This script is the command that the user actually runs.
It is stored in /etc/ppp/scripts/ppp-on, but I also have a link to it in /usr/local/bin, which is part of my standard path.
#! /bin/sh
# This script expects pppd version 2.2.0
# pppd uses /etc/ppp/options to configure itself
/usr/sbin/pppd /dev/cua0 38400
The script starts the pppd daemon. It reads the file /etc/ppp/options to find most of the parameters it needs to negotiate the connection.
Only two parameters need to be given on the command line: pppd is to connect to the first serial port (I have my mouse on the second) and is to use a baud rate to my modem of 38400.
For full and detailed information, see the man page on pppd.