Utilising USB modems with Linux

Universal Serial Bus, or USB, is fast becoming the standard in external device interfacing and, as a result, a range of USB modems is now available on the market. A key benefit of external USB modems is that they are bus powered and hence don't require an additional AC power supply. The good news for Linux users is that USB modems are supported by the kernel and you don't need to install third-party software or reboot your computer to get one working.


The first requirement is that the modem must comply with the Communication Device Class Abstract Control Model (CDC ACM) USB sub-class. There is a generic ACM driver in the kernel, which is used for modems and ISDN terminal adapters that conform to this specification. Therefore, make sure that the modem is ACM-compliant before you buy, and keep in mind that some USB modems are not ACM-compliant and have only Windows drivers. The Netcomm Roadster II 56 USB is a popular modem that complies with ACM.

The second major requirement is a Linux kernel version with USB support. The USB sub-system was developed and introduced with the 2.4 series and has since been back-ported to 2.2.18 and later kernels. A 2.4 series kernel is recommended to ensure that you have the latest drivers and features.


The great thing about using USB devices with Linux is that you can take advantage of the kernel's "hot plugging" ability. Hot plugging will load the appropriate module(s) upon detection of a device. If your distribution supports this, just plug in the USB modem and the ACM module will be loaded. Setting up hot plugging is beyond the scope of this article but Red Hat 7.2, Mandrake 8.1 and SuSE 7.3 all support it. If the modem lights flash and executing the # lsmod command reveals an ACM module, then your modem has been installed by hot plugging and you can skip ahead to the section on using the modem:


You will need at least three kernel modules loaded to get a USB modem working. They are usbcore - the base USB module; usb-uhci - the Universal Host Controller Interface (UHCI) root hub driver used in most Intel-based systems (use usb-ohci if yours is an Open HCI); and ACM - the Abstract Control Model driver. If these didn't come pre-compiled then re-compile your kernel with these drivers as modules.

Before you load the modules, as root, create a device node (if not already there) that will be used by the acm driver:

# mkmod /dev/usb/ttyACM0 c 166 0

Then do:

# modprobe -a usbcore usb-uhci acm

This will load all the required modules. In order to see your USB device details, mount the USB device file system with:

# mount -t usbdevfs usbdevfs /proc/bus/usbAnd then, with the modem plugged in, you can see it and the root hub's specifications with:

# cat /proc/bus/usb/devices


With all the USB modem drivers loaded, you are now ready to connect to the Internet. Insert (or replace) the device location of your USB modem - /dev/usb/ttyACM0 (Red Hat 7.2 uses /dev/input/ttyACM0) - into your Internet connection program or script. If you are accessing the Internet as a regular user, make sure all users can read and write to the device. As root, type in the following:

# chmod a+rw /dev/usb/ttyACM0

If you are accustomed to using /dev/modem then delete the old link and create a new one with:

# ln -s /dev/usb/ttyACM0 /dev/modem

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