the server and encapsulates them for movement over the POTS network. Encapsulation is just a fancy term for taking a particular frame type (such as an Ethernet frame) and placing it in a sort of electronic baggy. The frames can then move across the alien public phone network and still maintain their integrity as a particular frame type.
This electronic bagging or encapsulation of frames for movement across the phone network is handled by an access protocol . Two common access protocols are SLIP and PPP.
The Serial Line Internet Protocol (SLIP) is an access protocol that was developed primarily to allow PCs to connect to the Internet using a modem. It operates at the OSI model's Physical layer and allows data to flow across the telephone line to a remote system. SLIP provides no data compression or error checking and only supports connections to TCP/IP networks. The use of SLIP has all but evaporated because of the introduction of PPP.
The Point-to-Point Protocol (PPP) provides a reliable connection between computers, and PPP supports multiple network protocols, such as IP, IPX, and NetBEUI. PPP also provides compression and error checking and is therefore more reliable than a SLIP connection. PPP also supports password encryption, which allows for a more secure connection between the remote client and the RAS server.
You only really need to remember SLIP in terms of its historical significance; it was the first remote access protocol used for dial-in connections. PPP is used by default by most network operating systems and client operating systems. Whether you set up RAS for a NetWare network or a Windows network, PPP will be used as the access protocol for dial-up connections. PPP supports TCP/IP, IPX/SPX, and even NetBEUI. This means that the RAS client and the remote server can use a variety of LAN protocols to communicate over the PPP tunnel.