You will need to configure your modem correctly for PPP - to do this READ YOUR MODEM MANUAL! Most modems come with a factory default setting that selects the options required for PPP. The minimum configuration specifies:-
Other settings (in standard Hayes commands) you should investigate are:-
Use your communications software (eg minicom) to find out about your modem configuration and set it to what is required for PPP. Many modems report their current settings in response to AT&V, but you should consult your modem manual. If you completely mess up the settings, you can return to sanity (usually) by issuing an AT&F - return to factory settings. (For most modem modems I have encountered, the factory settings include all you need for PPP - but you should check).
Save your modem configuration in non-volatile RAM (usually the modem command AT&W will do this - but check in your modem manual).
With the correct modem configuration already in the modem, resetting the modem will activate this. Arringing things this way considerably simplifies the chat script necessary for the PPP connection.
When data is travelling on serial communication lines, it can happen that data arrives faster than a computer can handle it (the computer may be busy doing something else - remember, Linux is a multi-user, multi- tasking operating system). In order to ensure that data is not lost (data does not over run in the input buffer and hence get lost), some method of controlling the flow of data is necessary.
There are two ways of doing this on serial lines:-
Whilst the latter may be fine for a terminal (text) link, data on a PPP link is 8 bits - and it is quite probable that somewhere in the data there will be data bytes that translate as control S and control Q. So, if a modem is set up to use software flow control, things can rapidly go beserk!
For PPP (which uses 8 bits of data) hardware flow control is vital.