System Administration Guide: Network Services

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. The dial-out machine then sends instructions over the serial line to its modem, including the phone number of the dial-in server.

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

    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. 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.

  5. When modem-to-modem negotiation is completed, 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 dial-out machine can then run telnet or a similar command to a host that is reachable through the dial-in server.