What Happens During Dial-up Communications

PPP configuration files on both the dial-out and dial-in peers contain instructions for setting up the link. The following process occurs as the dial-up link is initiated:

1. User or process on the dial-out machine runs the pppd command to start the link.

2. Dial-out machine reads its PPP configuration files and sends instructions over the serial line to its modem, including the phone number of the dial-in server.

3. Modem dials the phone number and establishes a telephone connection with the modem on the dial-in server.

   If necessary, the dial-out machine sends commands to the dial-in server to invoke PPP on the server.

4. Modem attached to the dial-in server begins link negotiation with the modem on the dial-out machine.

   The series of text strings that the dial-out machine sends to the modem and dial-in server are contained in a file called a chat script.

5. When modem-to-modem negotiation completes, the modem on the dial-out machine reports “CONNECT.”

6. PPP on both peers enters Establish phase, where Link Control Protocol (LCP) negotiates basic link parameters and the use of authentication.

7. If necessary, the peers authenticate each other.

8. PPP's Network Control Protocols (NCPs) negotiate the use of network protocols, such as IPv4 or IPv6.

The user on the dial-out machine can then run rlogin, telnet, or a similar command to a host on a network reachable from the dial-in server.