Configuring a Serial Port Modem

Linux uses /dev/ttysx, /dev/ttyusBx, or /dev/usb/ttyusBx for serial ports, where x can range from 0 to 15. You can add many additional ports to a system using multiport cards or chained USB devices. A PC's integral serial ports are generally recognized at boot time.

To see a list of recognized ports for your system, pipe the dmesg command output through the fgrep command, as follows:

$ sudo dmesg | grep tty ttyS00 at 0x03f8 (irq = 4) is a 16550A

ttyS01 at 0x02f8 (irq = 3) is a 16550A

In this example, the grep command reports that two serial ports have been recognized in the dmesg output. Note that the device matching ttys00 is /dev/ttys0, despite the kernel output. The PC's external modem can be attached (most likely using a male DB9 adapter) to either port. Under Linux, nearly all modem-dependent software clients look for a symbolic link named /dev/modem that points to the desired device. This link is not created by default; as root, however, you can create this device manually using the ln command like this:

In this example, /dev/modem will point to the first serial port.

Ubuntu's network-admin (shown in Figure 4.12) will always detect the presence of a modem on the system. However, it does not activate the interface unless specifically told to do so. You can use the Auto-Detect button to find the correct modem port.

Figure 4.12. Ubuntu's network-admin will help you set up an appropriate modem.

Figure 4.12. Ubuntu's network-admin will help you set up an appropriate modem.

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