9. Configuring your modem

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:-

• Hardware flow control (RTS/CTS) (&K3 on many Hayes modems)

Other settings (in standard Hayes commands) you should investigate are:-

• E1 Command Echo ON (required for chat to operate)
• Q0 Report result codes (required for chat to operate)
• S0=0 Auto Answer OFF (unless you want your modem to answer the phone)
• &C1 Carrier Detect ON only after connect
• &S0 Data Set Ready (DSR) always ON
• (depends) Data Terminal Ready

It is also worth while investigating how the modem's serial interface between your computer and modem operates. Most modern modems allow you to run the serial interface at a FIXED speed whilst allowing the telephone line interface to change its speed to the highest speed it and the remote modem can both handle.

This is known as split speed operation. If your modem supports this, lock the modem's serial interface to its highest available speed (usually 115,200 baud but maybe 38,400 baud for 14,400 baud modems).

Use your communications software (e.g. 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. Arranging things this way considerably simplifies the chat script necessary for the PPP connection.

9.1 Note on Serial Flow Control

When data is traveling 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:-

• Using hardware signals (Clear To Send/Request to Send - CTS/RTS)
• Using software signals (control S and control Q).

Whilst the latter may be fine for a terminal (text) link, data on a PPP link uses all 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 berserk!

For PPP (which uses 8 bits of data) hardware flow control is vital.