Make an easy link to your modem device:
ln -s /dev/ttyS1 /dev/modem
Now change to the /usr/sbin directory and look for ppp files. Look for either a pppsetup or a pppconfig script. You may directly start this script to set up PPP on your system. On my Slackware system I typed pppsetup and started the script. This script asks for various parameters for connecting via the ISP.
It first asks for the phone number to dial -- enter "atdt#777", where 777 is replaced by the number that you need to dial.
It then asks for the modem -- select /dev/ttyS1
Baud Rate -- select "115200"
Callback -- Answer "NO"
Modem INIT String -- "ATZ OK "at+crm=1" OK"
ISP Domain Name -- Leave blank
DNS Server Address -- Enter a valid DNS Server IP, for instance "18.104.22.168" or "22.214.171.124", or leave blank
Authentication -- "PAP"
Refer to Section 3 for Username and Password.
Finally it shows you your configuration.
Some more work needs to be done. We need to edit the options file present in the /etc/ppp folder. We need to check if the following entries are present in the file:
lock defaultroute noipdefault modem /dev/ttyS1 115200 crtscts noauth passive asyncmap 0
|The noauth option|
Remember that noauth is by default commented; you need to uncomment it.
You can use egrep -v '#|^ *$' /etc/ppp/options to list only the options present in this file so as to quickly judge which ones are missing or incorrect.